text
stringlengths 105
4.44k
| label
int64 0
9
| label_text
stringclasses 10
values |
|---|---|---|
Tertiary carbons form the most stable carbocations due to a combination of factors. The three alkyl groups on the tertiary carbon contribute to a strong inductive effect. This is because each alkyl group will share its electron density with the central carbocation to stabilize it. Additionally, the surrounding sp3 hybridized carbons can stabilize the carbocation through hyperconjugation. This occurs when adjacent sp3 orbitals have a weak overlap with the vacant p orbital; since there are 3 surrounding carbons with sp3 hybridization, there are more opportunities for overlap, which contributes to increasing carbocation stability. | 0 | Organic Chemistry |
Polymers are high molecular mass compounds formed by polymerization of monomers. They are synthesized by the polymerization process and can be modified by the additive of monomers. The additives of monomers change polymers mechanical property, processability, durability and so on. The simple reactive molecule from which the repeating structural units of a polymer are derived is called a monomer. A polymer can be described in many ways: its degree of polymerisation, molar mass distribution, tacticity, copolymer distribution, the degree of branching, by its end-groups, crosslinks, crystallinity and thermal properties such as its glass transition temperature and melting temperature. Polymers in solution have special characteristics with respect to solubility, viscosity, and gelation. Illustrative of the quantitative aspects of polymer chemistry, particular attention is paid to the number-average and weight-average molecular weights and , respectively.
<br>
The formation and properties of polymers have been rationalized by many theories including Scheutjens–Fleer theory, Flory–Huggins solution theory, Cossee–Arlman mechanism, Polymer field theory, Hoffman Nucleation Theory, Flory–Stockmayer theory, and many others.
The study of polymer thermodynamics helps improve the material properties of various polymer-based materials such as polystyrene (styrofoam) and polycarbonate. Common improvements include toughening, improving impact resistance, improving biodegradability, and altering a material's solubility. | 7 | Physical Chemistry |
The isolation of trimyristin from powdered nutmeg is a common introductory-level college organic chemistry experiment. It is an uncommonly simple natural product extraction because nutmeg oil generally consists of over eighty percent trimyristin. Trimyristin makes up between 20-25% of the overall mass of dried, ground nutmeg. Separation is generally carried out by steam distillation and purification uses extraction from ether followed by distillation or rotary evaporation to remove the volatile solvent. The extraction of trimyristin can also be done with diethyl ether at room temperature, due to its high solubility in the ether. The experiment is frequently included in curricula, both for its relative ease and to provide instruction in these techniques. Trimyristin can then be used to prepare myristic acid or one of its salts as an example of saponification. | 1 | Biochemistry |
The AF4 experiment can be separated into three stages:
1. Sample Injection
:Samples are injected into the system using a known amount of sample volume. This volume will depend on the AF4 instrument being utilized in the experiment. Starting fractionation immediately after sample injection is not ideal because the sample is going to spread out randomly from the injection site, so the beginning velocity and place of the particles are not all the same. This leads to line broadening and insufficiency. In order to correct such an error, sample focusing is proposed.
2. Sample focusing<br />
:A current going opposite of the carrier solvent is used to focus all the particles in the sample to one specified area before fractionation begins. This corrects for any peak broadening that can occur due to particles being dispersed from the injection port to the channel outlet before fractionation begins. Sample preparation is another option that can be achieved in the focus step. Once all the particles are in the same area of the channel, fractionation can occur.
3. Fractionation<br />
:There are two components that make up the FFF system. Firstly, the laminar flow that carries the sample through the separation chamber and secondly the separation field applied perpendicular to the channel, against the sample flow.
:As particles flow along the channel the cross flow separation field pushes the molecules towards the bottom of the channel. As they pass by the bottom they undergo a counter acting diffusion back into the channel against the carrier flow. The extent to which the molecules can diffuse back into the channel is dictated by their natural Brownian motion, a characteristic based on size that is unique to each individual species. Smaller particles have a higher Brownian motion than larger ones and are able to diffuse higher into the channel against the carrier flow.
:The rate of laminar flow within the channel is not uniform. It travels in a parabolic pattern with the speed of the flow increasing towards the center of the channel and decreasing towards the sides. Therefore, the rate at which particles will be carried through will depend on their position within the channel. Those with a greater diffusion, located in the center of the channel, will be transported with a greater velocity. The larger particles in the shallow, slower moving stream are transported with lower flow velocity and elute later than smaller particles. This results in a gentle separation of particles based on mass with the elution order of smallest to largest. | 3 | Analytical Chemistry |
Geranylfarnesyl pyrophosphate is an intermediate used by organisms in the biosynthesis of sesterterpenoids. | 1 | Biochemistry |
There are two types of overlayers: commensurate and incommensurate. In the former the substrate-adsorbate interaction tends to dominate over any lateral adsorbate-adsorbate interaction, while in the latter the adsorbate-adsorbate interactions are of similar magnitude to those between adsorbate and substrate. | 7 | Physical Chemistry |
Prairie provinces (mainly Saskatchewan and Manitoba) contain high quantities of calcium and magnesium, often as dolomite, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide from the last glaciation. In these parts of Canada, the total hardness in ppm of calcium carbonate equivalent frequently exceeds 200 ppm, if groundwater is the only source of potable water. The west coast, by contrast, has unusually soft water, derived mainly from mountain lakes fed by glaciers and snowmelt.
Some typical values are:
* Montreal 116 ppm
* Calgary 165 ppm
* Regina 496 ppm
* Saskatoon 160–180 ppm
* Winnipeg 77 ppm
* Toronto 121 ppm
* Vancouver < 3 ppm
* Charlottetown, PEI 140–150 ppm
* Waterloo Region 400 ppm
* Guelph 460 ppm
* Saint John (West) 160–200 ppm
* Ottawa 30 ppm | 3 | Analytical Chemistry |
CDP-glucose is produced from CTP and glucose-1-phosphate by the enzyme glucose-1-phosphate cytidylyltransferase. | 1 | Biochemistry |
In general, biosignatures can be grouped into ten broad categories:
#Isotope patterns: Isotopic evidence or patterns that require biological processes.
#Chemistry: Chemical features that require biological activity.
#Organic matter: Organics formed by biological processes.
#Minerals: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., biomagnetite).
#Microscopic structures and textures: Biologically formed cements, microtextures, microfossils, and films.
#Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms.
#Temporal variability: Variations in time of atmospheric gases, reflectivity, or macroscopic appearance that indicates life's presence.
#Surface reflectance features: Large-scale reflectance features due to biological pigments could be detected remotely.
#Atmospheric gases: Gases formed by metabolic and/or aqueous processes, which may be present on a planet-wide scale.
#Technosignatures: Signatures that indicate a technologically advanced civilization. | 2 | Environmental Chemistry |
Pepscan is a procedure for mapping and characterizing epitopes involving the synthesis of overlapping peptides and analysis of the peptides in enzyme-linked immunosorbent assays (ELISAs). The method is based on combinatorial chemistry and was pioneered by Mario Geysen and coworkers.
Rob Meloen was one of Geysen's co-workers. He also played an important role in the development of numerous other new technologies, including vaccine and diagnostic product development for several viral diseases. From 1994 to 2010, Meloen was Professor of Special Appointment (Chair: Biomolecular Recognition) at Utrecht University. He was one of the co-founders of the company Pepscan (Lelystad, the Netherlands) and became Scientific Director (CSO). Pepscan is now part of the Biosynth Group.
Twenty-five years later, the Pepscan methodology, evolved and modernized with the latest insights, is still an important part of Pepscan’s epitope mapping platform, which is instrumental in therapeutic antibody development. | 1 | Biochemistry |
Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity. This method was pioneered by S. W. Benson. | 7 | Physical Chemistry |
Certain organic compounds in addition to metal redox catalysts can also produce reactive oxygen species. One of the most important classes of these is the quinones. Quinones can redox cycle with their conjugate semiquinones and hydroquinones, in some cases catalyzing the production of superoxide from dioxygen or hydrogen peroxide from superoxide. | 1 | Biochemistry |
An MC-ICP-MS instrument is a multiple collector mass spectrometer with a plasma source. MC-ICP-MS was developed to improve the precision achievable by ICP-MS during isotope-ratio measurements. Conventional ICP-MS analysis uses a quadrupole analyser, which only allows single-collector analysis. Due to the inherent instability of the plasma, this limits the precision of ICP-MS with a quadrupole analyzer to around 1%, which is insufficient for most radiogenic isotope systems.
Isotope-ratio analysis for radiometric dating has normally been determined by TIMS. However, some systems (e.g. Hf-W and Lu-Hf) are difficult or impossible to analyse by TIMS, due to the high ionization potential of the elements involved. Therefore, these methods can now be analysed using MC-ICP-MS.
The Ar-ICP produces an ion-beam with a large inherent kinetic energy distribution, which makes the design of the mass-spectrometer somewhat more complex than it is the case for conventional TIMS instruments. First, different from Quadrupole ICP-MS systems, magnetic sector instruments have to operate with a higher acceleration potential (several 1000 V) in order to minimize the energy distribution of the ion beam. Modern instruments operate at 6-10kV.
The radius of deflection of an ion within a magnetic field depends on the kinetic energy and the mass/charge ratio of the ion (strictly, the magnet is a momentum analyzer not just a mass analyzer). Because of the large energy distribution, ions with similar mass/charge ratio can have very different kinetic energies and will thus experience different deflection for the same magnetic field. In practical terms one would see that ions with the same mass/charge ratio focus at different points in space. However, in a mass-spectrometer one wants ions with the same mass/charge ratio to focus at the same point, e.g. where the detector is located. In order to overcome these limitations, commercial MC-ICP-MS are double-focusing instruments. In a double-focusing mass-spectrometer ions are focused due to kinetic energy by the ESA (electro-static-analyzer) and kinetic energy + mass/charge (momentum) in the magnetic field. Magnet and ESA are carefully chosen to match the energy focusing properties of one another and are arranged so that the direction of energy focusing is in opposite directions. To simplify, two components have an energy focus term, when arranged properly, the energy term cancels out and ions with the same mass/charge ratio focus at the same point in space. It is important to note, double-focusing does not reduce the kinetic energy distribution and different kinetic energies are not filtered or homogenized. Double-focusing works for single as well as multi-collector instruments. In single collector instruments ESA and magnet can be arranged in either forward geometry (first ESA then magnet) or reversed geometry (magnet first then ESA), as only point-to-point focusing is required. In multi-collector instruments, only forward geometry (ESA then magnet) is possible due to the array of detectors and the requirements of a focal plane rather than a focal point. | 9 | Geochemistry |
Lipids are a diverse and ubiquitous group of compounds which have many key biological functions, such as acting as structural components of cell membranes, serving as energy storage sources and participating in signaling pathways. Lipids may be broadly defined as hydrophobic or amphipathic small molecules that originate entirely or in part from two distinct types of biochemical subunits or "building blocks": ketoacyl and isoprene groups. The
huge structural diversity found in lipids arises from the biosynthesis of various combinations of these building blocks. For example, glycerophospholipids are composed of a glycerol backbone linked to one of approximately 10 possible headgroups and also to 2 fatty acyl/alkyl chains, which in turn may have 30 or more different molecular structures. In practice, not all possible permutations are detected experimentally, due to chain preferences depending on the cell type and also to detection limits - nevertheless several hundred distinct glycerophospholipid molecular species have been detected in mammalian cells.
Plant chloroplast thylakoid membranes however, have unique lipid composition as they are deficient in phospholipids. Also, their largest constituent, monogalactosyl diglyceride or MGDG, does not form aqueous bilayers. Nevertheless, dynamic studies reveal a normal lipid bilayer organisation in thylakoid membranes. | 1 | Biochemistry |
Aspects of the carbonate-silicate cycle have changed through Earth history as a result of biological evolution and tectonic changes. Generally, the formation of carbonates has outpaced that of silicates, effectively removing carbon dioxide from the atmosphere. The advent of carbonate biomineralization near the Precambrian-Cambrian boundary would have allowed more efficient removal of weathering products from the ocean. Biological processes in soils can significantly increase weathering rates. Plants produce organic acids that increase weathering. These acids are secreted by root and mycorrhizal fungi, as well as microbial plant decay. Root respiration and oxidation of organic soil matter also produce carbon dioxide, which is converted to carbonic acid, which increases weathering.
Tectonics can induce changes in the carbonate-silicate cycle. For example, the uplift of major mountain ranges, such as Himalayas and the Andes, is thought to have initiated the Late Cenozoic Ice Age due to increased rates of silicate weathering and draw down of carbon dioxide.
Seafloor weather is linked both to solar luminosity and carbon dioxide concentration. However, it presented a challenge to modelers who have tried to relate the rate of outgassing and subduction to the related rates of seafloor change. Proper, uncomplicated proxy data is difficult to attain for such questions. For example, sediment cores, from which scientists can deduce past sea levels, are not ideal because sea levels change as a result of more than just seafloor adjustment. Recent modeling studies have investigated the role of seafloor weathering on the early evolution of life, showing that relatively fast seafloor creation rates worked to draw down carbon dioxide levels to a moderate extent.
Observations of so-called deep time indicate that Earth has a relatively insensitive rock weathering feedback, allowing for large temperature swings. With about twice as much carbon dioxide in the atmosphere, paleoclimate records show that global temperatures reached up to 5 to 6 °C higher than current temperatures. However, other factors such as changes in orbital/solar forcing contribute to global temperature change in the paleo-record.
Human emissions of CO have been steadily increasing, and the consequent concentration of CO in the Earth system has reached unprecedented levels in a very short amount of time. Excess carbon in the atmosphere that is dissolved in seawater can alter the rates of carbonate-silicate cycle. Dissolved CO may react with water to form bicarbonate ions, HCO, and hydrogen ions, H. These hydrogen ions quickly react with carbonate, CO to produce more bicarbonate ions and reduce the available carbonate ions, which presents an obstacle to the carbon carbonate precipitation process. Put differently, 30% of excess carbon emitted into the atmosphere is absorbed by the oceans. Higher concentrations of carbon dioxide in the oceans work to push the carbonate precipitation process in the opposite direction (to the left), producing less CaCO. This process, which harms shell-building organisms, is called ocean acidification. | 9 | Geochemistry |
Stop codon suppression or translational readthrough occurs when in translation a stop codon is interpreted as a sense codon, that is, when a (standard) amino acid is encoded by the stop codon. Mutated tRNAs can be the cause of readthrough, but also certain nucleotide motifs close to the stop codon. Translational readthrough is very common in viruses and bacteria, and has also been found as a gene regulatory principle in humans, yeasts, bacteria and drosophila. This kind of endogenous translational readthrough constitutes a variation of the genetic code, because a stop codon codes for an amino acid. In the case of human malate dehydrogenase, the stop codon is read through with a frequency of about 4%. The amino acid inserted at the stop codon depends on the identity of the stop codon itself: Gln, Tyr, and Lys have been found for the UAA and UAG codons, while Cys, Trp, and Arg for the UGA codon have been identified by mass spectrometry. Extent of readthrough in mammals have widely variable extents, and can broadly diversify the proteome and affect cancer progression. | 1 | Biochemistry |
Succinate-Q oxidoreductase, also known as complex II or succinate dehydrogenase, is a second entry point to the electron transport chain. It is unusual because it is the only enzyme that is part of both the citric acid cycle and the electron transport chain. Complex II consists of four protein subunits and contains a bound flavin adenine dinucleotide (FAD) cofactor, iron–sulfur clusters, and a heme group that does not participate in electron transfer to coenzyme Q, but is believed to be important in decreasing production of reactive oxygen species. It oxidizes succinate to fumarate and reduces ubiquinone. As this reaction releases less energy than the oxidation of NADH, complex II does not transport protons across the membrane and does not contribute to the proton gradient.
In some eukaryotes, such as the parasitic worm Ascaris suum, an enzyme similar to complex II, fumarate reductase (menaquinol:fumarate
oxidoreductase, or QFR), operates in reverse to oxidize ubiquinol and reduce fumarate. This allows the worm to survive in the anaerobic environment of the large intestine, carrying out anaerobic oxidative phosphorylation with fumarate as the electron acceptor. Another unconventional function of complex II is seen in the malaria parasite Plasmodium falciparum. Here, the reversed action of complex II as an oxidase is important in regenerating ubiquinol, which the parasite uses in an unusual form of pyrimidine biosynthesis. | 1 | Biochemistry |
Carboranes have been prepared by many routes, the most common being addition of alkynyl reagents to boron hydride clusters to form dicarbon carboranes. For this reason, the great majority of carborane have two carbon vertices. | 7 | Physical Chemistry |
Extracellular polymeric substances (EPSs) are natural polymers of high molecular weight secreted by microorganisms into their environment. EPSs establish the functional and structural integrity of biofilms, and are considered the fundamental component that determines the physicochemical properties of a biofilm. EPS in the matrix of biofilms provides compositional support and protection of microbial communities from the harsh environments. Components of EPS can be of different classes of polysaccharides, lipids, nucleic acids, proteins, lipopolysaccharides, and minerals. | 1 | Biochemistry |
In alcohol fermentation, when a glucose molecule is oxidized, ethanol (ethyl alcohol) and carbon dioxide are byproducts. The organic molecule that is responsible for renewing the NAD+ supply in this type of fermentation is the pyruvate from glycolysis. Each pyruvate releases a carbon dioxide molecule, turning into acetaldehyde. The acetaldehyde is then reduced by the NADH produced from glycolysis, forming the alcohol waste product, ethanol, and forming NAD+, thereby replenishing its supply for glycolysis to continue producing ATP. | 1 | Biochemistry |
If a molecule contains two asymmetric centers, there are up to four possible configurations, and they cannot all be non-superposable mirror images of each other. The possibilities for different isomers continue to multiply as more stereocenters are added to a molecule. In general, the number of stereoisomers of a molecule can be determined by calculating 2, where n = the number of chiral centers in the molecule. This holds true except in cases where the molecule has meso forms. These meso compounds are molecules that contain stereocenters, but possess an internal plane of symmetry allowing it to be superposed on its mirror image. These equivalent configurations cannot be considered diastereomers.
For n = 3, there are eight stereoisomers. Among them, there are four pairs of enantiomers: R,R,R and S,S,S; R,R,S and S,S,R; R,S,S and S,R,R; and R,S,R and S,R,S. There are many more pairs of diastereomers, because each of these configurations is a diastereomer with respect to every other configuration excluding its own enantiomer (for example, R,R,R is a diastereomer of R,R,S; R,S,R; and R,S,S). For n = 4, there are sixteen stereoisomers, or eight pairs of enantiomers. The four enantiomeric pairs of aldopentoses and the eight enantiomeric pairs of aldohexoses (subsets of the five- and six-carbon sugars) are examples of sets of compounds that differ in this way. | 4 | Stereochemistry |
Quasicrystalline substances have potential applications in several forms.
Metallic quasicrystalline coatings can be applied by Thermal spraying or magnetron sputtering. A problem that must be resolved is the tendency for cracking due to the materials' extreme brittleness. The cracking could be suppressed by reducing sample dimensions or coating thickness. Recent studies show typically brittle quasicrystals can exhibit remarkable ductility of over 50% strains at room temperature and sub-micrometer scales (<500 nm).
An application was the use of low-friction Al–Cu–Fe–Cr quasicrystals as a coating for frying pans. Food did not stick to it as much as to stainless steel making the pan moderately non-stick and easy to clean; heat transfer and durability were better than PTFE non-stick cookware and the pan was free from perfluorooctanoic acid (PFOA); the surface was very hard, claimed to be ten times harder than stainless steel, and not harmed by metal utensils or cleaning in a dishwasher; and the pan could withstand temperatures of without harm. However, after an initial introduction the pans were a chrome steel, probably because of the difficulty of controlling thin films of the quasicrystal.
The Nobel citation said that quasicrystals, while brittle, could reinforce steel "like armor". When Shechtman was asked about potential applications of quasicrystals he said that a precipitation-hardened stainless steel is produced that is strengthened by small quasicrystalline particles. It does not corrode and is extremely strong, suitable for razor blades and surgery instruments. The small quasicrystalline particles impede the motion of dislocation in the material.
Quasicrystals were also being used to develop heat insulation, LEDs, diesel engines, and new materials that convert heat to electricity. Shechtman suggested new applications taking advantage of the low coefficient of friction and the hardness of some quasicrystalline materials, for example embedding particles in plastic to make strong, hard-wearing, low-friction plastic gears. The low heat conductivity of some quasicrystals makes them good for heat insulating coatings. One of the special properties of quasicrystals is their smooth surface, which despite the irregular atomic
structure, the surface of quasicrystals can be smooth and flat.
Other potential applications include selective solar absorbers for power conversion, broad-wavelength reflectors, and bone repair and prostheses applications where biocompatibility, low friction and corrosion resistance are required. Magnetron sputtering can be readily applied to other stable quasicrystalline alloys such as Al–Pd–Mn. | 3 | Analytical Chemistry |
Hydroperoxides are intermediates in the production of many organic compounds in industry. For example, the cobalt catalyzed oxidation of cyclohexane to cyclohexanone:
Drying oils, as found in many paints and varnishes, function via the formation of hydroperoxides. | 0 | Organic Chemistry |
Total synthesis of (+)-Absinthin was conducted in 2004 by Zhang, et al. The final yield reported for the synthesis was 18.6% over a course of 10 steps originating from Santonin (1), a commercially available reagent. The basis of the synthesis was the ring expansion of the original 6-membered carbon ring to the 7-membered ring, engendering the formation of the guaianolide monomer (2) scaffold, followed by Diels Alder coupling (3) and final stereochemical modifications resulting in (+)-Absinthin (4). | 0 | Organic Chemistry |
In organic chemistry, olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often creates fewer undesired by-products and hazardous wastes than alternative organic reactions. For their elucidation of the reaction mechanism and their discovery of a variety of highly active catalysts, Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock were collectively awarded the 2005 Nobel Prize in Chemistry. | 0 | Organic Chemistry |
To generate a traveling pulse with an ultrashort time duration, two key elements are needed: bandwidth and central wavelength of the electromagnetic wave.
From Fourier analysis, the more the available spectral bandwidth of a light pulse, the shorter, potentially, is its time duration.
There is, however, a lower-limit in the minimum duration exploitable for a given pulse central wavelength. This limit is the optical cycle.
Indeed, for a pulse centered in the low-frequency region, e.g. infrared (IR) 800 nm, its minimum time duration is around 2.67 fs, where is the speed of light; whereas, for a light field with central wavelength in the extreme ultraviolet (XUV) at 30 nm the minimum duration is around 100 as.
Thus, a smaller time duration requires the use of shorter, and more energetic wavelength, even down to the soft-X-ray (SXR) region.
For this reason, standard techniques to create attosecond light pulses are based on radiation sources with broad spectral bandwidths and central wavelength located in the XUV-SXR range.
The most common sources that fit these requirements are free-electron lasers (FEL) and high harmonic generation (HHG) setups. | 7 | Physical Chemistry |
Lead-zinc deposits are generally accompanied by silver, hosted within the lead sulfide mineral galena or within the zinc sulfide mineral sphalerite.
Lead and zinc deposits are formed by discharge of deep sedimentary brine onto the sea floor (termed sedimentary exhalative or SEDEX), or by replacement of limestone, in skarn deposits, some associated with submarine volcanoes (called volcanogenic massive sulfide ore deposits or VMS), or in the aureole of subvolcanic intrusions of granite. The vast majority of SEDEX lead and zinc deposits are Proterozoic in age, although there are significant Jurassic examples in Canada and Alaska.
The carbonate replacement type deposit is exemplified by the Mississippi valley type (MVT) ore deposits. MVT and similar styles occur by replacement and degradation of carbonate sequences by hydrocarbons, which are thought important for transporting lead. | 9 | Geochemistry |
On Saturns largest moon, Titan, lakes of liquid hydrocarbons comprising methane, ethane, propane and other constituents, occur naturally. Data collected by the space probe Cassini–Huygens yield an estimate that the visible lakes and seas of Titan contain about 300 times the volume of Earths proven oil reserves. Drilled samples from the surface of Mars taken in 2015 by the Curiosity rover's Mars Science Laboratory have found organic molecules of benzene and propane in 3-billion-year-old rock samples in Gale Crater. | 7 | Physical Chemistry |
Secondary antibodies provide signal detection and amplification along with extending the utility of an antibody through conjugation to proteins. Secondary antibodies are especially efficient in immunolabeling. Secondary antibodies bind to primary antibodies, which are directly bound to the target antigen(s). In immunolabeling, the primary antibodys Fab domain binds to an antigen and exposes its Fc domain to secondary antibody. Then, the secondary antibodys Fab domain binds to the primary antibody's Fc domain. Since the Fc domain is constant within the same animal class, only one type of secondary antibody is required to bind to many types of primary antibodies. This reduces the cost by labeling only one type of secondary antibody, rather than labeling various types of primary antibodies. Secondary antibodies help increase sensitivity and signal amplification due to multiple secondary antibodies binding to a primary antibody.
Whole Immunoglobulin molecule secondary antibodies are the most commonly used format, but these can be enzymatically processed to enable assay refinement. F(ab') fragments are generated by pepsin digestion to remove most of the Fc fragment, this avoids recognition by Fc receptors on live cells, or to Protein A or Protein G. Papain digestion generates Fab fragments, which removes the entire Fc fragment including the hinge region, yielding two monovalent Fab moieties. They can be used to block endogenous immunoglobulins on cells, tissues or other surfaces, and to block the exposed immunoglobulins in multiple labeling experiments using primary antibodies from the same species. | 1 | Biochemistry |
A common method of freezing lesions is by using liquid nitrogen as the cryogen. The liquid nitrogen may be applied to lesions using a variety of methods; such as dipping a cotton or synthetic material tipped applicator in liquid nitrogen and then directly applying the cryogen onto the lesion. The liquid nitrogen can also be sprayed onto the lesion using a spray canister. The spray canister may utilize a variety of nozzles for different spray patterns. A cryoprobe, which is a metal applicator that has been cooled using liquid nitrogen, can also be directly applied onto lesions. | 1 | Biochemistry |
Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools. Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity. In aluminium castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity. For casting that are a few kilograms in weight the pores are usually in size. In larger casting, they can be up to a millimetre (0.040 in) in diameter.
To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon or carbon dioxide, or under a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mould can introduce gases, so the moulds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance, oxygen can be removed from copper by adding phosphorus; aluminium or silicon can be added to steel to remove oxygen. A third source consists of reactions of the molten metal with grease or other residues in the mould.
Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mould. Drying the mould can eliminate this source of hydrogen formation.
Gas porosity can sometimes be difficult to distinguish from micro shrinkage because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation.
Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes or blisters. Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand, or other gasses from the melt or mould. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as blowholes). Proper foundry practices, including melt preparation and mould design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (e.g., industrial CT scanning) analysis. | 8 | Metallurgy |
It is an old joke in chemistry to draw a polycyclic hexagonal chemical structure and call this fictional compound chickenwire. By adding one or two simple chemical groups to this skeleton, the compound can then be named following the official chemical naming convention. An example is:
* 1,2-Dimethyl-chickenwire in a [https://web.archive.org/web/20070927205644/http://www.nearingzero.net/screen_res/nz079.jpg cartoon] by Nick D. Kim | 4 | Stereochemistry |
* Thionyl chloride can engage in a range of different electrophilic addition reactions. It adds to alkenes in the presence of to form an aluminium complex which can be hydrolysed to form a sulfinic acid. Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride in triflic acid or the presence of catalysts such as , , or .
* In the laboratory, a reaction between thionyl chloride and an excess of anhydrous alcohol can be used to produce anhydrous alcoholic solutions of HCl.
* Thionyl chloride undergoes halogen exchange reactions to give other thionyl species.
:Reactions with fluorinating agents such as antimony trifluoride give thionyl fluoride:
: A reaction with hydrogen bromide gives thionyl bromide:
:Thionyl iodide can likewise be prepared by a reaction with potassium iodide, but is reported to be highly unstable. | 0 | Organic Chemistry |
Symbiotic fermentation is a form of fermentation in which multiple organisms (yeasts, acetic acid bacteria, lactic acid bacteria and others) interact in symbiosis in order to produce the desired product. For example, a yeast may produce ethanol, which is then consumed by an acetic acid bacterium. Described early on as the fermentation of sugars following saccharification in a mixed fermentation process. | 1 | Biochemistry |
In some ores, particularly those that are partially sulfidized, aeration (prior to the introduction of cyanide) of the ore in water at high pH can render elements such as iron and sulfur less reactive to cyanide, therefore making the gold cyanidation process more efficient. Specifically, the oxidation of iron to iron (III) oxide and subsequent precipitation as iron hydroxide minimizes loss of cyanide from the formation of ferrous cyanide complexes. The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (SCN) byproduct. | 8 | Metallurgy |
A peptidomimetic is a small protein-like chain designed to mimic a peptide. They typically arise either from modification of an existing peptide, or by designing similar systems that mimic peptides, such as peptoids and β-peptides. Irrespective of the approach, the altered chemical structure is designed to advantageously adjust the molecular properties such as stability or biological activity. This can have a role in the development of drug-like compounds from existing peptides. Peptidomimetics can be prepared by cyclization of linear peptides or coupling of stable unnatural amino acids. These modifications involve changes to the peptide that will not occur naturally (such as altered backbones and the incorporation of nonnatural amino acids). Unnatural amino acids can be generated from their native analogs via modifications such as amine alkylation, side chain substitution, structural bond extension cyclization, and isosteric replacements within the amino acid backbone. Based on their similarity with the precursor peptide, peptidomimetics can be grouped into four classes (A – D) where A features the most and D the least similarities. Classes A and B involve peptide-like scaffolds, while classes C and D include small molecules (Figure 1). | 0 | Organic Chemistry |
In surface chemistry, disjoining pressure (symbol ) according to an IUPAC definition arises from an attractive interaction between two surfaces. For two flat and parallel surfaces, the value of the disjoining pressure (i.e., the force per unit area) can be calculated as the derivative of the Gibbs energy of interaction per unit area in respect to distance (in the direction normal to that of the interacting surfaces). There is also a related concept of disjoining force, which can be viewed as disjoining pressure times the surface area of the interacting surfaces.
The concept of disjoining pressure was introduced by Derjaguin (1936) as the difference between the pressure in a region of a phase adjacent to a surface confining it, and the pressure in the bulk of this phase. | 7 | Physical Chemistry |
To determine the optimal length of the stem, researchers modified its length and observed how quickly termination occurred. When the length of the stem was lengthened or shortened from the standard 8-9 base pair length, termination was less efficient, and if the changes were great enough, termination ceased completely.
Experiments determined that if an oligonucleotide sequence that is identical to the downstream portion of the stem is present, it will base pair with the upstream portion. This creates a structure that is analogous to the native stem-loop structure but is missing the loop at the end. Without the presence of the loop, intrinsic termination is still able to occur. This indicates that the loop is not inherently necessary for intrinsic termination.
Generally, the absence of the uracil-rich sequence following the stem-loop will result in a delay or pause in transcription, but termination will not cease completely. | 1 | Biochemistry |
Exorphins are exogenous opioid peptides, distinguished from endorphins, or endogenous opioid peptides.
Exorphins include opioid food peptides like gluten exorphin and microbial opioid peptides and any other opioid peptide foreign to a host that have metabolic efficacy for that host. Exorphins can be converted from plants and animals but also dairy products and certain vegetables like spinach and soy.
Exorphins can be released for many different kinds of proteins and thus can be isolated from various sources such as from plant proteins or from enzymes of the digestive system of animals. The study of exorphins as a bioactive peptide can be a source of discovery for new kinds of food and drugs to treat and prevent diseases associated with the accumulation of exorphins. | 1 | Biochemistry |
Properties of phycobiliproteins, such as their natural antioxidant, anti-inflammatory, food colourant, strong pigment and anti-aging activities, have given them considerable potential for use in food, cosmetics and medicinal applications. They have also proven to be therapeutic in treating diseases such as Alzheimers disease and cancer. Given their large range of applications and potential uses, researchers have been trying to find and develop ways to produce and purify phycobiliproteins to meet the growing demand for them. One such phycobiliprotein is C-phycocyanin (C-PC), which is found in spirulina. A limiting factor of C-PCs usage in these applications is its protein stability, given that in its natural form, C-PC is highly sensitive to light and heat when in aqueous solution, due to its photosensitive phycocyanobilin (PCB) chromophore, which also makes it prone to free-radical oxidation. Like other natural food colourants, C-PC is also sensitive to acidic conditions and oxidant exposure. This has prompted studies to develop methods of stabilising C-PC/PCB and expand their applications to other food systems.
More details on the applications of phycocyanin in food and medicine can be found here. | 1 | Biochemistry |
After the discovery of x-rays by Wilhelm Röntgen in 1895, and of the principles of X-ray diffraction by Laue and the Bragg family, it took several decades for the benefits of diffraction imaging to be fully recognized, and the first useful experimental techniques to be developed. The first systematic reports of laboratory topography techniques date from the early 1940s. In the 1950s and 1960s, topographic investigations played a role in detecting the nature of defects and improving crystal growth methods for germanium and (later) silicon as materials for semiconductor microelectronics.
For a more detailed account of the historical development of topography, see J.F. Kelly – "A brief history of X-ray diffraction topography".
From about the 1970s on, topography profited from the advent of synchrotron x-ray sources which provided considerably more intense x-ray beams, allowing to achieve shorter exposure times, better contrast, higher spatial resolution, and to investigate smaller samples or rapidly changing phenomena.
Initial applications of topography were mainly in the field of metallurgy, controlling the growth of better crystals of various metals. Topography was later extended to semiconductors, and generally to materials for microelectronics. A related field are investigations of materials and devices for X-ray optics, such as monochromator crystals made of Silicon, Germanium or Diamond, which need to be checked for defects prior to being used. Extensions of topography to organic crystals are somewhat more recent.
Topography is applied today not only to volume crystals of any kind, including semiconductor wafers, but also to thin layers, entire electronic devices, as well as to organic materials such as protein crystals and others. | 3 | Analytical Chemistry |
Modern ellipsometers are complex instruments that incorporate a wide variety of radiation sources, detectors, digital electronics and software. The range of wavelength employed is far in excess of what is visible so strictly these are no longer optical instruments. | 7 | Physical Chemistry |
Lightning strike protection minimizes damage to buildings during lightning terminations. This is usually accomplished by providing multiple interconnected pathways of low electrical impedance to the ground.
Copper and its alloys are the most common materials used in residential lightning protections, however in industrial, chemically corrosive environments, the copper may need to be clad in tin. Copper effectively facilitates the transmission of lightning energy to the ground because of its excellent electrical conductivity. Also, it bends easily compared to other conductor materials.
When copper roofing, gutters, and rain leaders are electrically bonded to an earth termination facility, a pathway of low electrical impedance to ground is provided, however without dedicated conduction pathways to concentrate the discharge channel, a disperse energized surface may not be the most desirable.
Because copper has a higher electrical conductivity than aluminium and its impedance during a lightning termination is less, copper allows for the use of less cross-sectional surface area per linear length, in its woven wires pathway than does aluminum. Also, aluminium cannot be used in poured concrete or for any component underground due to its galvanic properties.
To be effective, lightning protection systems generally maximize the surface area contact between the conductors and the earth through a ground grid of varying designs. To supplement grounding grids in low-conductivity earth, such as sand or rock, long, hollow copper tubes filled with metallic salts are available. These salts leach through holes in the tube, making the surrounding soil more conductive as well as increasing the overall surface area which decreases effective resistance.
Copper roofs may be used as part of a lightning protection scheme where the copper skin, gutters and rainwater pipes can be linked and bonded to an earth termination facility. The thickness of copper specified for roofing materials is usually adequate for lightning protection. A dedicated lightning protection system may be recommended to adequate lightning protection with an installed copper roof system. The system would include air terminals and intercepting conductors on the roof, a system of ground electrodes, and a system of down-conductors connecting the roof and ground components. It is recommended that the copper roof be bonded to the system of conductors. Bonding ensures that the conductors and roof remain at equipotential and reduce side flashing and possible roof damage. | 8 | Metallurgy |
:Pericline also refers to a doubly plunging anticline or syncline.
Pericline is a form of albite exhibiting elongate prismatic crystals.
Pericline twinning is a type of crystal twinning which show fine parallel twin laminae typically found in the alkali feldspars microcline. The twinning results from a structural transformation between high temperature and low temperature forms. | 3 | Analytical Chemistry |
Bacteria and archaea also can use chemiosmosis to generate ATP. Cyanobacteria, green sulfur bacteria, and purple bacteria synthesize ATP by a process called photophosphorylation. These bacteria use the energy of light to create a proton gradient using a photosynthetic electron transport chain. Non-photosynthetic bacteria such as E. coli also contain ATP synthase. In fact, mitochondria and chloroplasts are the product of endosymbiosis and trace back to incorporated prokaryotes. This process is described in the endosymbiotic theory. The origin of the mitochondrion triggered the origin of eukaryotes, and the origin of the plastid the origin of the Archaeplastida, one of the major eukaryotic supergroups.
Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule. This process is part of oxidative phosphorylation. | 1 | Biochemistry |
A ternary complex is a protein complex containing three different molecules that are bound together. In structural biology, ternary complex can also be used to describe a crystal containing a protein with two small molecules bound, such as a cofactor and a substrate; or a complex formed between two proteins and a single substrate. In Immunology, ternary complex can refer to the MHC–peptide–T-cell-receptor complex formed when T cells recognize epitopes of an antigen.
Another important example is the ternary complex formed during eukaryotic translation, in which ternary complex composed of eIF2 + GTP + Met-tRNA is formed.
A ternary complex can be a complex formed between two substrate molecules and an enzyme. This is seen in multi-substrate enzyme-catalyzed reactions where two substrates and two products can be formed. The ternary complex is an intermediate species in this type of enzyme-catalyzed reaction. An example for a ternary complex is seen in the random-order mechanism or the compulsory-order mechanism of enzyme catalysis for multiple substrates.
The term ternary complex can also refer to a polymer formed by electrostatic interactions. | 1 | Biochemistry |
Cyclamin has not been thoroughly investigated in terms of its effects on animals, given it is not a widely known compound. However, the effects of Cyclamin on the snail Biomphalaria glabrata (Say) in terms of molluscicidal activity were researched. The lowest concentration showing 100% mortality to snails was 21 mg/l. Beside cyclamin its effect on these snails, no further data is known about its effect on animals. | 0 | Organic Chemistry |
In 1849 a 6m radius steam-driven fan, designed by William Brunton, was made operational in the Gelly Gaer Colliery of South Wales. The model was exhibited at the Great Exhibition of 1851. Also in 1851 David Boswell Reid, a Scottish doctor, installed four steam-powered fans in the ceiling of St George's Hospital in Liverpool, so that the pressure produced by the fans would force the incoming air upward and through vents in the ceiling. Improvements in the technology were made by James Nasmyth, Frenchman Theophile Guibal and J. R. Waddle. | 7 | Physical Chemistry |
In cellular biology, P-bodies, or processing bodies, are distinct foci formed by phase separation within the cytoplasm of a eukaryotic cell consisting of many enzymes involved in mRNA turnover. P-bodies are highly conserved structures and have been observed in somatic cells originating from vertebrates and invertebrates, plants and yeast. To date, P-bodies have been demonstrated to play fundamental roles in general mRNA decay, nonsense-mediated mRNA decay, adenylate-uridylate-rich element mediated mRNA decay, and microRNA (miRNA) induced mRNA silencing. Not all mRNAs which enter P-bodies are degraded, as it has been demonstrated that some mRNAs can exit P-bodies and re-initiate translation. Purification and sequencing of the mRNA from purified processing bodies showed that these mRNAs are largely translationally repressed upstream of translation initiation and are protected from 5' mRNA decay.
P-bodies were originally proposed to be the sites of mRNA degradation in the cell and involved in decapping and digestion of mRNAs earmarked for destruction. Later work called this into question suggesting P bodies store mRNA until needed for translation.
In neurons, P-bodies are moved by motor proteins in response to stimulation. This is likely tied to local translation in dendrites. | 1 | Biochemistry |
*DAD1 Defender against cell death
*DAP3 Involved in mediating interferon-gamma-induced cell death.
*DAXX Death Associated Protein 6 | 1 | Biochemistry |
Marine primary production can be divided into new production from allochthonous nutrient inputs to the euphotic zone, and regenerated production from nutrient recycling in the surface waters. The total new production in the ocean roughly equates to the sinking flux of particulate organic matter to the deep ocean, about 4 × 10 tons of carbon annually. | 9 | Geochemistry |
Condensation of SAMP or RAMP with an aldehyde or ketone affords the (E)-hydrazine. Deprotonation with lithium diisopropylamide and addition of an alkyl halide affords the alkylated product. The auxiliary can be removed by ozonolysis or hydrolysis.
<br /> | 4 | Stereochemistry |
Stereochemistry also plays a role assigning faces to trigonal molecules such as ketones. A nucleophile in a nucleophilic addition can approach the carbonyl group from two opposite sides or faces. When an achiral nucleophile attacks acetone, both faces are identical and there is only one reaction product. When the nucleophile attacks butanone, the faces are not identical (enantiotopic) and a racemic product results. When the nucleophile is a chiral molecule diastereoisomers are formed. When one face of a molecule is shielded by substituents or geometric constraints compared to the other face the faces are called diastereotopic. The same rules that determine the stereochemistry of a stereocenter (R or S) also apply when assigning the face of a molecular group. The faces are then called the Re-face and Si-face. In the example displayed on the right, the compound acetophenone is viewed from the Re-face. Hydride addition as in a reduction process from this side will form the (S)-enantiomer and attack from the opposite Si-face will give the (R)-enantiomer. However, one should note that adding a chemical group to the prochiral center from the Re-face will not always lead to an (S)-stereocenter, as the priority of the chemical group has to be taken into account. That is, the absolute stereochemistry of the product is determined on its own and not by considering which face it was attacked from. In the above-mentioned example, if chloride (Z = 17) were added to the prochiral center from the Re-face, this would result in an (R)-enantiomer. | 4 | Stereochemistry |
The angle between the C-C bonds in each carbon atom is 108°, which is the angle between adjacent sides of a regular pentagon. That value is quite close to the 109.5° central angle of a regular tetrahedron—the ideal angle between the bonds on an atom that has sp hybridisation. As a result, there is minimal angle strain. However, the molecule has significant levels of torsional strain as a result of the eclipsed conformation along each edge of the structure.
The molecule has perfect icosahedral (I) symmetry, as evidenced by its proton NMR spectrum in which all hydrogen atoms appear at a single chemical shift of 3.38 ppm. Unlike buckminsterfullerene, dodecahedrane has no delocalized electrons and hence has no aromaticity. | 0 | Organic Chemistry |
Halohydrins may also be prepared from the reaction of an epoxide with a hydrohalic acid, or a metal halide.
This reaction is produced on an industrial scale for the production of chlorohydrin precursors to two important epoxides, epichlorohydrin and propylene oxide. At one time, 2-chloroethanol was produced on a large scale as a precursor to ethylene oxide, but the latter is now prepared by the direct oxidation of ethylene. | 0 | Organic Chemistry |
Extending their work on MOS technology, Atalla and Kahng next did pioneering work on hot carrier devices, which used what would later be called a Schottky barrier. The Schottky diode, also known as the Schottky-barrier diode, was theorized for years, but was first practically realized as a result of the work of Atalla and Kahng during 19601961. They published their results in 1962 and called their device the "hot electron" triode structure with semiconductor-metal emitter. It was one of the first metal-base transistors. The Schottky diode went on to assume a prominent role in mixer applications. | 7 | Physical Chemistry |
Isovaleryl-coenzyme A, also known as isovaleryl-CoA, is an intermediate in the metabolism of branched-chain amino acids. | 1 | Biochemistry |
A receptor agonist is a chemical that binds to a receptor with the end result of directly inducing a conformational change in the bound receptor and activating a downstream effect. Some common examples are opium derivates, such as heroin and Toll-like receptor agonists. Heroin functions in this manner, along with other opioids, when bound to µ-opioid receptors. Opioids' manner of action are both concentration- and receptor-dependent, which provides a key difference between agonists and partial agonists. Another example is insulin, which activates cell receptors to instigate blood glucose uptake. | 1 | Biochemistry |
Testing and/or inspection are typically included in component manufacturing lines to verify the product meets some set of standards to ensure the desired performance in the field. Improper testing or inspection would circumvent these quality checks and could allow a part with a defect that would normally disqualify the component from field use to be sold to a customer, potentially leading to a failure. | 8 | Metallurgy |
Industrial processes, such as oil refining, steel making or glass making are major sources of waste heat. | 7 | Physical Chemistry |
Because most deoxyribozymes suffer from product inhibition and thus exhibit single-turnover behavior, it is sometimes argued that deoxyribozymes do not exhibit "true" catalytic behavior since they cannot undergo multiple-turnover catalysis like most biological enzymes. However, the general definition of a catalyst requires only that the substance speeds up the rate of a chemical reaction without being consumed by the reaction (i.e. it is not permanently chemically altered and can be recycled). Thus, by this definition, single-turnover deoxyribozymes are indeed catalysts. Furthermore, many endogenous enzymes (both proteins and ribozymes) also exhibit single-turnover behavior, and so the exclusion of deoxyribozymes from the rank of "catalyst" simply because it does not feature multiple-turnover behavior seems unjustified. | 7 | Physical Chemistry |
Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement. Most natural crystals, however, do not display ideal habits and are commonly malformed. Hence, it is also important to describe the quality of the shape of a mineral specimen:
* Euhedral: a crystal that is completely bounded by its characteristic faces, well-formed. Synonymous terms: idiomorphic, automorphic;
* Subhedral: a crystal partially bounded by its characteristic faces and partially by irregular surfaces. Synonymous terms: hypidiomorphic, hypautomorphic;
* Anhedral: a crystal that lacks any of its characteristic faces, completely malformed. Synonymous terms: allotriomorphic, xenomorphic. | 3 | Analytical Chemistry |
Nitrogen and phosphorus grow a K. brevis red tide. Although K. brevis is initiated off shore, it will grow from nutrients (phosphorus and nitrogen) found on the shore. Along the southwest coast of Florida, when surface summer south winds blow phosphorus, nitrogen, green algae, and cyanobacteria into K. brevis that has come close to shore, there is a massive growth in the K. brevis red tide. The waves crashing break the cells open aerosolizing the subsequent brevetoxins which cause respiratory illnesses in humans. In 2018, MOTE Marine in Sarasota, FL updated their frequently asked questions to make it more clear that nutrients (nitrogen is a nutrient found in fertilizer) can grow K. brevis.
Along the west coast of Florida, the early phase of K. brevis blooms are initiated by northerly winds, resulting in upwelling events that cause nutrients to rise towards the surface of the water and transport multiple Karenia cell species towards the shore. Here they concentrate and either continue to grow or are taken up by onshore winds that spread the cells over beaches and near shore communities. It has been shown that K. brevis blooms are limited by available nitrogen (N) or phosphorus (P), but until recently it was not clear what sources K. brevis was utilizing for these key developmental nutrients. The most likely proposition is some combination of the upwelling of subsurface nutrients, land runoff (agricultural and sugar plantations, cattle ranches, golf courses, theme parks, septic systems, etc.) N2-fixation, drainage from phosphate mines and atmospheric deposition provides the necessary support for the blooms.
In addition to the breaking of the cells by waves, K. brevis cells can die because N-limitation directly affects the growth potential of blooms and the toxicity of K. brevis cells that comprise them. When N-limitation is present, intracellular brevetoxin concentrations (fg/μm) increased up to 2.5-fold in laboratory cultures, implying that during periods of N-limitation of algal growth, there is a higher chance of brevetoxin influx into the marine food web. The toxin content per cell increases when algal growth becomes P-limited. Various field measurements collected in the Gulf of Mexico have shown that the brevetoxin content of K. brevis cells is between 1 and 68 pg/cell; however, Hardison et al. discovered that during periods of transient P- and N-limitation, there is a 2- to 5-fold increase in brevetoxins per mole of cell carbon or unit of cell volume. Hardison concluded that this data suggest that the exposure of marine ecosystems to significantly different toxin levels depends on the nutrient status of the K. brevis cells. While brevetoxins remain intracellular during early stages of bloom development, the triggering of apoptosis and cell lysis with age release the toxins into the surrounding waters, implying that greater P-limitation that results in more cell death ultimately elevates brevetoxin levels. These high levels may persist in a food chain long after a bloom has subsided due to brevetoxin's high affinity for adsorbing to biological surfaces like sea grass fronds, and thereby accumulating in consuming organisms.
Overall, brevetoxins seem to increase under N- and P-limitation, however, the concentration of brevetoxins per cell under P-limitation has been reported to be roughly twice that under N-limitation. One major concern of this is that management of shellfish bed closures operating under the assumption that brevetoxin concentrations per cell do not vary may compromise public safety if a bloom became nutrient limited. | 0 | Organic Chemistry |
Gene targeting was developed in mammalian cells in the 1980s, with diverse applications possible as a result of being able to make specific sequence changes at a target genomic site, such as the study of gene function or human disease, particularly in mice models. Indeed, gene targeting has been widely used to study human genetic diseases by removing ("knocking out"), or adding ("knocking in"), specific mutations of interest. Previously used to engineer rat cell models, advances in gene targeting technologies enable a new wave of isogenic human disease models. These models are the most accurate in vitro models available to researchers and facilitate the development of personalized drugs and diagnostics, particularly in oncology. Gene targeting has also been investigated for gene therapy to correct disease-causing mutations. However the low efficiency of delivery of the gene-targeting machinery into cells has hindered this, with research conducted into viral vectors for gene targeting to try and address these challenges. | 1 | Biochemistry |
Selegiline is also delivered via a transdermal patch used as a treatment for major depressive disorder.
Administration of transdermal selegiline bypasses hepatic first pass metabolism. This avoids inhibition of gastrointestinal and hepatic MAO-A activity, which would result in an increase of food-borne tyramine in the blood and possible related adverse effects, while allowing for a sufficient amount of selegiline to reach the brain for an antidepressant effect.
A quantitative review published in 2015 found that for the pooled results of the pivotal trials, the number needed to treat (a sign of effect size, so a low number is better) for the patch for symptom reduction was 11, and for remission, was 9. The number needed to harm (inverse of the NNT, a high number here is better) ranged from 387 for sexual side effects to 7 for application site reaction. With regard to the likelihood to be helped or harmed (LHH), the analysis showed that the selegiline patch was 3.6 times as likely to lead to a remission vs. a discontinuation due to side effects; the LHH for remission vs. incidence of insomnia was 2.1; the LHH for remission vs. discontinuation due to insomnia was 32.7. The LHH for remission vs. insomnia and sexual dysfunction were both very low. | 4 | Stereochemistry |
In commercial or laboratory preparations, reagent-grade designates chemical substances meeting standards of purity that ensure the scientific precision and reliability of chemical analysis, chemical reactions or physical testing. Purity standards for reagents are set by organizations such as ASTM International or the American Chemical Society. For instance, reagent-quality water must have very low levels of impurities such as sodium and chloride ions, silica, and bacteria, as well as a very high electrical resistivity. Laboratory products which are less pure, but still useful and economical for undemanding work, may be designated as technical, practical, or crude grade to distinguish them from reagent versions. | 0 | Organic Chemistry |
A photosynthetic reaction center is a complex of several proteins, pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along the path of a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from HO or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy. | 5 | Photochemistry |
Silver-coated mirrors are optimum for use as steering mirrors for infrared pulses around 800 nm. Their reflectivity is higher than gold and much higher than aluminum at that wavelength. | 7 | Physical Chemistry |
In some places carbon dioxide bubbles out from the sea floor, locally changing the pH and other aspects of the chemistry of the seawater. Studies of these carbon dioxide seeps have documented a variety of responses by different organisms. Coral reef communities located near carbon dioxide seeps are of particular interest because of the sensitivity of some corals species to acidification. In Papua New Guinea, declining pH caused by carbon dioxide seeps is associated with declines in coral species diversity. However, in Palau carbon dioxide seeps are not associated with reduced species diversity of corals, although bioerosion of coral skeletons is much higher at low pH sites. | 9 | Geochemistry |
In dynamic reaction mixtures, multiple products exist in equilibrium. Reversible assembly of molecular components generates products and semi-stable intermediates. Reactions can proceed along kinetic or thermodynamic pathways. Initial concentrations of kinetic intermediates are greater than thermodynamic products because the lower barrier of activation (ΔG‡), compared to the thermodynamic pathway, gives a faster rate of formation. A kinetic pathway is represented in figure 1 as a purple energy diagram. With time, the intermediates equilibrate towards the global minimum, corresponding to the lowest overall Gibbs free energy (ΔG°), shown in red on the reaction diagram in figure 1. The driving force for products to re-equilibrate towards the most stable products is referred to as thermodynamic control. The ratio of products to at any equilibrium state is determined by the relative magnitudes of free energy of the products. This relationship between population and relative energies is called the Maxwell-Boltzmann distribution. | 6 | Supramolecular Chemistry |
Malonyl-CoA is a highly regulated molecule in fatty acid synthesis; as such, it inhibits the rate-limiting step in beta-oxidation of fatty acids. Malonyl-CoA inhibits fatty acids from associating with carnitine by regulating the enzyme carnitine acyltransferase, thereby preventing them from entering the mitochondria, where fatty acid oxidation and degradation occur. | 1 | Biochemistry |
In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold (usually by a crucible) that contains a negative impression (i.e., a three-dimensional negative image) of the intended shape. The metal is poured into the mold through a hollow channel called a sprue. The metal and mold are then cooled, and the metal part (the casting) is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.
Casting processes have been known for thousands of years, and have been widely used for sculpture (especially in bronze), jewelry in precious metals, and weapons and tools. Highly engineered castings are found in 90 percent of durable goods, including cars, trucks, aerospace, trains, mining and construction equipment, oil wells, appliances, pipes, hydrants, wind turbines, nuclear plants, medical devices, defense products, toys, and more.
Traditional techniques include lost-wax casting (which may be further divided into centrifugal casting, and vacuum assist direct pour casting), plaster mold casting and sand casting.
The modern casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure. | 8 | Metallurgy |
Parker is known for her work in the field of organic synthesis, where she works on the construction of natural products through methods that allow for efficient synthesis of known organic compounds. | 0 | Organic Chemistry |
Pioneer factors are transcription factors that can directly bind condensed chromatin. They can have positive and negative effects on transcription and are important in recruiting other transcription factors and histone modification enzymes as well as controlling DNA methylation. They were first discovered in 2002 as factors capable of binding to target sites on nucleosomal DNA in compacted chromatin and endowing competency for gene activity during hepatogenesis. Pioneer factors are involved in initiating cell differentiation and activation of cell-specific genes. This property is observed in histone fold-domain containing transcription factors (fork head box (FOX) and NF-Y) and other transcription factors that use zinc finger(s) for DNA binding (Groucho TLE, Gal4, and GATA).
The eukaryotic cell condenses its genome into tightly packed chromatin and nucleosomes. This ability saves space in the nucleus for only actively transcribed genes and hides unnecessary or detrimental genes from being transcribed. Access to these condensed regions is done by chromatin remodelling by either balancing histone modifications or directly with pioneer factors that can loosen the chromatin themselves or as a flag recruiting other factors. Pioneer factors are not necessarily required for assembly of the transcription apparatus and may dissociate after being replaced by other factors. | 1 | Biochemistry |
Dehydration of metal chlorides with trimethylsilyl chloride in THF gives the solvate as illustrated by the case of chromium trichloride: | 0 | Organic Chemistry |
Plantations typically consisted of a nearly self-sufficient community, including the head iron master, workers and their families, and other shopkeepers, blacksmiths, and agricultural workers needed to sustain mining and smelting operations as well as life on the plantation. Plantations were foremost land-intensive operations, commonly comprising thousands of acres. The grounds were typically defined by a conspicuous mansion, belonging to the iron master, which looked out on the charcoal furnace or iron forge from atop a geographically higher location.
The iron master was also in charge of hiring skilled labor and investing capital in construction and maintenance of charcoal furnaces and forges for the refining and working of iron. Workers on the plantation were often not paid directly in wages. Rather, the master tallied an employee’s earnings on a balance sheet, which he then offset with purchases of merchandise from the community’s stores. While an iron master lived a rather luxurious life with the opportunity to afford travel, tutors for his children, and expensive home furnishings, workers had few material possessions of their own. Workers were not well traveled outside of the plantations, and little news outside of the confines of the plantation concerned their daily lives. Notably, however, poverty was not well known on the plantations, even in times of economic depression, and workers’ wages in the United States greatly surpassed comparable wages in the European iron industry.
Work forces on iron plantations consisted of a wide array of labor and included indentured servants, slaves, and free laborers. Indentured servants composed the largest group. Indentured servants and slaves typically performed the least skilled tasks on plantations, serving as woodcutters to supply the charcoal furnaces or as miners to dig iron ore. Few opportunities were afforded to laborers for upwards mobility on plantations.
More efficient fossil fuels eventually substituted for wood-based charcoal, and “the semi-feudal iron plantation was replaced by the urban establishment and the company town” typically possessing a coke furnace.
The lack of nearby ore deposits additionally limited many plantations from being able to economically transport large quantities of ore over long distances to be smelted on the plantations themselves. Wagon transport of bar and pig iron to cities further added to costs and could run as high as forty percent of the market price per ton of pig iron in 1728, according to John Potts, a member of an iron plantation in Pennsylvania.
Iron plantations in Alabama, Tennessee, Georgia, Michigan, Wisconsin, and Missouri in particular better survived the evolving technological landscape by adopting practices that increased charcoal energy efficiency, that is, the amount of charcoal consumed per ton of iron smelted. One such technique was to raise the heights of furnaces to create a longer and more uniform reaction chamber to produce more homogeneous pig iron. Whereas antebellum furnaces were built with brick and mortar and reached only 30-35 feet in height, new furnaces remodeled in the 1840s reached as high as 65 feet. Continued demand for pig iron to be transported westward provided an additional competitive advantage to plantations in these states.
The iron industry shifted to one largely determined by the production of steel during the British Industrial Revolution and in the later half of the 19th and early 20th centuries. As such, blast furnaces, steam and electric power, and coke fuel replaced the largely land- and labor-intensive practices of iron making on plantations dependent on large tracts of land to produce charcoal and additional labor to sustain both the iron making operations and the community at large. Though iron produced on plantations remained practically useful for Westward Expansion, the eastern United States and Europe increasingly demanded more pliable and resistant steel for use in buildings, ships, engines, and railroads. Though demand still remained for pig iron as an ingredient in steel production, most iron plantations were no longer economically competitive with coke-powered smelters which located themselves increasingly closer to the major cities requiring their products. | 8 | Metallurgy |
Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) is a protein that in humans is encoded by the PPARGC1A gene. PPARGC1A is also known as human accelerated region 20 (HAR20). It may, therefore, have played a key role in differentiating humans from apes.
PGC-1α is the master regulator of mitochondrial biogenesis. PGC-1α is also the primary regulator of liver gluconeogenesis, inducing increased gene expression for gluconeogenesis. | 1 | Biochemistry |
Water meniscus forces are highly interesting for AFM measurements in air. Due to the ambient humidity, a thin layer of water is formed between the tip and the sample during air measurements. The resulting capillary force gives rise to a strong attractive force that pulls the tip onto the surface. In fact, the adhesion force measured between tip and sample in ambient air of finite humidity is usually dominated by capillary forces. As a consequence, it is difficult to pull the tip away from the surface. For soft samples including many polymers and in particular biological materials, the strong adhesive capillary force gives rise to sample degradation and destruction upon imaging in contact mode. Historically, these problems were an important motivation for the development of dynamic imaging in air (e.g. "tapping mode"). During tapping mode imaging in air, capillary bridges still form. Yet, for suitable imaging conditions, the capillary bridges are formed and broken in every oscillation cycle of the cantilever normal to the surface, as can be inferred from an analysis of cantilever amplitude and phase vs. distance curves. As a consequence, destructive shear forces are largely reduced and soft samples can be investigated.
In order to quantify the equilibrium capillary force, it is necessary to start from the Laplace equation for pressure:
where γ, is the surface energy and r and r are defined in the figure.
The pressure is applied on an area of
where θ is the angle between the tips surface and the liquids surface while h is the height difference between the surrounding liquid and the top of the miniscus.
The force that pulls together the two surfaces is
The same formula could also be calculated as a function of relative humidity.
Gao calculated formulas for different tip geometries. As an example, the force decreases by 20% for a conical tip with respect to a spherical tip.
When these forces are calculated, a difference must be made between the wet on dry situation and the wet on wet situation.
For a spherical tip, the force is:
: for dry on wet,
:for wet on wet,
where θ is the contact angle of the dry sphere and φ is the immersed angle, as shown in the figure
For a conical tip, the formula becomes:
: for dry on wet
: for wet on wet
where δ is the half cone angle and r and h are parameters of the meniscus profile. | 6 | Supramolecular Chemistry |
The cristae greatly increase the surface area of the inner membrane on which the above-mentioned reactions may take place. A widely accepted hypothesis for the function of the cristae is that the high surface area allows an increased capacity for ATP generation. However, the current model is that active ATP synthase complexes localize preferentially in dimers to the narrow edges of the cristae. Thus, the surface area of mitochondrial membranes allocated to ATP syntheses is actually quite modest.
Mathematical modelling suggested that the optical properties of the cristae in filamentous mitochondria may affect the generation and propagation of light within the tissue. | 1 | Biochemistry |
The cementation process is an obsolete technology for making steel by carburization of iron. Unlike modern steelmaking, it increased the amount of carbon in the iron. It was apparently developed before the 17th century. Derwentcote Steel Furnace, built in 1720, is the earliest surviving example of a cementation furnace. Another example in the UK is the cementation furnace in Doncaster Street, Sheffield. | 8 | Metallurgy |
Growing crystals for X-ray crystallography can be quite difficult. For X-ray analysis, single perfect crystals are required. Typically a small amount (5–100 mg) of a pure compound is used, and crystals are allowed to grow very slowly. Several techniques can be used to grow these perfect crystals:
* Slow evaporation of a single solvent - typically the compound is dissolved in a suitable solvent and the solvent is allowed to slowly evaporate. Once the solution is saturated crystals can be formed.
* Slow evaporation of a multi-solvent system - the same as above, however as the solvent composition changes due to evaporation of the more volatile solvent. The compound is more soluble in the volatile solvent, and so the compound becomes increasingly insoluble in solution and crystallizes.
* Slow diffusion - similar to the above. However, a second solvent is allowed to evaporate from one container into a container holding the compound solution (gas diffusion). As the solvent composition changes due to an increase in the solvent that has gas diffused into the solution, the compound becomes increasingly insoluble in the solution and crystallizes.
* Interface/slow mixing (often performed in an NMR tube). Similar to the above, but instead of one solvent gas-diffusing into another, the two solvents mix (diffuse) by liquid-liquid diffusion. Typically a second solvent is "layered" carefully on top of the solution containing the compound. Over time the two solution mix. As the solvent composition changes due to diffusion, the compound becomes increasingly insoluble in solution and crystallizes, usually at the interface. Additionally, it is better to use a denser solvent as the lower layer, and/or a hotter solvent as the upper layer because this results in the slower mixing of the solvents.
* Specialized equipment can be used in the shape of an "H" to perform the above, where one of the vertical lines of the "H" is a tube containing a solution of the compound, and the other vertical line of the "H" is a tube containing a solvent which the compound is not soluble in, and the horizontal line of the "H" is a tube which joins the two vertical tubes, which also has a fine glass sinter that restricts the mixing of the two solvents.
* Once single perfect crystals have been obtained, it is recommended that the crystals are kept in a sealed vessel with some of the liquid of crystallization to prevent the crystal from drying out. Single perfect crystals may contain solvent of crystallization in the crystal lattice. Loss of this internal solvent from the crystals can result in the crystal lattice breaking down, and the crystals turning to powder. | 3 | Analytical Chemistry |
Any of the image correlation spectroscopy methods can also be performed on a spinning disk confocal microscope, which in practice can obtain faster imaging speeds compared to a laser scanning confocal microscope. This approach has recently been applied to diffusion in a spatially varying complex environment, producing a pixel resolution map of a diffusion coefficient. The spatial mapping of diffusion with FCS has subsequently been extended to the TIRF system. Spatial mapping of dynamics using correlation techniques had been applied before, but only at sparse points or at coarse resolution. | 7 | Physical Chemistry |
A mutation that produces small (petite" > petite) anaerobic-like colonies had shown first in Yeast Saccharomyces cerevisiae and described by Boris Ephrussi and his co-workers in 1949 in Gif-sur-Yvette, France. The cells of petite colonies were smaller than those of wild-type colonies, but the term “petite” refers only to colony size and not the individual cell size. | 1 | Biochemistry |
Double bond isomers are always considered diastereomers, not enantiomers. Diastereomerism can also occur at a double bond, where the cis vs trans relative positions of substituents give two non-superposable isomers. Many conformational isomers are diastereomers as well.
In the case of diastereomerism occurring at a double bond, E-Z, or entgegen and zusammen (German), is used in notating nomenclature of alkenes. | 4 | Stereochemistry |
Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. | 0 | Organic Chemistry |
Methyl fluoroacetate is produced and used as a chemical reagent and it can be released to the environment through several waste streams. When it was used as a rodenticide, it was released directly to the environment where it would be broken down in the air. If released to air, an estimated vapor pressure of 31 mmHg at 25 °C indicates methyl fluoroacetate will exist solely as a vapor in the atmosphere. Vapor-phase methyl fluoroacetate will be degraded in the atmosphere by reaction with photochemically produced hydroxyl radicals. The half-life for this reaction in air is estimated to be 98 days.
MFA does not contain chromophores that absorb at wavelengths > 290 nm and therefore it's not expected to be susceptible to direct photolysis by sunlight. | 1 | Biochemistry |
Hydrozirconation is a form of hydrometalation. Substrates for hydrozirconation are alkenes and alkynes. With terminal alkynes the terminal vinyl zirconium product is predominantly formed. Secondary reactions are nucleophilic additions, transmetalations, conjugate additions, coupling reactions, carbonylation and halogenation.
Computational studies indicate that hydrozirconation occurs from the interior portion.
When treated with one equivalent of CpZrClH, diphenylacetylene gives the corresponding alkenylzirconium as a mixture of cis and trans isomers. With two equivalents of hydride, the endproduct was a mixture of erythro and threo zircono alkanes:
In 1974 Hart and Schwartz reported that the organozirconium intermediates react with electrophiles such as hydrochloric acid, bromine and acid chlorides to give the corresponding alkane, bromoalkanes, and ketones:
The corresponding organoboron and organoaluminum compounds were already known, but these are air-sensitive and/or pyrophoric whereas organozirconium compounds are not. | 0 | Organic Chemistry |
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.
The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food. This stage includes the mechanical breakdown of food by chewing, and the chemical breakdown by digestive enzymes, that takes place in the mouth. Saliva contains the digestive enzymes amylase, and lingual lipase, secreted by the salivary and serous glands on the tongue. Chewing, in which the food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which is swallowed down the esophagus to enter the stomach.
The second stage, the gastric phase, happens in the stomach. Here, the food is further broken down by mixing with gastric acid until it passes into the duodenum, the first part of the small intestine.
The third stage, the intestinal phase, begins in the duodenum. Here, the partially digested food is mixed with a number of enzymes produced by the pancreas.
Digestion is helped by the chewing of food carried out by the muscles of mastication, the tongue, and the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
Peristalsis is the rhythmic contraction of muscles that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of chyme which when fully broken down in the small intestine is absorbed as chyle into the lymphatic system. Most of the digestion of food takes place in the small intestine. Water and some minerals are reabsorbed back into the blood in the colon of the large intestine. The waste products of digestion (feces) are defecated from the rectum via the anus. | 1 | Biochemistry |
Image formation is a plotting method that produces a color mapping through changing the x–y position of the tip while scanning and recording the measured variable, i.e. the intensity of control signal, to each x–y coordinate. The color mapping shows the measured value corresponding to each coordinate. The image expresses the intensity of a value as a hue. Usually, the correspondence between the intensity of a value and a hue is shown as a color scale in the explanatory notes accompanying the image.
Operation mode of image forming of the AFM are generally classified into two groups from the viewpoint whether it uses z-Feedback loop (not shown) to maintain the tip-sample distance to keep signal intensity exported by the detector. The first one (using z-Feedback loop), said to be "constant XX mode" (XX is something which kept by z-Feedback loop).
Topographic image formation mode is based on abovementioned "constant XX mode", z-Feedback loop controls the relative distance between the probe and the sample through outputting control signals to keep constant one of frequency, vibration and phase which typically corresponds to the motion of cantilever (for instance, voltage is applied to the Z-piezoelectric element and it moves the sample up and down towards the Z direction. | 6 | Supramolecular Chemistry |
All chemical elements except for hydrogen and helium derive from stellar nucleosynthesis. The basic chemical ingredients of life – the carbon-hydrogen molecule (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – were produced by ultraviolet light from stars. Complex molecules, including organic molecules, form naturally both in space and on planets. Organic molecules on the early Earth could have had either terrestrial origins, with organic molecule synthesis driven by impact shocks or by other energy sources, such as ultraviolet light, redox coupling, or electrical discharges; or extraterrestrial origins (pseudo-panspermia), with organic molecules formed in interstellar dust clouds raining down on to the planet. | 9 | Geochemistry |
MALDI-TOF spectra have been used for the detection and identification of various parasites such as trypanosomatids, Leishmania and Plasmodium. In addition to these unicellular parasites, MALDI/TOF can be used for the identification of parasitic insects such as lice or cercariae, the free-swimming stage of trematodes. | 1 | Biochemistry |
RNA polymerase then starts to synthesize the initial DNA-RNA heteroduplex, with ribonucleotides base-paired to the template DNA strand according to Watson-Crick base-pairing interactions. As noted above, RNA polymerase makes contacts with the promoter region. However these stabilizing contacts inhibit the enzyme's ability to access DNA further downstream and thus the synthesis of the full-length product. In order to continue RNA synthesis, RNA polymerase must escape the promoter. It must maintain promoter contacts while unwinding more downstream DNA for synthesis, "scrunching" more downstream DNA into the initiation complex. During the promoter escape transition, RNA polymerase is considered a "stressed intermediate." Thermodynamically the stress accumulates from the DNA-unwinding and DNA-compaction activities. Once the DNA-RNA heteroduplex is long enough (~10 bp), RNA polymerase releases its upstream contacts and effectively achieves the promoter escape transition into the elongation phase. The heteroduplex at the active center stabilizes the elongation complex.
However, promoter escape is not the only outcome. RNA polymerase can also relieve the stress by releasing its downstream contacts, arresting transcription. The paused transcribing complex has two options: (1) release the nascent transcript and begin anew at the promoter or (2) reestablish a new 3′-OH on the nascent transcript at the active site via RNA polymerase's catalytic activity and recommence DNA scrunching to achieve promoter escape. Abortive initiation, the unproductive cycling of RNA polymerase before the promoter escape transition, results in short RNA fragments of around 9 bp in a process known as abortive transcription. The extent of abortive initiation depends on the presence of transcription factors and the strength of the promoter contacts. | 1 | Biochemistry |
The spontaneous assembly of a single layer of molecules at interfaces is usually referred to as two-dimensional self-assembly. One of the common examples of such assemblies are Langmuir-Blodgett monolayers and multilayers of surfactants. Non-surface active molecules can assemble into ordered structures as well. Early direct proofs showing that non-surface active molecules can assemble into higher-order architectures at solid interfaces came with the development of scanning tunneling microscopy and shortly thereafter. Eventually two strategies became popular for the self-assembly of 2D architectures, namely self-assembly following ultra-high-vacuum deposition and annealing and self-assembly at the solid-liquid interface. The design of molecules and conditions leading to the formation of highly-crystalline architectures is considered today a form of 2D crystal engineering at the nanoscopic scale. | 6 | Supramolecular Chemistry |
In the early years, people from the surrounding community used the mill tailings as fill dirt in various properties, such as their yards and gardens. A woman who played in such a yard as a child later sued Kerr-McGee over her Hodgkin's disease and settled out of court in 1988.
Radioactive waste from the plant was put in a local landfill that later became a public park called Reed-Keppler Park.
Kress Creek and West Branch Dupage River (including sediments, banks, and floodplains) were contaminated by years of rainwater runoff from REF going into a storm sewer and then into the creek. The floodplain includes people's yards.
The West Chicago Sewage Treatment Plant was contaminated when mill tailings from REF were used as fill dirt there. This also resulted in pollution of the West Branch Dupage River from runoff and erosion.
In 1991, the Illinois Department of Public Health found elevated cancer rates in the community. | 8 | Metallurgy |
The reactions best for temperature (geothermometers) are ones that have a large enthalpy of reaction, which means they release or consume a lot of heat. Higher temperatures allow the reaction to consume that heat while lower temperatures cause the reaction to release heat. Similarly to geobarometers, the proportion of minerals that are formed by releasing heat versus consuming heat can be used to calculate the temperature, as long as the reaction is calibrated. | 9 | Geochemistry |
When heated, n-BuLi, analogously to other alkyllithium reagents with "β-hydrogens", undergoes β-hydride elimination to produce 1-butene and lithium hydride (LiH):
: CHLi → LiH + CHCHCH=CH | 0 | Organic Chemistry |
Static light scattering is a technique in physical chemistry that measures the intensity of the scattered light to obtain the average molecular weight M of a macromolecule like a polymer or a protein in solution. Measurement of the scattering intensity at many angles allows calculation of the root mean square radius, also called the radius of gyration R. By measuring the scattering intensity for many samples of various concentrations, the second virial coefficient, A, can be calculated.
Static light scattering is also commonly utilized to determine the size of particle suspensions in the sub-μm and supra-μm ranges, via the Lorenz-Mie (see Mie scattering) and Fraunhofer diffraction formalisms, respectively.
For static light scattering experiments, a high-intensity monochromatic light, usually a laser, is launched into a solution containing the macromolecules. One or many detectors are used to measure the scattering intensity at one or many angles. The angular dependence is required to obtain accurate measurements of both molar mass and size for all macromolecules of radius above 1–2% of the incident wavelength. Hence simultaneous measurements at several angles relative to the direction of the incident light, known as multi-angle light scattering (MALS) or multi-angle laser light scattering (MALLS), are generally regarded as the standard implementation of static light scattering. Additional details on the history and theory of MALS may be found in multi-angle light scattering.
To measure the average molecular weight directly without calibration from the light scattering intensity, the laser intensity, the quantum efficiency of the detector, and the full scattering volume and solid angle of the detector need to be known. Since this is impractical, all commercial instruments are calibrated using a strong, known scatterer like toluene since the Rayleigh ratio of toluene and a few other solvents were measured using an absolute light scattering instrument. | 7 | Physical Chemistry |
Mutations in TCF4 cause Pitt-Hopkins Syndrome (PTHS). These mutations cause TCF4 proteins to not bind to DNA properly and control the differentiation of the nervous system. It has been suggested that TCF4 loss-of-function leads to decreased Wnt signaling and, consequently, a reduced neural progenitor proliferation. In most cases that have been studied, the mutations were de novo, meaning it was a new mutation not found in other family members of the patient. Common symptoms of Pitt-Hopkins Syndrome include a wide mouth, gastrointestinal problems, developmental delay of fine motor skills, speech and breathing problems, epilepsy, and other brain defects. | 1 | Biochemistry |
Cytochrome c belongs to class I of the c-type cytochrome family and contains a characteristic CXXCH (cysteine-any-any-cysteine-histidine) amino acid motif that binds heme. This motif is located towards the N-terminus of the peptide chain and contains a histidine as the 5th ligand of the heme iron. The 6th ligand is provided by a methionine residue found towards the C-terminus. The protein backbone is folded into five α-helices that are numbered α1-α5 from N-terminus to C-terminus. Helices α3, α4 and α5 are referred to as 50s, 60s and 70s helices, respectively, when referring to mitochondrial cytochrome c. | 1 | Biochemistry |
Hammond's postulate is useful for understanding the relationship between the rate of a reaction and the stability of the products.
While the rate of a reaction depends just on the activation energy (often represented in organic chemistry as ΔG “delta G double dagger”), the final ratios of products in chemical equilibrium depends only on the standard free-energy change ΔG (“delta G”). The ratio of the final products at equilibrium corresponds directly with the stability of those products.
Hammond's postulate connects the rate of a reaction process with the structural features of those states that form part of it, by saying that the molecular reorganizations have to be small in those steps that involve two states that are very close in energy. This gave birth to the structural comparison between the starting materials, products, and the possible "stable intermediates" that led to the understanding that the most stable product is not always the one that is favored in a reaction process. | 7 | Physical Chemistry |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.