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The number average molar mass is a way of determining the molecular mass of a polymer. Polymer molecules, even ones of the same type, come in different sizes (chain lengths, for linear polymers), so the average molecular mass will depend on the method of averaging. The number average molecular mass is the ordinary arithmetic mean or average of the molecular masses of the individual macromolecules. It is determined by measuring the molecular mass of polymer molecules, summing the masses, and dividing by .
The number average molecular mass of a polymer can be determined by gel permeation chromatography, viscometry via the (Mark–Houwink equation), colligative methods such as vapor pressure osmometry, end-group determination or proton NMR.
High number-average molecular mass polymers may be obtained only with a high fractional monomer conversion in the case of step-growth polymerization, as per the Carothers' equation. | 7 | Physical Chemistry |
Common laboratory conductivity meters employ a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically. The electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of the electrodes. The potential between the inner pair is measured. Conductivity could in principle be determined using the distance between the electrodes and their surface area using Ohm's law but generally, for accuracy, a calibration is employed using electrolytes of well-known conductivity.
Industrial conductivity probes often employ an inductive method, which has the advantage that the fluid does not wet the electrical parts of the sensor. Here, two inductively-coupled coils are used. One is the driving coil producing a magnetic field and it is supplied with accurately-known voltage. The other forms a secondary coil of a transformer. The liquid passing through a channel in the sensor forms one turn in the secondary winding of the transformer. The induced current is the output of the sensor.
Another way is to use four-electrode conductivity sensors that are made from corrosion-resistant materials. A benefit of four-electrode conductivity sensors compared to inductive sensors is scaling compensation and the ability to measure low (below 100 μS/cm) conductivities (a feature especially important when measuring near-100% hydrofluoric acid). | 7 | Physical Chemistry |
Despite its limitations, the OR technique benefited from its close association with the development of the polymerase chain reaction. Kary Mullis, who also worked at Cetus, had synthesized the oligonucleotide probes being tested by Saiki and Erlich. Aware of the problems they were encountering, he envisioned an alternative method for analyzing the SCA mutation that would use components of the Sanger DNA sequencing technique. Realizing the difficulty of hybridizing an oligonucleotide primer to a single location in the genome, he considered using a second primer on the opposite strand. He then generalized that process and realized that repeated extensions of the two primers would lead to an exponential increase in the segment of DNA between the primers - a chain reaction of replication catalyzed by DNA polymerase.
As Mullis encountered his own difficulties in demonstrating PCR, he joined an existing group of researchers that were addressing the problems with OR. Together, they developed the combined PCR-OR assay. Thus, OR became the first method used to analyze PCR-amplified genomic DNA.
Mullis also encountered difficulties in publishing the basic idea of PCR (scientific journals rarely publish concepts without accompanying results). When his manuscript for the journal Nature was rejected, the basic description of PCR was hurriedly added to the paper originally intended to report the OR method (Mullis was also a co-author there). This OR paper thus became the first publication of PCR, and for several years would become the report most cited by other researchers. | 1 | Biochemistry |
Phytobenthos form biofilm with other microbial populations, including heterotrophic bacteria, which can also produce extracellular polymeric substance to help establish biofilm. Within these diverse communities, phytobenthos sustains the heterotrophs and mixotrophs not only by serving as food themselves. Phytobenthos can fix organic matters as primary producers, and the extracellular polymeric substance they produced to attach themselves to surfaces can also be utilized by bacteria as another potential carbon source. The presence of consumers are not the only biotic factors driving changes to the phytobenthos composition in the community. Photosynthetic populations that demonstrate themselves to be competitive can also change the benthic community makeup. The diatom D. geminata can proliferate quickly and are readily adaptive to changes to the aquatic environment.
Researchers have assigned trophic values or indicators based on the Periphyton Index of Trophic status (PIT) to phytobenthos as another means to determine the ecological status of water bodies. Researchers have also taken into consideration of the water chemistry, richness of the community, and biomass in their studies. Depending on the site of study, researchers also account for the activities from the phytobenthos when calculating for primary productivity. | 5 | Photochemistry |
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Walter Sidney Metcalf (18 May 1918 – 25 July 2008) was a New Zealand physical chemist.
Metcalf was born in Kaitangata on 18 May 1918, the son of George Metcalf, and was educated at Napier Boys High School. He earned a bachelors degree in music in parallel with his first science degree at the University of Otago, and completed a DPhil degree with E. J. Bowen at England's Oxford University.
Metcalf married Mary Glen Simmers, and the couple went on to have two children.
Metcalf initially worked at Victoria University College, then moved to Canterbury University College (now the University of Canterbury) in 1954. He retired as a reader in 1975. He worked mainly on photochemistry and was awarded the T. K. Sidey Medal by the Royal Society of New Zealand for his research in 1966. Towards the end of his career, he worked on calcium metabolism. | 7 | Physical Chemistry |
Typically, an ultrashort pulsed laser is used in the terahertz pulse generation process. In the use of low-temperature grown GaAs as an antenna, the ultrashort pulse creates charge carriers that are accelerated to create the terahertz pulse. In the use of non-linear crystals as a source, a high-intensity ultrashort pulse produces THz radiation from the crystal. A single terahertz pulse can contain frequency components covering much of the terahertz range, often from 0.05 to 4 THz, though the use of an air plasma can yield frequency components up to 40 THz. After THz pulse generation, the pulse is directed by optical techniques, focused through a sample, then measured.
THz-TDS requires generation of an ultrafast (thus, large bandwidth) terahertz pulse from an even faster femtosecond optical pulse, typically from a Ti-sapphire laser. That optical pulse is first split to provide a probe pulse whose path length is adjusted using an optical delay line. The probe pulse strobes the detector that is sensitive to the electric field of the resulting terahertz signal at the time of the optical probe pulse sent to it. By varying the path length traversed by the probe pulse, the test signal is thereby measured as a function of time—the same principle as a sampling oscilloscope (technically, the measurement obtains the convolution of the test signal and the time-domain response of the strobed detector). To obtain the resulting frequency domain response using the Fourier transform, the measurement must cover each point in time (delay-line offset) of the resulting test pulse. The response of a test sample can be calibrated by dividing its spectrum so obtained by the spectrum of the terahertz pulse obtained with the sample removed, for instance. | 7 | Physical Chemistry |
The first total synthesis of carpanone was the biomimetic approach published by Chapman et al. in 1971. The required desmethylcarpacin (2-allylsesamol), shown below as the starting molecule in the scheme, is acquired in two high-yield steps involving three transformations:
* allylation of the phenolic anion generated after treatment of sesamol with potassium carbonate and allyl bromide,
* followed by a thermal Claisen rearrangement to move the O-allyl group onto the adjacent site on the aromatic ring, and then
* thermal isomerization of the Claisen product, to move the terminal olefin (alkene) into conjugation with the ring (with e.g., potassium tert-butoxide as base).
This procedure is one of several that gives the required desmethylcarpacin (carpacin with the methyl of its methoxy group removed). Though oxidative dimerizations of phenols normally used a 1-electron oxidant, Chapman then followed a precedent involving a 2-electron oxidant and treated desmethylcarpacin with PdCl in the presence of sodium acetate (e.g., dissolved in a mixture of methanol and water); the reaction was perceived to proceed via a complexation of a pair of carpacins to the Pd(II) metal via their phenolic anions (as shown in scheme, below right), followed by a classic 8-8 (β-β) oxidative phenolic coupling of the two olefin tails—shown crossing in the image—to give a dimeric trans-ortho-quinone methide-type of lignan intermediate. A particular conformation of this dimer then places a 4-electron enone of one ring over the 2-electron enol of the other (shown adjacent in image for clarity), setting the state for a variant of the Diels-Alder reaction termed an inverse demand Diels-Alder reaction (see curved arrows in image), which closes the 2 new rings and generates the 5 contiguous stereocenters. The carpanone is produced in yields of ≈50% by the original method, and in yields >90% in modern variants (see below). The synthesis of a single diastereomer was confirmed in the original Chapman work, using X-ray crystallography.
For the elegance of its "one-pot construction of a tetracyclic scaffold with complete stereocontrol of five contiguous stereo centers", the original Chapman design and synthesis is "[n]ow considered a classic in total synthesis" that "highlights the power of biomimetic synthesis". | 0 | Organic Chemistry |
RajanBabu has over 160 publications to date and has co-authored several reviews and patents. His H-index is 56.
Notable publications include:
* Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators
* Selective Generation of Free Radicals from Epoxides Using a Transition-Metal Radical. A Powerful New Tool for Organic Synthesis
* Transition-metal-centered radicals in organic synthesis. Titanium(III)-induced cyclization of epoxy olefins
* Ligand Electronic Effects in Asymmetric Catalysis: Enhanced Enantioselectivity in the Asymmetric Hydrocyanation of Vinylarenes
* Beyond Nature's Chiral Pool - Enantioselective Catalysis in Industry | 0 | Organic Chemistry |
An increased thyroxine-binding globulin results in an increased total thyroxine and total triiodothyronine without an actual increase in hormonal activity of thyroid hormones.
Reference ranges: | 1 | Biochemistry |
CRP is used mainly as an inflammation marker. Apart from liver failure, there are few known factors that interfere with CRP production. Interferon alpha inhibits CRP production from liver cells which may explain the relatively low levels of CRP found during viral infections compared to bacterial infections
Measuring and charting CRP values can prove useful in determining disease progress or the effectiveness of treatments. ELISA, immunoturbidimetry, nephelometry, radial immunodiffusion
* low: hs-CRP level under 1.0 mg/L
* average: between 1.0 and 3.0 mg/L
* high: above 3.0 mg/L
Normal levels increase with aging. Higher levels are found in late pregnant women, mild inflammation and viral infections (10–40 mg/L), active inflammation, bacterial infection (40–200 mg/L), severe bacterial infections and burns (>200 mg/L).
CRP cut-off levels indicating bacterial from non-bacterial illness can vary due to co-morbidities such as malaria, HIV and malnutrition and the stage of disease presentation.
CRP is a more sensitive and accurate reflection of the acute phase response than the ESR (erythrocyte sedimentation rate). ESR may be normal while CRP is elevated. CRP returns to normal more quickly than ESR in response to therapy. | 1 | Biochemistry |
The major difference between atomic force microscopy and competing technologies such as optical microscopy and electron microscopy is that AFM does not use lenses or beam irradiation. Therefore, it does not suffer from a limitation in spatial resolution due to diffraction and aberration, and preparing a space for guiding the beam (by creating a vacuum) and staining the sample are not necessary.
There are several types of scanning microscopy including scanning probe microscopy (which includes AFM, scanning tunneling microscopy (STM) and near-field scanning optical microscope (SNOM/NSOM), STED microscopy (STED), and scanning electron microscopy and electrochemical AFM, EC-AFM). Although SNOM and STED use visible, infrared or even terahertz light to illuminate the sample, their resolution is not constrained by the diffraction limit. | 6 | Supramolecular Chemistry |
In thermodynamics, a spontaneous process is a process which occurs without any external input to the system. A more technical definition is the time-evolution of a system in which it releases free energy and it moves to a lower, more thermodynamically stable energy state (closer to thermodynamic equilibrium). The sign convention for free energy change follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in the free energy of the system and a positive change in the free energy of the surroundings.
Depending on the nature of the process, the free energy is determined differently. For example, the Gibbs free energy change is used when considering processes that occur under constant pressure and temperature conditions, whereas the Helmholtz free energy change is used when considering processes that occur under constant volume and temperature conditions. The value and even the sign of both free energy changes can depend upon the temperature and pressure or volume.
Because spontaneous processes are characterized by a decrease in the system's free energy, they do not need to be driven by an outside source of energy.
For cases involving an isolated system where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in entropy.
A spontaneous reaction is a chemical reaction which is a spontaneous process under the conditions of interest. | 7 | Physical Chemistry |
Impermeable plasma is a type of thermal plasma which acts like an impermeable solid with respect to gas or cold plasma and can be physically pushed. Interaction of cold gas and thermal plasma was briefly studied by a group led by Hannes Alfvén in 1960s and 1970s for its possible applications in insulation of fusion plasma from the reactor walls. However, later it was found that the external magnetic fields in this configuration could induce kink instabilities in the plasma and subsequently lead to an unexpectedly high heat loss to the walls. In 2013, a group of materials scientists reported that they have successfully generated stable impermeable plasma with no magnetic confinement using only an ultrahigh-pressure blanket of cold gas. While spectroscopic data on the characteristics of plasma were claimed to be difficult to obtain due to the high pressure, the passive effect of plasma on synthesis of different nanostructures clearly suggested the effective confinement. They also showed that upon maintaining the impermeability for a few tens of seconds, screening of ions at the plasma-gas interface could give rise to a strong secondary mode of heating (known as viscous heating) leading to different kinetics of reactions and formation of complex nanomaterials. | 7 | Physical Chemistry |
The vertical axis is labeled E for the voltage potential with respect to the standard hydrogen electrode (SHE) as calculated by the Nernst equation. The "H" stands for hydrogen, although other standards may be used, and they are for room temperature only.
For a reversible redox reaction described by the following chemical equilibrium:
With the corresponding equilibrium constant :
The Nernst equation is:
sometimes formulated as:
or, more simply directly expressed numerically as:
where:
* volt is the thermal voltage or the "Nernst slope" at standard temperature
* λ = ln(10) ≈ 2.30, so that volt.
The horizontal axis is labeled pH for the −log function of the H ion activity.
The lines in the Pourbaix diagram show the equilibrium conditions, that is, where the activities are equal, for the species on each side of that line. On either side of the line, one form of the species will instead be said to be predominant.
In order to draw the position of the lines with the Nernst equation, the activity of the chemical species at equilibrium must be defined. Usually, the activity of a species is approximated as equal to the concentration (for soluble species) or partial pressure (for gases). The same values should be used for all species present in the system.
For soluble species, the lines are often drawn for concentrations of 1 M or 10 M. Sometimes additional lines are drawn for other concentrations.
If the diagram involves the equilibrium between a dissolved species and a gas, the pressure is usually set to P = 1 atm = , the minimum pressure required for gas evolution from an aqueous solution at standard conditions.
In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation.
The diagrams also do not take kinetic effects into account, meaning that species shown as unstable might not react to any significant degree in practice.
A simplified Pourbaix diagram indicates regions of "immunity", "corrosion" and "passivity", instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while corrosion shows that general attack will occur. Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on its surface when exposed to air. | 7 | Physical Chemistry |
Hardy worked as assistant professor of organic chemistry at Bryn Mawr College in 1939 and 1940. In the years 1942–1958, Hardy worked as a chemist at Calco Chemical Division, subsequently she worked as a literature chemist at Lederle Labs from 1958 to 1975. After working for Lederle, Hardy worked as a senior resident literature chemist for American Cyanamid Company from 1975 on. She was a member of the American Association for the Advancement of Science, American Chemical Society and Chemical Institute of Canada. Hardy worked in a number of different research areas including molecular rearrangements, preparation of unsaturated esters and ketones, vat dyestuffs, esterification of leuco vat dyes, organosulfur compounds, and pharmaceutical chemistry. | 0 | Organic Chemistry |
* RRS effect (Resonance Raman Scaterring)
The Raman resonance effect produces an increase in Raman intensity up to 10 times. In this phenomenon, the monochromatic light interaction with the sample produces the transition of the molecules from the fundamental state to an excited electronic state, instead of a virtual state as in normal Raman spectroscopy. This phenomenon of increased intensity could be observed in materials such as carbon nanotubes.
* SERS (Surface-Enhanced Raman Scattering)
Surface-Enhanced Raman Scattering (SERS) is a technique capable of increasing Raman signal intensity up to 10 times. This phenomenon is based on the interaction of monochromatic light with materials that exhibit plasmonic properties. The most common metals used in SERS are nanostructured metals with plasmonic band (gold, silver or copper). Nanostructured electrode surfaces can be generated by depositing metallic nanostructures of these materials. A disadvantage of this phenomenon is, sometimes, the lack of reproducibility of the spectra due to the difficulty of obtaining identical nanostructured surfaces in each experiment.
* SOERS (Surface-Oxidation-Enhanced Raman Scattering)
Surface-oxidation enhanced Raman scattering (SOERS) is a process similar to SERS, which allows the Raman signal to be enhanced when a silver electrode is oxidized in a particular electrolyte composition. This process is carried out at sufficiently positive potentials to ensure the oxidation of the electrode surface. There are significant differences with the SERS effect, but it is a phenomenon that also enhances the Raman signal.
* SHINERS (Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy)
In SHINERS, metallic nanoparticles with plasmonic properties are coated with ultra-thin homogeneous silica or alumina layers, forming isolated nanoparticles. The metallic nucleus (Au or Ag) is responsible of the enhancement of the Raman signals of the nearby molecules, while the coating layers eliminate the influence of the metallic nucleus on the Raman and electrochemical signals by preventing the molecules from being directly adsorbed onto them. Silica and alumina coating can improve the chemical and thermal stability of nanoparticles. This fact has great importance in the in-situ study of catalytic reactions. The high sensitivity of the SHINERS surfaces makes these nanostructures a promising tool for the study of liquid-solid interfaces, especially in spectroelectrochemistry.
* TERS (Tip-Enhanced Raman Scattering)
Tip-enhanced Raman scattering (TERS) is a technique that provides molecular information at nanoscale. In these experiments, metal nanostructures are replaced by a sharp metal tip of nanometric size, concentrating the roughness directly on a small region that improves the spatial resolution of scanning techniques in Raman spectroscopy.
<br /> | 7 | Physical Chemistry |
Micro pitting is a fatigue failure of the surface of a material commonly seen in rolling bearings and gears.
It is also known as grey staining, micro spalling or frosting. | 8 | Metallurgy |
The enantioselective version of the Tsuji–Trost reaction is called the Trost asymmetric allylic alkylation (Trost AAA) or simply, asymmetric allylic alkylation (AAA). These reactions are often used in asymmetric synthesis. The reaction was originally developed with a palladium catalyst supported by the Trost ligand, although suitable conditions have greatly expanded since then.
Enantioselectivity can be imparted to the reaction during any of the steps aside from the decomplexation of the palladium from the alkene since the stereocenter is already set at that point. Five main ways have been conceptualized to take advantage of these steps and yield enantioselective reaction conditions.
These methods of enantiodiscrimination were previously reviewed by Trost:
# Preferential ionization via enantioselective olefin complexation
# Enantiotopic ionization of leaving groups
# Attack at enantiotopic ends of the allyl complex
# Enantioface exchange in the -allyl complex
# Differentiation of prochiral nucleophile faces
The favored method for enantiodiscrimination is largely dependent on the substrate of interest, and in some cases, the enantioselectivity may be influenced by several of these factors. | 0 | Organic Chemistry |
The insertion of alkenes into metal-hydrogen bonds is a key step in hydrogenation and hydroformylation reactions. The reaction involves the alkene and the hydride ligands combining within the coordination sphere of a catalyst. In hydrogenation, the resulting alkyl ligand combines with a second hydride to give the alkane. Analogous reactions apply to the hydrogenation of alkynes: an alkenyl ligand combines with a hydride to eliminate an alkene. | 0 | Organic Chemistry |
The lower rim of the cyclooctane B ring containing the first 5 carbon atoms was synthesized in a semisynthesis starting from naturally occurring L-serine (scheme 1). This route started with conversion of the amino group of the serine methyl ester (1) to the diol ester 2 via diazotization (sodium nitrite/sulfuric acid). After protection of the primary alcohol group to a (t-butyldimethyl) TBS silyl ether (TBSCl / imidazole) and that of the secondary alcohol group with a (Bn) benzyl ether (benzyl imidate, triflic acid), the aldehyde 3 was reacted with the methyl ester of isobutyric acid (4) in an Aldol addition to alcohol 5 with 65% stereoselectivity. This group was protected as a PMB (p-methoxybenzyl) ether (again through an imidate) in 6 which enabled organic reduction of the ester to the aldehyde in 7 with DIBAL.
Completing the cyclooctane ring required 3 more carbon atoms that were supplied by a C2 fragment in an aldol addition and a Grignard C1 fragment (scheme 2). A Mukaiyama aldol addition (magnesium bromide / toluene) took place between aldehyde 7 and ketene silyl acetal 8 with 71% stereoselectivity to alcohol 9 which was protected as the TBS ether 10 (TBSOTf, 2,6-lutidine). The ester group was reduced with DIBAL to an alcohol and then back oxidized to aldehyde 11 by Swern oxidation. Alkylation by methyl magnesium bromide to alcohol 12 and another Swern oxidation gave ketone 13. This group was converted to the silyl enol ether 14 (LHMDS, TMSCl) enabling it to react with NBS to alkyl bromide 15. The C20 methyl group was introduced as methyl iodide in a nucleophilic substitution with a strong base (LHMDS in HMPA) to bromide 16. Then in preparation to ring-closure the TBS ether was deprotected (HCl/THF) to an alcohol which was converted to the aldehyde 17 in a Swern oxidation. The ring-closing reaction was a Reformatskii reaction with Samarium(II) iodide and acetic acid to acetate 18. The stereochemistry of this particular step was of no consequence because the acetate group is dehydrated to the alkene 19 with DBU in benzene. | 0 | Organic Chemistry |
The NRTL parameters are fitted to activity coefficients that have been derived from experimentally determined phase equilibrium data (vapor–liquid, liquid–liquid, solid–liquid) as well as from heats of mixing. The source of the experimental data are often factual data banks like the Dortmund Data Bank. Other options are direct experimental work and predicted activity coefficients with UNIFAC and similar models.
Noteworthy is that for the same liquid mixture several NRTL parameter sets might exist. The NRTL parameter set to use depends on the kind of phase equilibrium (i.e. solid–liquid (SL), liquid–liquid (LL), vapor–liquid (VL)). In the case of the description of a vapor–liquid equilibria it is necessary to know which saturated vapor pressure of the pure components was used and whether the gas phase was treated as an ideal or a real gas. Accurate saturated vapor pressure values are important in the determination or the description of an azeotrope. The gas fugacity coefficients are mostly set to unity (ideal gas assumption), but for vapor-liquid equilibria at high pressures (i.e. > 10 bar) an equation of state is needed to calculate the gas fugacity coefficient for a real gas description.
Determination of NRTL parameters from LLE data is more complicated than parameter regression from VLE data as it involves solving isoactivity equations which are highly non-linear. In addition, parameters obtained from LLE may not always represent the real activity of components due to lack of knowledge on the activity values of components in the data regression. For this reason it is necessary to confirm the consistency of the obtained parameters in the whole range of compositions (including binary subsystems, experimental and calculated lie-lines, Hessian matrix, etc.). | 7 | Physical Chemistry |
Splicing of all intron-containing RNA molecules is superficially similar, as described above. However, different types of introns were identified through the examination of intron structure by DNA sequence analysis, together with genetic and biochemical analysis of RNA splicing reactions. At least four distinct classes of introns have been identified:
*Introns in nuclear protein-coding genes that are removed by spliceosomes (spliceosomal introns)
* Introns in nuclear and archaeal transfer RNA genes that are removed by proteins (tRNA introns)
* Self-splicing group I introns that are removed by RNA catalysis
* Self-splicing group II introns that are removed by RNA catalysis
Group III introns are proposed to be a fifth family, but little is known about the biochemical apparatus that mediates their splicing. They appear to be related to group II introns, and possibly to spliceosomal introns. | 1 | Biochemistry |
In materials where the maximum applied-stress-intensity factor exceeds the stress-corrosion cracking-threshold value, stress corrosion adds to crack-growth velocity. This is shown in the schematic on the right. In a corrosive environment, the crack grows due to cyclic loading at a lower stress-intensity range; above the threshold stress intensity for stress corrosion cracking, additional crack growth (the red line) occurs due to SCC. The lower stress-intensity regions are not affected, and the threshold stress-intensity range for fatigue-crack propagation is unchanged in the corrosive environment. In the most-general case, corrosion-fatigue crack growth may exhibit both of the above effects; crack-growth behavior is represented in the schematic on the left. | 8 | Metallurgy |
eIF2 is an essential factor for protein synthesis that forms a ternary complex (TC) with GTP and the initiator Met-tRNA. After its formation, the TC binds the 40S ribosomal subunit to form the 43S preinitiation complex (43S PIC). 43S PIC assembly is believed to be stimulated by the initiation factors eIF1, eIF1A, and the eIF3 complex according to in vitro experiments. The 43S PIC then binds mRNA that has previously been unwound by the eIF4F complex. The 43S PIC and the eIF4F proteins form a new 48S complex on the mRNA, which starts searching along the mRNA for the start codon (AUG). Upon base pairing of the AUG-codon with the Met-tRNA, eIF5 (which is a GTPase-activating protein , or GAP) is recruited to the complex and induces eIF2 to hydrolyse its GTP. This causes eIF2-GDP to be released from this 48S complex and translation begins after recruitment of the 60S ribosomal subunit and formation of the 80S initiation complex. Finally, with the help of the guanine nucleotide exchange factor (GEF) eIF2B, the GDP in eIF2 is exchanged for a GTP and the ternary complex reforms for a new round of translation initiation. | 1 | Biochemistry |
Underpotential deposition (UPD), in electrochemistry, is a phenomenon of electrodeposition of a species (typically reduction of a metal cation to a solid metal) at a potential less negative than the equilibrium (Nernst) potential for the reduction of this metal. The equilibrium potential for the reduction of a metal in this context is the potential at which it will deposit onto itself. Underpotential deposition can then be understood to be when a metal can deposit onto another material more easily than it can deposit onto itself. | 7 | Physical Chemistry |
In December 2007, Takeda submitted a New Drug Application (NDA) for alogliptin to the United States Food and Drug Administration (FDA), after positive results from Phase III clinical trials. In September 2008, the company also filed for approval in Japan, winning approval in April 2010. The company also filed a Marketing Authorization Application elsewhere outside the United States, which was withdrawn in June 2009 needing more data. The first NDA failed to gain approval and was followed by a pair of NDAs (one for alogliptin and a second for a combination of alogliptin and pioglitazone) in July 2011. In 2012, Takeda received a negative response from the FDA on both of these NDAs, citing a need for additional data.
In 2013, the FDA approved the drug in three formulations: as a stand-alone with the brand-name Nesina, combined with metformin using the name Kazano, and when combined with pioglitazone as Oseni. | 4 | Stereochemistry |
Cannabis (/ˈkænəbɪs/) is commonly known as marijuana or hemp and has two known strains: Cannabis sativa and Cannabis indica, both of which produce chemicals to deter herbivory. The chemical composition includes specialized terpenes and cannabinoids, mainly tetrahydrocannabinol (THC), and cannabidiol (CBD). These substances play a role in defending the plant from pathogens including insects, fungi, viruses and bacteria. THC and CBD are stored mostly in the trichomes of the plant, and can cause psychological and physical impairment in the user, via the endocannabinoid system and unique receptors. THC increases dopamine levels in the brain, which attributes to the euphoric and relaxed feelings cannabis provides. As THC is a secondary metabolite, it poses no known effects towards plant development, growth, and reproduction. However, some studies show secondary metabolites such as cannabinoids, flavonoids, and terpenes are used as defense mechanisms against biotic and abiotic environmental stressors. | 1 | Biochemistry |
The resulting flow from applying a voltage is a plug flow. Unlike a parabolic profile flow generated from a pressure differential, a plug flow’s velocity profile is approximately planar, with slight variation near the electric double layer. This offers significantly less deleterious dispersive effects and can be controlled without valves, offering a high-performance method for fluid separation, although many complex factors prove this control to be difficult. Because of difficulties measuring and monitoring flow in microfluidic channels, primarily disrupting the flow pattern, most analysis is done through numerical methods and simulation.
Electroosmotic flow through microchannels can be modeled after the Navier-Stokes equation with the driving force deriving from the electric field and the pressure differential. Thus it is governed by the continuity equation
and momentum
where is the velocity vector, is the density of the fluid, is the material derivative, is the viscosity of the fluid, is the electric charge density, is the applied electric field, is the electric field due to the zeta potential at the walls and is the fluid pressure.
Laplace’s equation can describe the external electric field
while the potential within the electric double layer is governed by
where is the dielectric constant of the electrolyte solution and is the vacuum permittivity. This equation can be further simplified using the Debye-Hückel approximation
where is the Debye length, used to describe the characteristic thickness of the electric double layer. The equations for potential field within the double layer can be combined as
The transport of ions in space can be modeled using the Nernst–Planck equation:
Where is the ion concentration, is the magnetic vector potential, is the diffusivity of the chemical species, is the valence of ionic species, is the elementary charge, is the Boltzmann constant, and is the absolute temperature. | 7 | Physical Chemistry |
In step-growth polymerization, the Carothers equation (or Carothers' equation) gives the degree of polymerization, , for a given fractional monomer conversion, .
There are several versions of this equation, proposed by Wallace Carothers, who invented nylon in 1935. | 7 | Physical Chemistry |
The percent yield is a comparison between the actual yield—which is the weight of the intended product of a chemical reaction in a laboratory setting—and the theoretical yield—the measurement of pure intended isolated product, based on the chemical equation of a flawless chemical reaction, and is defined as,
The ideal relationship between products and reactants in a chemical reaction can be obtained by using a chemical reaction equation. Stoichiometry is used to run calculations about chemical reactions, for example, the stoichiometric mole ratio between reactants and products. The stoichiometry of a chemical reaction is based on chemical formulas and equations that provide the quantitative relation between the number of moles of various products and reactants, including yields. Stoichiometric equations are used to determine the limiting reagent or reactant—the reactant that is completely consumed in a reaction. The limiting reagent determines the theoretical yield—the relative quantity of moles of reactants and the product formed in a chemical reaction. Other reactants are said to be present in excess. The actual yield—the quantity physically obtained from a chemical reaction conducted in a laboratory—is often less than the theoretical yield. The theoretical yield is what would be obtained if all of the limiting reagent reacted to give the product in question. A more accurate yield is measured based on how much product was actually produced versus how much could be produced. The ratio of the theoretical yield and the actual yield results in a percent yield.
When more than one reactant participates in a reaction, the yield is usually calculated based on the amount of the limiting reactant, whose amount is less than stoichiometrically equivalent (or just equivalent) to the amounts of all other reactants present. Other reagents present in amounts greater than required to react with all the limiting reagent present are considered excess. As a result, the yield should not be automatically taken as a measure for reaction efficiency.
In their 1992 publication General Chemistry, Whitten, Gailey, and Davis described the theoretical yield as the amount predicted by a stoichiometric calculation based on the number of moles of all reactants present. This calculation assumes that only one reaction occurs and that the limiting reactant reacts completely.
According to Whitten, the actual yield is always smaller (the percent yield is less than 100%), often very much so, for several reasons. As a result, many reactions are incomplete and the reactants are not completely converted to products. If a reverse reaction occurs, the final state contains both reactants and products in a state of chemical equilibrium. Two or more reactions may occur simultaneously, so that some reactant is converted to undesired side products. Losses occur in the separation and purification of the desired product from the reaction mixture. Impurities are present in the starting material which do not react to give desired product. | 0 | Organic Chemistry |
The most frequently introduced forms are:where:
* is the absolute pressure of the gas,
* is the volume of the gas,
* is the amount of substance of gas (also known as number of moles),
* is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,
* is the Boltzmann constant,
* is the Avogadro constant,
* is the absolute temperature of the gas,
* is the number of particles (usually atoms or molecules) of the gas.
In SI units, p is measured in pascals, V is measured in cubic metres, n is measured in moles, and T in kelvins (the Kelvin scale is a shifted Celsius scale, where 0.00 K = −273.15 °C, the lowest possible temperature). R has for value 8.314 J/(mol·K) = 1.989 ≈ 2 cal/(mol·K), or 0.0821 L⋅atm/(mol⋅K). | 7 | Physical Chemistry |
Fish generally use bioluminescence for camouflage to hide from predators. Endogenous photocytes are more commonly used for bioluminescence than other means like bacteria. Some fish may use the bioluminescence produced by their photocytes as a means of communication. | 1 | Biochemistry |
Hemichromes form an insoluble macromolecule (macromolecular aggregate) by copolymerization with the cytoplasm of band 3. Covalent bonds reinforce the aggregate interactions of the hemichromes which are accumulated on the surface of the membrane. However, hemichromes are less stable than their native form. | 1 | Biochemistry |
Modafinil is occasionally prescribed off-label for individuals with attention deficit hyperactivity disorder (ADHD). It has shown no efficacy in treating adult ADHD, especially when compared to other treatments such as lisdexamfetamine. In children, modafinil is efficient in treating ADHD symptoms.
Given its approved status in the US to treat narcolepsy, physicians can also prescribe modafinil for off-label uses, such as treating ADHD in both children and adults.
The Canadian Network for Mood and Anxiety Treatments (CANMAT) suggests modafinil as a second-line choice for ADHD, after the first-line choices such as bupropion are exhausted. | 4 | Stereochemistry |
The eudysmic ratio (also spelled eudismic ratio) represents the difference in pharmacologic activity between the two enantiomers of a drug. In most cases where a chiral compound is biologically active, one enantiomer is more active than the other. The eudysmic ratio is the ratio of activity between the two. A eudysmic ratio significantly differing from 1 means that they are statistically different in activity. Eudisimic ratio (ER) reflects the degree of enantioselectivity of the biological systems. For example, (S)-propranolol (ER = 130) meaning that (S)-propranolol is 130 times more active than its (R)-enantiomer. | 4 | Stereochemistry |
K. C. Nicolaou was born on July 5, 1946, in Karavas, Cyprus where he grew up and went to school until the age of 18. In 1964, he went to England where he spent two years learning English and preparing to enter University. He studied chemistry at the University of London (B.Sc., 1969, Bedford College; Ph.D. 1972, University College London, with Professors F. Sondheimer and P. J. Garratt). In 1972, he moved to the United States and, after postdoctoral appointments at Columbia University (1972–1973, Professor T. J. Katz) and Harvard University (1973–1976, Professor E. J. Corey), he joined the faculty at the University of Pennsylvania where he became the Rhodes-Thompson Professor of Chemistry. While at Penn, he won the prestigious Sloan Fellowship.
In 1989, he relocated to San Diego, where he took up a joint appointment at the University of California, San Diego, where he served as Professor of Chemistry, and The Scripps Research Institute, where he was Darlene Shiley Professor of Chemistry and Chairman of the Department of Chemistry. In 1996, he was appointed Aline W. and L.S. Skaggs Professor of Chemical Biology in The Skaggs Institute for Chemical Biology, The Scripps Research Institute. From 2005 to 2011, he directed Chemical Synthesis Laboratory @ ICES-A*STAR, Singapore. In 2013, Nicolaou moved to Rice University.
The Nicolaou group is active in the field of organic chemistry with research interests in methodology development and total synthesis. He is responsible for the synthesis of many complex molecules found in nature, such as Taxol and vancomycin. His groups route to Taxol, completed in 1994 at roughly the same time as a synthesis by the group of Robert A. Holton, attracted national news media attention due to Taxols structural complexity and its potent anti-cancer activity. | 0 | Organic Chemistry |
The Bouguer-Lambert law may be applied to describe the attenuation of solar or stellar radiation as it travels through the atmosphere. In this case, there is scattering of radiation as well as absorption. The optical depth for a slant path is , where refers to a vertical path, is called the relative airmass, and for a plane-parallel atmosphere it is determined as where is the zenith angle corresponding to the given path. The Bouguer-Lambert law for the atmosphere is usually written
where each is the optical depth whose subscript identifies the source of the absorption or scattering it describes:
* refers to aerosols (that absorb and scatter);
* are uniformly mixed gases (mainly carbon dioxide (CO) and molecular oxygen (O) which only absorb);
* is nitrogen dioxide, mainly due to urban pollution (absorption only);
* are effects due to Raman scattering in the atmosphere;
* is water vapour absorption;
* is ozone (absorption only);
* is Rayleigh scattering from molecular oxygen () and nitrogen () (responsible for the blue color of the sky);
* the selection of the attenuators which have to be considered depends on the wavelength range and can include various other compounds. This can include tetraoxygen, HONO, formaldehyde, glyoxal, a series of halogen radicals and others.
is the optical mass or airmass factor, a term approximately equal (for small and moderate values of ) to where is the observed objects zenith angle (the angle measured from the direction perpendicular to the Earths surface at the observation site). This equation can be used to retrieve , the aerosol optical thickness, which is necessary for the correction of satellite images and also important in accounting for the role of aerosols in climate. | 7 | Physical Chemistry |
In situ infrared spectroscopy may be used to monitor the course of a reaction, provided a reagent or product shows distinctive absorbance in the IR spectral region. The rate of reactant consumption and/or product formation may be abstracted from the change of absorbance over time (by application of Beers Law). Even when reactant and product spectra display some degree of overlap, modern instrumentation software is generally able to accurately deconvolute the relative contributions provided there is a dramatic change in the absolute absorbance of the peak of interest over time. In situ IR may be classified as an integral technique as the primary data collected are proportional to concentration vs. time. From these data, the starting material or product concentration over time may be obtained by simply taking the integral of a polynomial fit to the experimental curve. With increases in the availability of spectrometers with in situ' monitoring capabilities, FT-IR has seen increasing use in recent years. Examples of note include mechanistic analysis of the amido-thiourea catalyzed asymmetric Strecker synthesis of unnatural amino acids and of the Lewis base catalyzed halolactonization and cycloetherification. | 7 | Physical Chemistry |
During the next thousand years of the Early Bronze Age (end of 4th–end of 3rd millennium BCE) the same unalloyed copper production of the Chalcolithic (group 2, above) continued for the production of short blades and points. The same metal technique was used for the novel production of long blade weapons
(riveted daggers and knives, heavy tanged swords, and epsilon-shaped axes). The same copper production
technique of casting into an open mould and then hammering and annealing, was used to produce all other
metals as well, including jewelry of thin plates, sometimes decorated, and elongated thin wires (mainly for rings and bracelets) made of unalloyed copper as well as from silver (first appearance) and gold.
Archaeological remains of Early Bronze copper mining and copper smelting in the vicinity of the
mines were found in Trans-Jordan (Feinan), the Arava Valley (Timnah), and southern Sinai. The only production remains of metal are those of copper and include copper slag, prills, and amorphous copper lumps and small shallow ball-shaped clay crucibles with a socket. In the Early Bronze I site of the Ashkelon Marina, in the southern part of the Israeli coast, small shallow open pits, probably for the melting of copper in a crucible, were found next to copper industrial remains.
All pits showed a similar structure of a red soil burnt layer covered within by a white thin layer of calcite. No archaeological material was found associated with these man-made rudimentary remains. In the vicinity, and detached from the installations, scattered remains of pottery sherds, bone fragments, copper slag remains, and some pieces of clay crucibles were found. They were dated to Early Bronze Age I. Optically stimulated luminescence (OSL) ages of the fill of the pits and thermoluminescence (TL) ages of quartz grains extracted from the hardened red layer of the pits showed that the last burning activity was conducted during the same period: 5260 ± 380 years ago (OSL) and 5180 ± 380 years ago (TL).
Most of the metal products are found in burials, and are mainly from the Early Bronze I. The same types of metals continue in sites and tombs throughout the entire Early Bronze and all over the local distribution map of Early Bronze sites in Israel, from the Upper Galilee in the north to Ein Besor and Malhata in the northern Negev. A single hoard of copper objects probably from the Early Bronze I was found with no related archaeological context in the
fields of Kfar Monash.
During the Early Bronze period the Southern Levants metal industry became more specialized and organized in separated places for the different parts of production, and the products became more homogenous, as did the different materials and modes of production. For the first time in the Southern Levants metal history, significant typological and technological affiliations to the growing metal industries in the two major imperial centers (Egypt and Mesopotamia) on both sides of the “Fertile Crescent” were visible. During all the Early Bronze Age there is no archaeometallurgical evidence for bronze production and no bronze objects were found during this period in the Southern Levant as opposed to unalloyed copper. | 8 | Metallurgy |
The first monosaccharide attached in the synthesis of O-linked glycans is N-acetyl-galactosamine. After this, several different pathways are possible. A Core 1 structure is generated by the addition of galactose. A Core 2 structure is generated by the addition of N-acetyl-glucosamine to the N-acetyl-galactosamine of the Core 1 structure. Core 3 structures are generated by the addition of a single N-acetyl-glucosamine to the original N-acetyl-galactosamine. Core 4 structures are generated by the addition of a second N-acetyl-glucosamine to the Core 3 structure. Other core structures are possible, though less common.
Images:
https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=glyco.figgrp.561 : Core 1 and Core 2 generation. White square = N-acetyl-galactosamine; black circle = galactose; Black square = N-acetyl-glucosamine. Note: There is a mistake in this diagram. The bottom square should always be white in each image, not black.
https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=glyco.figgrp.562 : Core 3 and Core 4 generation.
A common structural theme in O-linked glycans is the addition of polylactosamine units to the various core structures. These are formed by the repetitive addition of galactose and N-acetyl-glucosamine units. Polylactosamine chains on O-linked glycans are often capped by the addition of a sialic acid residue (similar to neuraminic acid). If a fucose residue is also added, to the next to penultimate residue, a Sialyl-Lewis X (SLex) structure is formed. | 0 | Organic Chemistry |
The exponential amplification via reverse transcription polymerase chain reaction provides for a highly sensitive technique in which a very low copy number of RNA molecules can be detected. RT-PCR is widely used in the diagnosis of genetic diseases and, semiquantitatively, in the determination of the abundance of specific different RNA molecules within a cell or tissue as a measure of gene expression. | 1 | Biochemistry |
The EC set maximum residue levels for diphenylamine in 2005. (Annex II and Part B of Annex III to Regulation (EC) No 396/2005). Diphenylamine was one of 84 substances of a European Commission (EC) review program covered by a regulation from 2002 requiring the European Food Safety Authority (EFSA) upon EC request to organize a peer review of the initial evaluation, i.e. a draft risk assessment, and to provide the EC within 6 months with a conclusion. The assessment, received by the EFSA in 2007 started the peer review in October 2007 by dispatching it for consultation of the EC member states and the applicants, the two manufacturers, Cerexagri s.a., Italian subsidiary of United Phosphorus Ltd (UPL), and Pace International LLC. As a result of the peer review, mostly lacking data about risk to consumers, and particularly the levels and toxicity of unidentified metabolites of the substance, the possible formation of nitrosamines during storage of the active substance and during processing of treated apples, and the lack of data on the potential breakdown product of diphenylamine residues in processed commodities, the EC decided on 30 November 2009 to withdraw authorizations for plant protection products containing diphenylamine.(2009/859/EC)
The European Diphenylamine Task Force resubmitted an application to the EC with more data, and an additional report was received by the EFSA on 3 December 2010. EFSA concluded the risk assessment did not eliminate the concerns on 5 December 2011, published this opinion in 2012 and it became law in 2013. | 3 | Analytical Chemistry |
In 1926, Yakov Frenkel suggested that in an ionic crystal like AgI, in thermodynamic equilibrium, a small fraction of the cations, α, are displaced from their regular lattice sites into interstitial positions. He related α with the Gibbs energy for the formation of one mol of Frenkel pairs, ΔG, as α = exp(-ΔG/2RT), where T is temperature and R is the gas constant; for a typical value of ΔG = 100 kJ/mol, α ~ 1 at 100 °C and ~6 at 400 °C. This idea naturally explained the presence of an appreciable fraction of mobile ions in otherwise defect-free ionic crystals, and thus the ionic conductivity in them.
Frenkel’s idea was expanded by Carl Wagner and Walter Schottky in their 1929 theory, which described the equilibrium thermodynamics of point defects in ionic crystals. In particular, Wagner and Schottky related the deviations from stoichiometry in those crystals with the chemical potentials of the crystal components, and explained the phenomenon of mixed electronic and ionic conduction.
Wagner and Schottky considered four extreme cases of point-defect disorder in a stoichiometric binary ionic crystal of type AB:
# Pairs of interstitial cations A and lattice vacancies (Frenkel defects)
#Pairs of interstitial anions B and lattice vacancies (anti-Frenkel defects)
#Pairs of interstitial cations A and interstitial anions B with no vacancies
# Pairs of A and B-type lattice vacancies with no interstitials (Schottky disorder).
Type-3 disorder does not occur in practice, and type 2 is observed only in rare cases when anions are smaller than cations, while both types 1 and 4 are common and show the same exp(-ΔG/2RT) temperature dependence.
Later in 1933, Wagner suggested that in metal oxides an excess of metal would result in extra electrons, while a deficit of metal would produce electron holes, i.e., that atomic non-stoichiometry would result in a mixed ionic-electronic conduction. | 7 | Physical Chemistry |
The model proposed by Roberto Germano and his collaborators, who have first observed the effect is based on the known concept of the exclusion zone.
The first observations of a different behaviour of water molecules close to the walls of its container date back to late ‘60s and early ‘70s, when Drost-Hansen, upon reviewing many experimental articles, came to the conclusion that interfacial water shows structural difference with respect to the bulk liquid water.
In 2006 Gerald Pollack published a seminal work on the exclusion zone and those observations were subsequently reported by several other groups, in which a hydrophilic material creates a coherent water region at the boundary between its surface and the water.
Further elaborating on the work of Pollack, the model describes liquid water as a system made of two phases: a matrix of non-coherent water molecules hosting many “Coherence Domains” (CDs), about 0.1 um in size, found in the exclusion zone, but also in the bulk volume.
In this model the behaviour of the coherence domains is also considered as the cause for the formation of xerosydryle.
The two phases, are characterized by different thermodynamic parameters, and are in a stable non-equilibrium state.
The coherent phase should be described by a quantum state, and in particular a state oscillating between a fundamental state, where electrons are firmly bound (ionization energy of 12.60 eV), and an excited state characterized by a quasi-free electron configuration. The energy of the excited state is 12.06 eV, which means that only a small amount of energy as small as (12.60 - 12.06) eV = 0.54 eV (Infrared range) is sufficient to extract an electron.
Then, at a fixed temperature and for molecules density exceeding a threshold, the transition of the non-coherent water molecules to the coherence state is spontaneous because it is driving the system to a lower energy configuration.
More exactly, the almost free electrons have to cross an energy barrier of (0.54 - Χ) eV, where Χ ~ 0.1 eV is the electric potential difference at the CD boundary with the non-coherent water.
This small amount of energy, ~ 0.44 eV, necessary for the electron extraction, makes the coherent water a reservoir of quasi-free electrons that can be easily released by Infrared stimulation, or quantum tunnel effect or by small external perturbation.
The two water phases, with their different potentials behave as the two components of a photovoltaic cell based on semiconductors.
Then, in the cell described in the patent, one of the two sectors has sheets of hydrophilic material, which create (more) coherent domains in that sector, with respect to the other sector. | 7 | Physical Chemistry |
* Dr. Vahan Simonyan and Dr. Raja Mazumder presented at the NIH Frontiers in Data Science about HIVE acting as a bridge between research and regulatory analytics. Simonyan also presented on the topic at the 2014 Bio-IT World Expo.
* HIVE was additionally discussed in FedScoop.
* Inside the HIVE, the FDA's Multi-Omics Compute Architecture, BioIT World. | 1 | Biochemistry |
A crop, or croup, is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds.
Certain insects may have a crop or enlarged esophagus. | 1 | Biochemistry |
There are two types of thermodynamic instruments, the meter and the reservoir. A thermodynamic meter is any device which measures any parameter of a thermodynamic system. In some cases, the thermodynamic parameter is actually defined in terms of an idealized measuring instrument. For example, the zeroth law states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This principle, as noted by James Maxwell in 1872, asserts that it is possible to measure temperature. An idealized thermometer is a sample of an ideal gas at constant pressure. From the ideal gas law pV=nRT, the volume of such a sample can be used as an indicator of temperature; in this manner it defines temperature. Although pressure is defined mechanically, a pressure-measuring device, called a barometer may also be constructed from a sample of an ideal gas held at a constant temperature. A calorimeter is a device which is used to measure and define the internal energy of a system.
A thermodynamic reservoir is a system which is so large that its state parameters are not appreciably altered when it is brought into contact with the system of interest. When the reservoir is brought into contact with the system, the system is brought into equilibrium with the reservoir. For example, a pressure reservoir is a system at a particular pressure, which imposes that pressure upon the system to which it is mechanically connected. The Earth's atmosphere is often used as a pressure reservoir. The ocean can act as temperature reservoir when used to cool power plants. | 7 | Physical Chemistry |
In chromatography, resolution is a measure of the separation of two peaks of different retention time t in a chromatogram. | 3 | Analytical Chemistry |
It is common for two monochromators to be connected in series, with their mechanical systems operating in tandem so that they both select the same color. This arrangement is not intended to improve the narrowness of the spectrum, but rather to lower the cutoff level. A double monochromator may have a cutoff about one millionth of the peak value, the product of the two cutoffs of the individual sections. The intensity of the light of other colors in the exit beam is referred to as the stray light level and is the most critical specification of a monochromator for many uses. Achieving low stray light is a large part of the art of making a practical monochromator. | 7 | Physical Chemistry |
An important functional feature of genes is the phenotype caused by mutations. Mutants can be produced by random mutations or by directed mutagenesis, including site-directed mutagenesis, deleting complete genes, or other techniques. | 1 | Biochemistry |
The Hajos–Parrish–Eder–Sauer–Wiechert reaction, reported in 1971 by several research teams, is an early example of an enantioselective catalytic reaction in organic chemistry. Its scope has been modified and expanded through the development of related reactions including the Michael addition, asymmetric aldol reaction, and the Mannich reaction. This reaction has likewise been used to perform asymmetric Robinson annulations. The general scheme of this reaction follows:
This example illustrates a 6-enolendo aldolization. In the , proline catalyses an asymmetric aldol reaction. The zwitterionic character and the H-bonding of proline in the transition state determine the reaction outcome. An enamine is formed during the reaction and only one proline molecule is involved in forming the transition state.
Asymmetric synthesis of the Wieland-Miescher ketone is also based on proline. Additional reactions include aldol reactions, Mannich reaction, Michael reaction, amination, α-oxyamination, and α-halogenation.
Modifications on the basic proline structure improved the enantioselectivity and regioselectivity of the catalysis. These proline-derived auxiliaries and catalysts, including the Enders hydrazone reaction and Corey–Itsuno reduction, have been reviewed, as have MacMillan’s iminium catalysts, Miller catalysts, and CBS-oxazaborolidines.
Illustrating an enolexo intramolecular aldolization, dicarbonyl (dials,diketones) can be converted to anti-aldol products with a 10% L-proline catalyst loading.
A prominent example of proline catalysis is the addition of acetone or hydroxyacetone to a diverse set of aldehydes catalyzed by 20-30% proline catalyst loading with high (>99%) enantioselectivity yielding diol products. As refined by List and Notz, the aforementioned reaction produces diol products as follows: | 0 | Organic Chemistry |
Methylotrophy refers to the ability of an organism to use C1-compounds as energy sources. These compounds include methanol, methyl amines, formaldehyde, and formate. Several other less common substrates may also be used for metabolism, all of which lack carbon-carbon bonds. Examples of methylotrophs include the bacteria Methylomonas and Methylobacter. Methanotrophs are a specific type of methylotroph that are also able to use methane () as a carbon source by oxidizing it sequentially to methanol (), formaldehyde (), formate (), and carbon dioxide initially using the enzyme methane monooxygenase. As oxygen is required for this process, all (conventional) methanotrophs are obligate aerobes. Reducing power in the form of quinones and NADH is produced during these oxidations to produce a proton motive force and therefore ATP generation. Methylotrophs and methanotrophs are not considered as autotrophic, because they are able to incorporate some of the oxidized methane (or other metabolites) into cellular carbon before it is completely oxidized to (at the level of formaldehyde), using either the serine pathway (Methylosinus, Methylocystis) or the ribulose monophosphate pathway (Methylococcus), depending on the species of methylotroph.
In addition to aerobic methylotrophy, methane can also be oxidized anaerobically. This occurs by a consortium of sulfate-reducing bacteria and relatives of methanogenic Archaea working syntrophically (see below). Little is currently known about the biochemistry and ecology of this process.
Methanogenesis is the biological production of methane. It is carried out by methanogens, strictly anaerobic Archaea such as Methanococcus, Methanocaldococcus, Methanobacterium,
Methanothermus, Methanosarcina, Methanosaeta and Methanopyrus. The biochemistry of methanogenesis is unique in nature in its use of a number of unusual cofactors to sequentially reduce methanogenic substrates to methane, such as coenzyme M and methanofuran. These cofactors are responsible (among other things) for the establishment of a proton gradient across the outer membrane thereby driving ATP synthesis. Several types of methanogenesis occur, differing in the starting compounds oxidized. Some methanogens reduce carbon dioxide () to methane () using electrons (most often) from hydrogen gas () chemolithoautotrophically. These methanogens can often be found in environments containing fermentative organisms. The tight association of methanogens and fermentative bacteria can be considered to be syntrophic (see below) because the methanogens, which rely on the fermentors for hydrogen, relieve feedback inhibition of the fermentors by the build-up of excess hydrogen that would otherwise inhibit their growth. This type of syntrophic relationship is specifically known as interspecies hydrogen transfer. A second group of methanogens use methanol () as a substrate for methanogenesis. These are chemoorganotrophic, but still autotrophic in using as only carbon source. The biochemistry of this process is quite different from that of the carbon dioxide-reducing methanogens. Lastly, a third group of methanogens produce both methane and carbon dioxide from acetate () with the acetate being split between the two carbons. These acetate-cleaving organisms are the only chemoorganoheterotrophic methanogens. All autotrophic methanogens use a variation of the reductive acetyl-CoA pathway to fix and obtain cellular carbon. | 1 | Biochemistry |
Ketenes are generally very reactive, and participate in various cycloadditions. One important process is the dimerization to give propiolactones. A specific example is the dimerization of the ketene of stearic acid to afford alkyl ketene dimers, which are widely used in the paper industry. AKD's react with the hydroxyl groups on the cellulose via esterification reaction.
They will also undergo [2+2] cycloaddition reactions with electron-rich alkynes to form cyclobutenones, or carbonyl groups to form beta-lactones. With imines, beta-lactams are formed. This is the Staudinger synthesis, a facile route to this important class of compounds. With acetone, ketene reacts to give isopropenyl acetate.
A variety of hydroxylic compounds can add as nucleophiles, forming either enol or ester products. As examples, a water molecule easily adds to ketene to give 1,1-dihydroxyethene and acetic anhydride is produced by the reaction of acetic acid with ketene. Reactions between diols () and bis-ketenes () yield polyesters with a repeat unit of ().
Ethyl acetoacetate, an important starting material in organic synthesis, can be prepared using a diketene in reaction with ethanol. They directly form ethyl acetoacetate, and the yield is high when carried out under controlled circumstances; this method is therefore used industrially. | 0 | Organic Chemistry |
One modern-day method of chiral resolution is used in the organic synthesis of the drug duloxetine:
In one of its steps the racemic alcohol 1 is dissolved in a mixture of toluene and methanol to which solution is added optically active (S)-mandelic acid 3. The alcohol (S)-enantiomer forms an insoluble diastereomeric salt with the mandelic acid and can be filtered from the solution. Simple deprotonation with sodium hydroxide liberates free (S)-alcohol. In the meanwhile the (R)-alcohol remains in solution unaffected and is recycled back to the racemic mixture by epimerization with hydrochloric acid in toluene. This process is known as RRR synthesis in which the R's stand for Resolution-Racemization-Recycle. | 4 | Stereochemistry |
A comet assay can determine the degree of DNA fragmentation in sperm cells. The degree of DNA fragmentation has been associated with outcomes of in vitro fertilization.
The comet has been modified for use with sperm cells as a tool for male infertility diagnosis
To break down these tightly bound protamine proteins in order to use the comet for sperm, additional steps in the de-condensation protocol are required. | 1 | Biochemistry |
Goniophotometers measure the spatial distribution of light visible to the human eye (often luminous intensity) at specific angular positions, usually covering all spherical angles. | 7 | Physical Chemistry |
Electropositivity is a measure of an element's ability to donate electrons, and therefore form positive ions; thus, it is antipode to electronegativity.
Mainly, this is an attribute of metals, meaning that, in general, the greater the metallic character of an element the greater the electropositivity. Therefore, the alkali metals are the most electropositive of all. This is because they have a single electron in their outer shell and, as this is relatively far from the nucleus of the atom, it is easily lost; in other words, these metals have low ionization energies.
While electronegativity increases along periods in the periodic table, and decreases down groups, electropositivity decreases along periods (from left to right) and increases down groups. This means that elements in the upper right of the periodic table of elements (oxygen, sulfur, chlorine, etc.) will have the greatest electronegativity, and those in the lower-left (rubidium, caesium, and francium) the greatest electropositivity. | 3 | Analytical Chemistry |
The two main approaches to synthesis of N-H, N-alkyl, and N-aryloxaziridines are oxidation of imines with peracids (A) and amination of carbonyls (B).
Additionally, oxidation of chiral imines and oxidation of imines with chiral peracids may yield enantiopure oxaziridines. Some oxaziridines have the unique property of configurationally stable nitrogen atoms at room temperature due to an inversion barrier of 100 to 130 kJ/mol. Enantiopure oxaziridines where stereochemistry is entirely due to configurationally stable nitrogen are reported. | 0 | Organic Chemistry |
Following initial observations that some peptide-peptide complexes could survive MALDI deposition and ionization, studies of large protein complexes using MALDI-MS have been reported. | 1 | Biochemistry |
Within the citric acid cycle in humans, propionyl-CoA, which interacts with oxaloacetate to form methylcitrate, can also catalyzed into methylmalonyl-CoA through carboxylation by propionyl-CoA carboxylase (PCC). Methylmalonyl-CoA is later transformed to succinyl-CoA to be further used in the tricarboxylic acid cycle. PCC not only catalyzes the carboxylation of propionyl-CoA to methylmalonyl-CoA, but also acts on several different acyl-CoAs. Nevertheless, its highest binding affinity is to propionyl-CoA. It was further shown that propionyl-CoA transformation is inhibited by the absence of several TCA markers, such as glutamate. The mechanism is shown by the figure to the left. | 1 | Biochemistry |
Although quite stable, the blue ammonia solutions containing solvated electrons degrade rapidly in the presence of catalysts to give colorless solutions of sodium amide:
:2 [Na(NH)]e → H + 2 NaNH + 10 NH
Electride salts can be isolated by the addition of macrocyclic ligands such as crown ether and cryptands to solutions containing solvated electrons. These ligands strongly bind the cations and prevent their re-reduction by the electron.
:[Na(NH)]e + cryptand → [Na(cryptand)]e+ 6 NH
The solvated electron reacts with oxygen to form a superoxide radical (O). With nitrous oxide, solvated electrons react to form hydroxyl radicals (HO). | 0 | Organic Chemistry |
Born and raised in New Orleans, Louisiana, to a professional musician-father and a housewife-mother, Ted Ellis earliest hints of artistic talent began to show at five-years-old. Ellis first attempt at art was a third-grade freehand sketch of a dog from Archie Comics, which he drew so accurately that friends and family believed it had been traced. Growing up, one of his favorite characters to draw was Wile E. Coyote, and as an adult he continues to enjoy comic books as "refreshing".
When he was old enough, Ted would ride the bus alone to downtown New Orleans so as to be exposed to and spend time with the area artists. He and his friends would in their spare time compete with one another to see who could draw the best designs, and Ted continued developing his art skills throughout primary school despite only receiving "satisfactory" marks in art class. In elementary school he attended a summer program at the New Orleans Center for Creative Arts, and later attended an after-school program at Lawless High School. Ellis says that he knew he wanted to be an artist in the seventh grade, and credits his teacher in that class for keeping him focused.
Ted worked for a time with charcoal and pastel before settling on oil and acrylic. He took art classes during high school and enrolled in four months of private art lessons, but is otherwise self-taught. Ellis followed advice from Anna Torregano, his mentor, friend and high school art teacher, and his parents, all of whom advised him to pursue an academic career so as not to become a "starving artist".
His mother especially stressed university and earning a professional degree.
Ted Ellis earned a B.Sc. in chemistry at Dillard University on a United States Army ROTC scholarship as well as academic scholarship, and went on to be commissioned a second lieutenant in the United States Army's Field Artillery Branch. Ellis spent the next ten years working in the field of chemistry, eight years of which were as an environmental chemist at Rollins Engineering Services.
Ellis has lived in Louisiana and California, and currently resides in Friendswood, Texas with his wife, Erania. They have a daughter, Chaney, and a son, Tanner.
Chaney is an aspiring rap artist and has produced a positive, parent-friendly CD about school and drugs titled Off the Chain. | 3 | Analytical Chemistry |
Typically the copper oxhide ingots are highly pure (approximately 99 weight percent copper) with trace element content of less than one weight percent. The few tin oxhide ingots that have been available to study are also exceptionally pure. Microscopic analysis of the Uluburun copper oxhide ingots reveals that they are highly porous. This feature results from the effervescence of gases as the molten metal cooled. Slag inclusions are also present. Their existence implies that slag was not fully removed from the smelted metal and thus that the ingots were made from remelted copper.
Macroscopic observation of the Uluburun copper ingots indicates that they were cast through multiple pours; there are distinct layers of metal in each ingot. Furthermore, the relatively high weight and high purity of the ingots would be difficult to achieve even today in only one pour.
The porosity of the copper ingots and the natural brittleness of tin suggest that both metal ingots were easy to break. As Bass et al. proposes, a metalsmith could simply break off a piece of the ingot whenever he liked for a new casting. | 8 | Metallurgy |
The far-eastern blot is for the detection of lipid-linked oligosaccharides. High-performance thin-layer chromatography is first used to separate the lipids by physical and chemical characteristics, then transferred to a blotting matrix before the oligosaccharides are detected by a specific binding protein (i.e. antibodies or lectins). | 1 | Biochemistry |
It has been shown that the integration of a given synthetic ultrasensitive module with upstream and downstream components often alters its information-processing capabilities. This effects must be taken into account in the design process. | 1 | Biochemistry |
In the case of photochemical reactions, light provides the activation energy. Simplistically, light is one mechanism for providing the activation energy required for many reactions. If laser light is employed, it is possible to selectively excite a molecule so as to produce a desired electronic and vibrational state. Equally, the emission from a particular state may be selectively monitored, providing a measure of the population of that state. If the chemical system is at low pressure, this enables scientists to observe the energy distribution of the products of a chemical reaction before the differences in energy have been smeared out and averaged by repeated collisions.
The absorption of a photon by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise-inaccessible reaction path, as described by the Woodward–Hoffmann selection rules. A [2+2] cycloaddition reaction is one example of a pericyclic reaction that can be analyzed using these rules or by the related frontier molecular orbital theory.
Some photochemical reactions are several orders of magnitude faster than thermal reactions; reactions as fast as 10 seconds and associated processes as fast as 10 seconds are often observed.
The photon can be absorbed directly by the reactant or by a photosensitizer, which absorbs the photon and transfers the energy to the reactant. The opposite process, when a photoexcited state is deactivated by a chemical reagent, is called quenching.
Most photochemical transformations occur through a series of simple steps known as primary photochemical processes. One common example of these processes is the excited state proton transfer. | 5 | Photochemistry |
An ova bank, or cryobank, or egg cell bank is a facility that collects and stores human ova, mainly from ova donors, primarily for the purpose of achieving pregnancies of either the donor, at a later time (i.e. to overcome issues of infertility), or through third party reproduction, notably by artificial insemination. Ova donated in this way are known as donor ova. | 1 | Biochemistry |
Because of concerns over Peak oil, pollution and climate change, as well the oil price increases since 2003, non-petroleum alternatives have become more popular. This has led to the introduction of biobitumen alternatives that are more environmentally friendly and nontoxic.
For millions of people living in and around cities, heat islands are of growing concern. This phenomenon describes urban and suburban temperatures that are 1 to 6 °C (2 to 10 °F) hotter than nearby rural areas. Elevated temperatures can impact communities by increasing peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality. There are common-sense measures that communities can take to reduce the negative effects of heat islands, such as replacing conventional black asphalt road surfaces with the new pigmentable bitumen that gives lighter colors. | 7 | Physical Chemistry |
When electrodes are placed in an electrolyte and a voltage is applied, the electrolyte will conduct electricity. Lone electrons normally cannot pass through the electrolyte; instead, a chemical reaction occurs at the cathode, providing electrons to the electrolyte. Another reaction occurs at the anode, consuming electrons from the electrolyte. As a result, a negative charge cloud develops in the electrolyte around the cathode, and a positive charge develops around the anode. The ions in the electrolyte neutralize these charges, enabling the electrons to keep flowing and the reactions to continue.
For example, in a solution of ordinary table salt (sodium chloride, NaCl) in water, the cathode reaction will be
:2 HO + 2e → 2 OH + H
and hydrogen gas will bubble up; the anode reaction is
:2 NaCl → 2 Na + Cl + 2e
and chlorine gas will be liberated into solution where it reacts with the sodium and hydroxyl ions to produce sodium hypochlorite - household bleach. The positively charged sodium ions Na will react toward the cathode, neutralizing the negative charge of OH there, and the negatively charged hydroxide ions OH will react toward the anode, neutralizing the positive charge of Na there. Without the ions from the electrolyte, the charges around the electrode would slow down continued electron flow; diffusion of H and OH through water to the other electrode takes longer than movement of the much more prevalent salt ions.
Electrolytes dissociate in water because water molecules are dipoles and the dipoles orient in an energetically favorable manner to solvate the ions.
In other systems, the electrode reactions can involve the metals of the electrodes as well as the ions of the electrolyte.
Electrolytic conductors are used in electronic devices where the chemical reaction at a metal-electrolyte interface yields useful effects.
* In batteries, two materials with different electron affinities are used as electrodes; electrons flow from one electrode to the other outside of the battery, while inside the battery the circuit is closed by the electrolyte's ions. Here, the electrode reactions convert chemical energy to electrical energy.
* In some fuel cells, a solid electrolyte or proton conductor connects the plates electrically while keeping the hydrogen and oxygen fuel gases separated.
* In electroplating tanks, the electrolyte simultaneously deposits metal onto the object to be plated, and electrically connects that object in the circuit.
* In operation-hours gauges, two thin columns of mercury are separated by a small electrolyte-filled gap, and, as charge is passed through the device, the metal dissolves on one side and plates out on the other, causing the visible gap to slowly move along.
* In electrolytic capacitors the chemical effect is used to produce an extremely thin dielectric or insulating coating, while the electrolyte layer behaves as one capacitor plate.
* In some hygrometers the humidity of air is sensed by measuring the conductivity of a nearly dry electrolyte.
* Hot, softened glass is an electrolytic conductor, and some glass manufacturers keep the glass molten by passing a large current through it. | 7 | Physical Chemistry |
Leiv Kristen Sydnes (born 9 July 1948) is a Norwegian chemist, specializing in organic chemistry.
He was born in Haugesund, and took his education at the University of Oslo. He has the dr.philos. degree from 1978. He was hired as an associate professor at the University of Tromsø in 1978, and was later promoted to professor. In 1993 he moved to the University of Bergen. He presided over the Norwegian Chemical Society from 1992 to 1996 and the International Union of Pure and Applied Chemistry (IUPAC) from 2004 to 2005. He is a member of the Norwegian Academy of Science and Letters and the Norwegian Academy of Technological Sciences.
Sydnes stood for election as rector of the University of Bergen in 2005, but lost the election to Sigmund Grønmo. In 2009 he applied for the position as rector of the Norwegian University of Science and Technology; here the rectors are hired rather than elected. | 0 | Organic Chemistry |
Biocrystallization is the formation of crystals from organic macromolecules by living organisms. This may be a stress response, a normal part of metabolism such as processes that dispose of waste compounds, or a pathology. Template mediated crystallization is qualitatively different from in vitro crystallization. Inhibitors of biocrystallization are of interest in drug design efforts against lithiasis and against pathogens that feed on blood, since many of these organisms use this process to safely dispose of heme. | 1 | Biochemistry |
Chiral oxazolidinones have been employed most widely in stereoselective aldol reactions.
Soft enolization with the Lewis acid dibutylboron triflate and the base diisopropylethylamine gives the (Z)-enolate, which undergoes a diastereoselective aldol reaction with an aldehyde substrate. The transformation is particularly powerful because it establishes two contiguous stereocenters simultaneously.
A model for the observed stereoselectivity can be found below. The syn-stereo relationship between the methyl group and the new secondary alcohol results from a six-membered ring Zimmerman-Traxler transition state, wherein the enolate oxygen and the aldehyde oxygen both coordinate boron. The aldehyde is oriented such that the hydrogen is placed in a pseudo-axial orientation to minimize 1,3-diaxial interactions. The absolute stereochemistry of the two stereocenters is controlled by the chirality in the auxiliary. In the transition structure, the auxiliary carbonyl is oriented away from the enolate oxygen so as to minimize the net dipole of the molecule; one face of the enolate is blocked by the substituent on the chiral auxiliary.
<br /> | 4 | Stereochemistry |
Development of the Albion process started during the early nineties led by Mount Isa Mines. It was first patented in 1993. Several pilot plant projects were conducted in 1994 and 1995 which tested the feasibility of using the technology to process high arsenic gold and copper ore.
The Albion Process has been successfully installed in seven projects globally:
* GPM Gold Project (Gold, Armenia)
* Las Lagunas Tailings (Gold, Dominican Republic)
* Sable Copper Project (Copper, Chalcopyrite, Zambia)
* Asturiana de Zinc (Zinc, Spain)
* Nordenham Zinc Refinery (Zinc, Germany)
* McArthur River (Zinc, Australia) | 8 | Metallurgy |
Grignard reagents react with a variety of carbonyl derivatives.
The most common application of Grignard reagents is the alkylation of aldehydes and ketones, i.e. the Grignard reaction:
Note that the acetal functional group (a protected carbonyl) does not react.
Such reactions usually involve an aqueous acidic workup, though this step is rarely shown in reaction schemes. In cases where the Grignard reagent is adding to an aldehyde or a prochiral ketone, the Felkin-Anh model or Cram's Rule can usually predict which stereoisomer will be formed. With easily deprotonated 1,3-diketones and related acidic substrates, the Grignard reagent RMgX functions merely as a base, giving the enolate anion and liberating the alkane RH.
Grignard reagents are nucleophiles in nucleophilic aliphatic substitutions for instance with alkyl halides in a key step in industrial Naproxen production:
Grignard reagents also react with many "carbonyl-like" compounds and other electrophiles: | 0 | Organic Chemistry |
The mathematical similarities between the expressions for shear viscocity, thermal conductivity and diffusion coefficient of the ideal (dilute) gas is not a coincidence; It is a direct result of the Onsager reciprocal relations (i.e. the detailed balance of the reversible dynamics of the particles), when applied to the convection (matter flow due to temperature gradient, and heat flow due to pressure gradient) and advection (matter flow due to the velocity of particles, and momentum transfer due to pressure gradient) of the ideal (dilute) gas. | 7 | Physical Chemistry |
Aluminothermic reactions are exothermic chemical reactions using aluminium as the reducing agent at high temperature. The process is industrially useful for production of alloys of iron. The most prominent example is the thermite reaction between iron oxides and aluminium to produce iron itself:
: FeO + 2 Al → 2 Fe + AlO
This specific reaction is however not relevant to the most important application of aluminothermic reactions, the production of ferroalloys. For the production of iron, a cheaper reducing agent, coke, is used instead via the carbothermic reaction. | 8 | Metallurgy |
Liquid chromatography-mass spectrometry (LC/MS) couples high resolution chromatographic separation with MS detection. As the system adopts the high separation of HPLC, analytes which are in the liquid mobile phase are often ionized by various soft ionization methods including atmospheric pressure chemical ionization (APCI), electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI), which attains the gas phase ionization required for the coupling with MS. These ionization methods allow the analysis of a wider range of biological molecules, including those with larger masses, thermally unstable or nonvolatile compounds where GC-MS is typically incapable of analyzing.
LC-MS provides high selectivity as unresolved peaks can be isolated by selecting a specific mass. Furthermore, better identification is also attained by mass spectra and the user does not have to rely solely on the retention time of analytes. As a result, molecular mass and structural information as well as quantitative data can all be obtained via LC-MS. LC-MS can therefore be applied to various fields, such as impurity identification and profiling in drug development and pharmaceutical manufacturing, since LC provides efficient separation of impurities and MS provides structural characterization for impurity profiling.
Common solvents used in normal or reversed phase LC such as water, acetonitrile, and methanol are all compatible with ESI, yet a LC grade solvent may not be suitable for MS. Furthermore, buffers containing inorganic ions should be avoided as they may contaminate the ion source. Nonetheless, the problem can be resolved by 2D LC-MS, as well as other various issues including analyte coelution and UV detection responses. | 3 | Analytical Chemistry |
Heterodimeric diatomic complexes involving a noble gas and a halide, such as xenon chloride, are common in the construction of excimer lasers, which are excimers' most common application. These lasers take advantage of the fact that excimer components have attractive interactions in the excited state and repulsive interactions in the ground state. Emission of excimer molecules is also used as a source of spontaneous ultraviolet light (excimer lamps).
The molecule pyrene is another canonical example of an excimer that has found applications in biophysics to evaluate the distance between biomolecules.
In organic chemistry, many reactions occur through an exciplex, for example, those of simple arene compounds with alkenes. The reactions of benzene and their products depicted are a [2+2]cycloaddition to the ortho product (A), a [2+3]cycloaddition to the meta product (B) and the [2+4]cycloaddition to the para product (C) with simple alkenes such as the isomers of 2-butene. In these reactions, it is the arene that is excited.
As a general rule, the regioselectivity is in favor of the ortho adduct at the expense of the meta adduct when the amount of charge transfer taking place in the exciplex increases. | 5 | Photochemistry |
In the mathematical description it is assumed that no clustering occurs within the alloy. Then, for a binary alloy of the form , e.g. , the standard deviation of the composition is given by:
where is the number of atoms within the excitons' volume, i.e. with being the atoms per volume.
In general, the band gap energy of a semiconducting alloy depends on the composition, i.e. . The band gap energy can be considered to be the fluorescence energy. Therefore, the standard deviation in fluorescence is
As the alloy broadening belongs to the group of inhomogeneous broadenings the line shape of the fluorescence intensity is Gaussian: | 7 | Physical Chemistry |
The spectroscopic notation of molecules uses Greek letters to represent the modulus of the orbital angular momentum along the internuclear axis.
The quantum number that represents this angular momentum is Λ.
: Λ = 0, 1, 2, 3, ...
: Symbols: Σ, Π, Δ, Φ
For Σ states, one denotes if there is a reflection in a plane containing the nuclei (symmetric), using the + above. The − is used to indicate that there is not.
For homonuclear diatomic molecules, the index g or u denotes the existence of a center of symmetry (or inversion center) and indicates the symmetry of the vibronic wave function with respect to the point-group inversion operation i. Vibronic states that are symmetric with respect to i are denoted g for (German for "even"), and unsymmetric states are denoted u for (German for "odd"). | 7 | Physical Chemistry |
Steroid 11β-hydroxylase, also known as steroid 11β-monooxygenase, is a steroid hydroxylase found in the zona glomerulosa and zona fasciculata of the adrenal cortex. Named officially the cytochrome P450 11B1, mitochondrial, it is a protein that in humans is encoded by the CYP11B1 gene. The enzyme is involved in the biosynthesis of adrenal corticosteroids by catalyzing the addition of hydroxyl groups during oxidation reactions. | 1 | Biochemistry |
Bromocresol purple (BCP) or 5′,5″-dibromo-o-cresolsulfophthalein, is a dye of the triphenylmethane family (triarylmethane dyes) and a pH indicator. It is colored yellow below pH 5.2, and violet above pH 6.8. In its cyclic sulfonate ester form, it has a pK value of 6.3, and is usually prepared as a 0.04% aqueous solution. | 3 | Analytical Chemistry |
Lithium is primarily used as a maintenance drug in the treatment of bipolar disorder to stabilize mood and prevent manic episodes, but it may also be helpful in the acute treatment of manic episodes. Although recommended by treatment guidelines for the treatment of depression in bipolar disorder, the evidence that lithium is superior to placebo for acute depression is low-quality; atypical antipsychotics are considered more effective for treating acute depressive episodes. Lithium carbonate treatment was previously considered to be unsuitable for children; however, more recent studies show its effectiveness for treatment of early-onset bipolar disorder in children as young as eight. The required dosage is slightly less than the toxic level (representing a low therapeutic index), requiring close monitoring of blood levels of lithium carbonate during treatment. Within the therapeutic range there is a dose response relationship. A limited amount of evidence suggests lithium carbonate may contribute to treatment of substance use disorders for some people with bipolar disorder. Although it is believed that lithium prevents suicide in people with bipolar disorder, a 2022 systematic review found that "Evidence from randomised trials is inconclusive and does not support the idea that lithium prevents suicide or suicidal behaviour." | 1 | Biochemistry |
The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The hypothesis is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released methane clathrate on upper continental slopes. This would have had an immediate impact on the global temperature, as methane is a much more powerful greenhouse gas than carbon dioxide. Despite its atmospheric lifetime of around 12 years, methane's global warming potential is 72 times greater than that of carbon dioxide over 20 years, and 25 times over 100 years (33 when accounting for aerosol interactions). It is further proposed that these warming events caused the Bond Cycles and individual interstadial events, such as the Dansgaard–Oeschger interstadials.
The hypothesis was supported for the Bølling–Allerød warming and Preboreal periods, but not for Dansgaard–Oeschger interstadials, although there are still debates on the topic. While it may be important on the millennial timescales, it is no longer considered relevant for the near future climate change: the IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century". | 6 | Supramolecular Chemistry |
In organic chemistry, a methylidyne group or just methylidyne is a neutral part of a molecule (a substituent or functional group) with formula , consisting of a carbon atom bonded to a hydrogen atom by one single bond and to the rest of the molecule by one triple bond. For example, a methylidyne group is present in n-methylidyne-1-hexanaminium, .
The name "methylidyne" is also used for the methylidyne radical (carbyne) , the same two atoms not bound to any other atom. | 0 | Organic Chemistry |
In a conventional oil refinery, isobutane is alkylated with low-molecular-weight alkenes (primarily a mixture of propene and butene) in the presence of a Brønsted acid catalyst, which can include solid acids (zeolites). The catalyst protonates the alkenes (propene, butene) to produce carbocations, which alkylate isobutane. The product, called "alkylate", is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons (mostly isoheptane and isooctane). Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. Alkylate is also a key component of avgas. By combining fluid catalytic cracking, polymerization, and alkylation, refineries can obtain a gasoline yield of 70 percent. The widespread use of sulfuric acid and hydrofluoric acid in refineries poses significant environmental risks. Ionic liquids are used in place of the older generation of strong Bronsted acids. | 0 | Organic Chemistry |
Stereochemistry has important applications in the field of medicine, particularly pharmaceuticals. An often cited example of the importance of stereochemistry relates to the thalidomide disaster. Thalidomide is a pharmaceutical drug, first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug was discovered to be teratogenic, causing serious genetic damage to early embryonic growth and development, leading to limb deformation in babies. Some of the several proposed mechanisms of teratogenicity involve a different biological function for the (R)- and the (S)-thalidomide enantiomers. In the human body however, thalidomide undergoes racemization: even if only one of the two enantiomers is administered as a drug, the other enantiomer is produced as a result of metabolism. Accordingly, it is incorrect to state that one stereoisomer is safe while the other is teratogenic. Thalidomide is currently used for the treatment of other diseases, notably cancer and leprosy. Strict regulations and controls have been enabled to avoid its use by pregnant women and prevent developmental deformations. This disaster was a driving force behind requiring strict testing of drugs before making them available to the public. | 4 | Stereochemistry |
Transcription is when RNA is copied from DNA. During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process differs slightly in eukaryotes and prokaryotes. One notable difference is that prokaryotic RNA polymerase associates with DNA-processing enzymes during transcription so that processing can proceed during transcription. Therefore, this causes the new mRNA strand to become double stranded by producing a complementary strand known as the tRNA strand, which when combined are unable to form structures from base-pairing. Moreover, the template for mRNA is the complementary strand of tRNA, which is identical in sequence to the anticodon sequence that the DNA binds to. The short-lived, unprocessed or partially processed product is termed precursor mRNA, or pre-mRNA; once completely processed, it is termed mature mRNA. | 1 | Biochemistry |
An SNP array is a useful tool for studying slight variations between whole genomes. The most important clinical applications of SNP arrays are for determining disease susceptibility and for measuring the efficacy of drug therapies designed specifically for individuals. In research, SNP arrays are most frequently used for genome-wide association studies. Each individual has many SNPs. SNP-based genetic linkage analysis can be used to map disease loci, and determine disease susceptibility genes in individuals. The combination of SNP maps and high density SNP arrays allows SNPs to be used as markers for genetic diseases that have complex traits. For example, genome-wide association studies have identified SNPs associated with diseases such as rheumatoid arthritis and prostate cancer. A SNP array can also be used to generate a virtual karyotype using software to determine the copy number of each SNP on the array and then align the SNPs in chromosomal order.
SNPs can also be used to study genetic abnormalities in cancer. For example, SNP arrays can be used to study loss of heterozygosity (LOH). LOH occurs when one allele of a gene is mutated in a deleterious way and the normally-functioning allele is lost. LOH occurs commonly in oncogenesis. For example, tumor suppressor genes help keep cancer from developing. If a person has one mutated and dysfunctional copy of a tumor suppressor gene and his second, functional copy of the gene gets damaged, they may become more likely to develop cancer.
Other chip-based methods such as comparative genomic hybridization can detect genomic gains or deletions leading to LOH. SNP arrays, however, have an additional advantage of being able to detect copy-neutral LOH (also called uniparental disomy or gene conversion). Copy-neutral LOH is a form of allelic imbalance. In copy-neutral LOH, one allele or whole chromosome from a parent is missing. This problem leads to duplication of the other parental allele. Copy-neutral LOH may be pathological. For example, say that the mothers allele is wild-type and fully functional, and the fathers allele is mutated. If the mothers allele is missing and the child has two copies of the fathers mutant allele, disease can occur.
High density SNP arrays help scientists identify patterns of allelic imbalance. These studies have potential prognostic and diagnostic uses. Because LOH is so common in many human cancers, SNP arrays have great potential in cancer diagnostics. For example, recent SNP array studies have shown that solid tumors such as gastric cancer and liver cancer show LOH, as do non-solid malignancies such as hematologic malignancies, ALL, MDS, CML and others. These studies may provide insights into how these diseases develop, as well as information about how to create therapies for them.
Breeding in a number of animal and plant species has been revolutionized by the emergence of SNP arrays. The method is based on the prediction of genetic merit by incorporating relationships among individuals based on SNP array data. This process is known as genomic selection. Crop-specific arrays find use in agriculture. | 1 | Biochemistry |
Structurally, the lac repressor protein is a homotetramer. More precisely, the tetramer contains two DNA-binding subunits composed of two monomers each (a dimer of dimers). Each monomer consists of four distinct regions:
*An N-terminal DNA-binding domain (in which two LacI proteins bind a single operator site)
*A regulatory domain (sometimes called the core domain, which binds allolactose, an allosteric effector molecule)
*A linker that connects the DNA-binding domain with the core domain (sometimes called the hinge helix, which is important for allosteric communication)
*A C-terminal tetramerization region (which joins four monomers in an alpha-helix bundle)
DNA binding occurs via an N-terminal helix-turn-helix structural motif and is targeted to one of several operator DNA sequences (known as O, O and O). The O operator sequence slightly overlaps with the promoter, which increases the affinity of RNA polymerase for the promoter sequence such that it cannot enter elongation and remains in abortive initiation. Additionally, because each tetramer contains two DNA-binding subunits, binding of multiple operator sequences by a single tetramer induces DNA looping. | 1 | Biochemistry |
DNA can also be introduced into cells using viruses as a carrier. In such cases, the technique is called transduction, and the cells are said to be transduced. Adenoviral vectors can be useful for viral transfection methods because they can transfer genes into a wide variety of human cells and have high transfer rates. Lentiviral vectors are also helpful due to their ability to transduce cells not currently undergoing mitosis.
Protoplast fusion is a technique in which transformed bacterial cells are treated with lysozyme in order to remove the cell wall. Following this, fusogenic agents (e.g., Sendai virus, PEG, electroporation) are used in order to fuse the protoplast carrying the gene of interest with the target recipient cell. A major disadvantage of this method is that bacterial components are non-specifically introduced into the target cell as well. | 1 | Biochemistry |
Akin to complexation, phosphines are readily alkylated. For example, methyl bromide converts triphenylphosphine to the methyltriphenylphosphonium bromide, a "quat salt":
:PPh + CHBr → [CHPPh]Br
Phosphines are nucleophilic catalysts in organic synthesis, e.g. the Rauhut–Currier reaction and Baylis-Hillman reaction. | 0 | Organic Chemistry |
Strong hydrogen bonds are revealed by downfield shifts in the H NMR spectrum. For example, the acidic proton in the enol tautomer of acetylacetone appears at 15.5, which is about 10 ppm downfield of a conventional alcohol.
In the IR spectrum, hydrogen bonding shifts the stretching frequency to lower energy (i.e. the vibration frequency decreases). This shift reflects a weakening of the bond. Certain hydrogen bonds - improper hydrogen bonds - show a blue shift of the stretching frequency and a decrease in the bond length. H-bonds can also be measured by IR vibrational mode shifts of the acceptor. The amide I mode of backbone carbonyls in α-helices shifts to lower frequencies when they form H-bonds with side-chain hydroxyl groups. The dynamics of hydrogen bond structures in water can be probed by this OH stretching vibration. In the hydrogen bonding network in protic organic ionic plastic crystals (POIPCs), which are a type of phase change material exhibiting solid-solid phase transitions prior to melting, variable-temperature infrared spectroscopy can reveal the temperature dependence of hydrogen bonds and the dynamics of both the anions and the cations. The sudden weakening of hydrogen bonds during the solid-solid phase transition seems to be coupled with the onset of orientational or rotational disorder of the ions. | 6 | Supramolecular Chemistry |
Ferrier was born in Edinburgh on 7 August 1932. Following the family's idiosyncratic naming tradition, although he was named Robert John, he was always known as Robin. Likewise his father Edward was known as William and his mother Sophia was known as Rita. William was a policeman and became head of Edinburgh CID, while Rita was a housewife.
His only sibling was a fraternal twin sister Dr Barbara M. Ferrier (d. 2006), known as Ray, who likewise became an organic chemist, becoming professor emeritus of the Department of Biochemistry and Biomedical Sciences at McMaster University. A polycyclic ketone "barbaralone", related to bullvalene was named after her.
Ferrier attended George Heriot's School for all of his schooling, apart from a brief time in Traquair, to where he was evacuated during the war with his mother and sister.
He gained a Bachelor of Science with first class honours in 1954 and a PhD in plant polysaccharides in 1957, under Professor Gerald Aspinall. | 0 | Organic Chemistry |
Cation–pi interactions involve the positive charge of a cation interacting with the electrons in a π-system of a molecule. This interaction is surprisingly strong (as strong or stronger than H-bonding in some contexts), and has many potential applications in chemical sensors. For example, the sodium ion can easily sit atop the π cloud of a benzene molecule, with C symmetry (See figure 4).
Anion–π interactions are very similar to cation–π interactions, but reversed. In this case, an anion sits atop an electron-poor π-system, usually established by the placement of electron-withdrawing substituents on the conjugated molecule | 6 | Supramolecular Chemistry |
Retroviruses have been the focus of several recent claims and assertions which have been largely discredited by the science community. An initial study in 2009 seemed to make new findings which might change some of the established knowledge on this topic. However, although later research disproved some of the claims made about retroviruses, there are several controversial figures who continue to make claims which overall are considered to not have any valid basis or consensus in support of these claims. | 1 | Biochemistry |
During the more than 80 years of technical development of sensor-based ore sorting equipment, various types of machines have been developed. This includes the channel-type, bucket-wheel type and cone type sorters. The main machine types being installed in the mining industry today are belt-type and chute-type machines. Harbeck made a good comparison of both disadvantages and advantages of the systems for different sorting applications. The selection of a machine-type for an application depends various case-dependent factors, including the detection system applied, particle size, moisture, yield amongst others. | 3 | Analytical Chemistry |
The enzymatic process that produces heme is properly called porphyrin synthesis, as all the intermediates are tetrapyrroles that are chemically classified as porphyrins. The process is highly conserved across biology. In humans, this pathway serves almost exclusively to form heme. In bacteria, it also produces more complex substances such as cofactor F430 and cobalamin (vitamin B).
The pathway is initiated by the synthesis of δ-aminolevulinic acid (dALA or δALA) from the amino acid glycine and succinyl-CoA from the citric acid cycle (Krebs cycle). The rate-limiting enzyme responsible for this reaction, ALA synthase, is negatively regulated by glucose and heme concentration. Mechanism of inhibition of ALAs by heme or hemin is by decreasing stability of mRNA synthesis and by decreasing the intake of mRNA in the mitochondria. This mechanism is of therapeutic importance: infusion of heme arginate or hematin and glucose can abort attacks of acute intermittent porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase.
The organs mainly involved in heme synthesis are the liver (in which the rate of synthesis is highly variable, depending on the systemic heme pool) and the bone marrow (in which rate of synthesis of Heme is relatively constant and depends on the production of globin chain), although every cell requires heme to function properly. However, due to its toxic properties, proteins such as emopexin (Hx) are required to help maintain physiological stores of iron in order for them to be used in synthesis. Heme is seen as an intermediate molecule in catabolism of hemoglobin in the process of bilirubin metabolism. Defects in various enzymes in synthesis of heme can lead to group of disorder called porphyrias, which include acute intermittent porphyria, congenital erythropoetic porphyria, porphyria cutanea tarda, hereditary coproporphyria, variegate porphyria, and erythropoietic protoporphyria. | 1 | Biochemistry |
The electronic conductivity of purified distilled water in electrochemical laboratory settings at room temperature is often between 0.05 and 1 μS/cm. Environmental influences during the peparation of salt solutions as gas absorption due to storing the water in an unsealed beaker may immediately increase the conductivity from and lead to values between 0.5 and 1 . <br>
When distilled water is heated during the preparation of salt solutions, the conductivity increases even without adding salt. This is often not taken into account.
In a typical experiment under the fume hood in an unsealed beaker the conductivity of purified water increases typically non linearly from values below 1 μS/cm to values close 3.5 μS/cm at . This temperature dependence has to be taken into account particularly in dilute salt solutions. | 7 | Physical Chemistry |
* New Discussions on Nanocatalysts in Organic Chemistry, Ilam: Ilam University Press 2018, ISBN 9786006184395
* Drug Delivery Systems and Their Effectiveness Through Nanotechnology, Ilam: Havar 2019, ISBN 9786008473855 | 0 | Organic Chemistry |
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