text
stringlengths 105
4.44k
| label
int64 0
9
| label_text
stringclasses 10
values |
|---|---|---|
Megaphone is a cytotoxic neolignan obtained from Aniba megaphylla, a flowering plant of Laurel family which gave the compound its name. Megaphone has also been prepared synthetically.
Studies carried out in the 1960s demonstrated that an alcoholic extract of the ground root of Aniba megaphylla inhibited, in vitro, growth of cells derived from human carcinoma of the nasopharynx. In 1978, the active components of the extract were isolated using silica gel chromatography, characterized and named as megaphone (CHO, solid), megaphone acetate (CHO, oily liquid) and megaphyllone acetate (CHO, oily liquid). For comparison, megaphone acetate was also produced synthetically by reacting megaphone with acetic anhydride at 50 °C for 6 hours. Stirring an alcoholic solution of megaphone (megaphone acetate), with added palladium catalyst, in hydrogen atmosphere, followed by evaporation of the solvent yields tetrahydromegaphone (tetrahydromegaphone acetate) as an oil. Millimeter-sized crystals of megaphone can be grown from an ether-chloroform solution. They have monoclinic symmetry with space group P2, lattice constants a = 0.8757 nm, b = 1.1942 nm and c = 1.0177 nm and two formula units per unit cell. Megaphone and megaphone acetate molecules are chiral and the reported extraction and synthesis procedures yielded their racemic mixtures. Megaphone acetate was also isolated from the root of Endlicheria dysodantha, another plant of Laurel family, using chromatography of ethanolic solution. It showed inhibitory activity against cells of crown gall tumor and human lung, breast and colon carcinomas. | 0 | Organic Chemistry |
Noric steel was a steel from Noricum, a kingdom located in modern Austria and Slovenia.
The proverbial hardness of Noric steel is expressed by Ovid: "...durior [...] ferro quod noricus excoquit ignis..." which roughly translates to "...harder than iron which Noric fire tempers [was Anaxarete towards the advances of Iphis]..." and it was widely used for the weapons of the Roman military after Noricum joined the Empire in 16 BC.
The iron ore was quarried at two mountains in modern Austria still called Erzberg "ore mountain" today, one at Hüttenberg, Carinthia and the other at Eisenerz, Styria, separated by . The latter is the site of the modern Erzberg mine.
Buchwald identifies a sword of found in Krenovica, Moravia as an early example of Noric steel due to a chemical composition consistent with Erzberg ore. A more recent sword, dating to and found in Zemplin, eastern Slovakia, is of extraordinary length for the period (95 cm, 37 in) and carries a stamped Latin inscription (?V?TILICI?O), identified as a "fine sword of Noric steel" by Buchwald. A center of manufacture was at Magdalensberg. | 8 | Metallurgy |
Also in 2006 the group of Masakatsu Shibasaki of the University of Tokyo published a synthesis again bypassing shikimic acid.
An improved method published in 2007 starts with the enantioselective desymmetrization of aziridine 1 with trimethylsilyl azide (TMSN) and a chiral catalyst to the azide 2. The amide group is protected as a BOC group with Boc anhydride and DMAP in 3 and iodolactamization with iodine and potassium carbonate first gives the unstable intermediate 4 and then stable cyclic carbamate 5 after elimination of hydrogen iodide with DBU.
The amide group is reprotected as BOC 6 and the azide group converted to the amide 7 by reductive acylation with thioacetic acid and 2,6-lutidine. Caesium carbonate accomplishes the hydrolysis of the carbamate group to the alcohol 8 which is subsequently oxidized to ketone 9 with Dess-Martin periodinane. Cyanophosphorylation with diethyl phosphorocyanidate (DEPC) modifies the ketone group to the cyanophosphate 10 paving the way for an intramolecular allylic rearrangement to unstable β-allyl phosphate 11 (toluene, sealed tube) which is hydrolyzed to alcohol 12 with ammonium chloride. This hydroxyl group has the wrong stereochemistry and is therefore inverted in a Mitsunobu reaction with p-nitrobenzoic acid followed by hydrolysis of the p-nitrobenzoate to 13.
A second Mitsunobu reaction then forms the aziridine 14 available for ring-opening reaction with 3-pentanol catalyzed by boron trifluoride to ether 15. In the final step the BOC group is removed (HCl) and phosphoric acid added to objective 16. | 0 | Organic Chemistry |
The Chichibabin pyridine synthesis was reported in 1924 and the basic approach underpins several industrial routes. In its general form, the reaction involves the condensation reaction of aldehydes, ketones, α,β-unsaturated carbonyl compounds, or any combination of the above, in ammonia or ammonia derivatives. Application of the Chichibabin pyridine synthesis suffer from low yields, often about 30%, however the precursors are inexpensive. In particular, unsubstituted pyridine is produced from formaldehyde and acetaldehyde. First, acrolein is formed in a Knoevenagel condensation from the acetaldehyde and formaldehyde. The acrolein then condenses with acetaldehyde and ammonia to give dihydropyridine, which is oxidized to pyridine. This process is carried out in a gas phase at 400–450 °C. Typical catalysts are modified forms of alumina and silica. The reaction has been tailored to produce various methylpyridines. | 0 | Organic Chemistry |
The interaction of the tip with the sample modifies the amplitudes, phase shifts and frequency resonances of the excited modes. Those changes are detected and processed by the feedback of the instrument. Several features make bimodal AFM a very powerful surface characterization method at the nanoscale. (i) Resolution. Atomic, molecular or nanoscale spatial resolution was demonstrated. (ii) Simultaneity. Maps of different properties are generated at the same time. (iii) Efficiency. A maximum number of four data points per pixel are needed to generate material property maps. (iv) Speed. Analytical solutions link observables with material properties. | 6 | Supramolecular Chemistry |
Henry's law describes the tendency of a compound to join air in the vapor phase or dissolve in water. The Henry’s Law constant, sometimes called coefficient, is specific to each compound and depends on the system temperature. The constant is used to predict the amount of contaminant what will remain in the vapor phase (or transfer to the liquid phase), upon exiting the condenser. | 2 | Environmental Chemistry |
Live copepods are used in the saltwater aquarium hobby as a food source and are generally considered beneficial in most reef tanks. They are scavengers and also may feed on algae, including coralline algae. Live copepods are popular among hobbyists who are attempting to keep particularly difficult species such as the mandarin dragonet or scooter blenny. They are also popular to hobbyists who want to breed marine species in captivity. In a saltwater aquarium, copepods are typically stocked in the refugium. | 2 | Environmental Chemistry |
Biotin PEG2 amine is used as a linker or cross linker. This allows to attach specific compounds to proteins or to antibodies.
A common use of biotin PEG2 amine is to use EDC and crosslink the amine in the biotin PEG2 amine to carboxyl groups on protein residues that are either aspartate or glutamate or the carboxy-terminus of proteins.
A particular example of use is labeling red cells, which in turn allows for the detection of these labeled cells in small samples using flow cytometry. | 1 | Biochemistry |
The NCEC was formed in 1973 as a government agency. On 1 March 1979 the Centre launched, in cooperation with the Home Office, its Hazfile computer database, made available to fifteen British fire services, listing over 10,000 chemical compounds; this was later replaced by the Chemdata system. A similar system in the USA is called RTECS (Registry of Toxic Effects of Chemical Substances). | 2 | Environmental Chemistry |
A crystal system is a set of point groups in which the point groups themselves and their corresponding space groups are assigned to a lattice system. Of the 32 crystallographic point groups that exist in three dimensions, most are assigned to only one lattice system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two lattice systems, rhombohedral and hexagonal, because both exhibit threefold rotational symmetry. These point groups are assigned to the trigonal crystal system. | 3 | Analytical Chemistry |
In 1982, Koiti Titanis lab identified an "N-terminal blocking group" on the catalytic subunit of cyclic AMP-dependent protein kinase in cows as n-tetradecanoyl. Almost simultaneously in Claude B. Klees lab, this same N-terminal blocking group was further characterized as myristic acid. Both labs made this discovery utilizing similar techniques: mass spectrometry and gas chromatography. | 1 | Biochemistry |
The Ribose repressor (RbsR) is a bacterial DNA-binding transcription repressor protein and a member of the LacI/GalR protein family. In Escherichia coli, RbsR is responsible for regulation of genes involved in D-ribose metabolism. In Bacillus subtilis, RbsR was shown to interact with Histidine-containing protein (HPr), an allosteric effector of the related LacI/GalR protein Catabolite Control Protein A (CcpA). | 1 | Biochemistry |
Detonation spray coatings are applied using a detonation gun (D-gun) which is composed of a long-water-cooled metal barrel containing inlet valves for introducing gases and powders into the chamber. A preselected amount of the desired protective coating material known as feedstock (in powder form of particle size 5–60μm) is introduced into the chamber (at common powder flow rates of 16–40 g/min). Here oxygen and fuel (generally acetylene) are ignited by a spark plug to create a supersonic shock wave that propels the mixture of melted and/or partially-melted and/or solid feedstock (depending on the type of material used) out of the barrel and onto the subject being sprayed. The barrel is then cleared using a short burst of nitrogen before the D-gun is ready to be fired again. This is an important step because the heat from the residual gases can cause the new fuel mixture to combust which would in turn cause an uncontrollable reaction. Also a small amount of inert nitrogen gas inserted between the two mixtures of fuel and feedstock prior to firing, helps to prevent backfiring. D-guns typically operate at firing rates of between 1–10 Hz. Many different mixtures of coating powders and D-gun settings can be used during detonation gun spraying of a material, all of which influence the final surface characteristics of the sprayed coating. Common powder materials used include but are not limited to: alumina-titania, alumina, tungsten carbide-tungsten-chromium carbide mixture with nickel-chromium alloy binder, chromium carbide, tungsten carbide with cobalt binder.
Metallurgists consider the measurements of surface oxygen content, macro and micro-hardness, porosity, bond strength and surface roughness when determining the quality of a thermally sprayed coating. | 8 | Metallurgy |
Diffraction topography with neutron radiation has been in use for several decades, mainly at research reactors with high neutron beam intensities. Neutron topography can make use of contrast mechanisms that are partially different from the X-ray case, and thus serve e.g. to visualize magnetic structures. However, due to the comparatively low neutron intensities, neutron topography requires long exposure times. Its use is therefore rather limited in practice.
Literature: | 3 | Analytical Chemistry |
Emission microspectroscopy is a sensitive technique with excitation and emission ranging from the ultraviolet, visible and NIR regions. As such, it has numerous biomedical, biotechnological and agricultural applications. There are several powerful, highly specific and sensitive fluorescence techniques that are currently in use, or still being developed; among the former are FLIM, FRAP, FRET and FLIM-FRET; among the latter are NIR fluorescence and probe-sensitivity enhanced NIR fluorescence microspectroscopy and nanospectroscopy techniques (see "Further reading" section). Fluorescence emission microspectroscopy and imaging are also commonly used to locate protein crystals in solution, for the characterization of metamaterials and biotechnology devices. | 7 | Physical Chemistry |
Phred uses a four-phase procedure as outlined by Ewing et al. to determine a sequence of base calls from the processed DNA sequence tracing:
# Predicted peak locations are determined, based on the assumption that fragments are relatively evenly spaced, on average, in most regions of the gel, to determine the correct number of bases and their idealized evenly spaced locations in regions where the peaks are not well resolved, noisy, or displaced (as in compressions)
# Observed peaks are identified in the trace
# Observed peaks are matched to the predicted peak locations, omitting some peaks and splitting others; as each observed peak comes from a specific array and is thus associated with 1 of the 4 bases (A, G, T, or C), the ordered list of matched observed peaks determines a base sequence for the trace.
# The unmatched observed peaks are checked for any peak that appears to represent a base but could not be assigned to a predicted peak in the third phase and if found, the corresponding base is inserted into the read sequence.
The entire procedure is rapid, usually taking less than half a second per trace. The results can be output as a PHD file, which contains base data as triples consisting of the base call, quality, and position. | 1 | Biochemistry |
The DMACA is any of a number of acidified DMACA solutions:
* 0.117 g of DMACA, 39 mL of ethanol, 5 mL of conc hydrochloric acid and diluted to 50mL with water
* 1 g DMACA, 1 mL conc. hydrochloric acid and 99 mL water
* 1 g DMACA in 99 mL conc. hydrochloric acid.
It is primarily used as a histological dye used to detect indoles, particularly for production in cells. It is used for the rapid identification of bacteria containing tryptophanase enzyme systems. It is also particularly useful for localization of proanthocyanidins compounds in plants, resulting in a blue staining. It has been used for grapevine fruit or for legumes foliage histology.
A colorimetric assay based upon the reaction of A-rings with the chromogen. p-Dimethylaminocinnamaldehyde has been developed for flavanoids in beer that can be compared with the vanillin procedure. The DMACA reagent may be superior to the vanillin procedure for the detection of catechins.
The DMACA reagent changes color over several days when exposed to air but when refrigerated can be stored for up to two weeks.
The DMACA reagent may also be referred to as the Renz and Loew reagent. | 3 | Analytical Chemistry |
Another example of a demethylase is protein-glutamate methylesterase, also known as CheB protein (EC 3.1.1.61), which demethylates MCPs (methyl-accepting chemotaxis proteins) through hydrolysis of carboxylic ester bonds. The association of a chemotaxis receptor with an agonist leads to the phosphorylation of CheB. Phosphorylation of CheB protein enhances its catalytic MCP demethylating activity resulting in adaption of the cell to environmental stimuli. MCPs respond to extracellular attractants and repellents in bacteria like E. coli in chemotaxis regulation. CheB is more specifically termed a methylesterase, as it removes methyl groups from methylglutamate residues located on the MCPs through hydrolysis, producing glutamate accompanied by the release of methanol.
CheB is of particular interest to researchers as it may be a therapeutic target for mitigating the spread of bacterial infections. | 1 | Biochemistry |
Curvibacter sp. is a gram-negative curved rod-formed bacterium which is the main colonizer of the epithelial cell surfaces of the early branching metazoan Hydra vulgaris. Sequencing the complete genome uncovered a circular chromosome (4.37 Mb), a plasmid (16.5 kb), and two operons coding each for an AHL (N-acyl-homoserine lactone) synthase (curI1 and curI2) and an AHL receptor (curR1 and curR2). Moreover, a study showed that these host associated Curvibacter bacteria produce a broad spectrum of AHL, explaining the presence of those operons. As mentioned before, AHL are the quorum sensing molecules of gram-negative bacteria, which means Curvibacter has a quorum sensing activity.
Even though their function in host-microbe interaction is largely unknown, Curvibacter quorum-sensing signals are relevant for host-microbe interactions. Indeed, due to the oxidoreductase activity of Hydra, there is a modification of AHL signalling molecules (3-oxo-homoserine lactone into 3-hydroxy-homoserine lactone) which leads to a different host-microbe interaction. On one hand, a phenotypic switch of the colonizer Curvibacter takes place. The most likely explanation is that the binding of 3-oxo-HSL and 3-hydroxy-HSL causes different conformational changes in the AHL receptors curR1 and curR2. As a result, there is a different DNA-binding motif affinity and thereby different target genes are activated. On the other hand, this switch modifies its ability to colonize the epithelial cell surfaces of Hydra vulgaris. Indeed, one explanation is that with a 3-oxo-HSL quorum-sensing signal, there is an up-regulation of flagellar assembly. Yet, flagellin, the main protein component of flagella, can act as an immunomodulator and activate the innate immune response in Hydra. Therefore, bacteria have less chance to evade the immune system and to colonize host tissues. Another explanation is that 3-hydroxy-HSL induces carbon metabolism and fatty acid degradation genes in Hydra. This allows the bacterial metabolism to adjust itself to the host growth conditions, which is essential for the colonization of the ectodermal mucus layer of Hydrae. | 1 | Biochemistry |
Nuclear receptor-interacting protein 1 (NRIP1) also known as receptor-interacting protein 140 (RIP140) is a protein that in humans is encoded by the NRIP1 gene. | 1 | Biochemistry |
β-Neoendorphin is an endogenous opioid peptide with a nonapeptide structure and the amino acid sequence Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro (YGGFLRKYP).
β-Neoendorphins (β-NEP) have the capability to stimulate wound healing by accelerating keratinocyte migration. This is achieved by β-NEP's activation of mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinases 1 and 2 (ERK 1 and ERK 2); along with the upregulation of matrix metalloproteinase 2 and 9 (MMP-2 and MMP-9). Wound healing by β-NEP results in migration without consequences on proliferation in human keratinocytes. | 1 | Biochemistry |
An iniferter is a chemical compound that simultaneously acts as initiator, transfer agent, and terminator (hence the name ini-fer-ter) in controlled free radical iniferter polymerizations, the most common is the dithiocarbamate type. | 7 | Physical Chemistry |
Caesium carbonate can be prepared by thermal decomposition of caesium oxalate. Upon heating, caesium oxalate is converted to caesium carbonate with emission of carbon monoxide.
It can also be synthesized by reacting caesium hydroxide with carbon dioxide. | 0 | Organic Chemistry |
* Liquid–liquid extraction
** Acid-base extraction
** Supercritical fluid extraction
* Solid-liquid extraction
** Solid-phase extraction
** Maceration
** Ultrasound-assisted extraction
** Microwave-assisted extraction
** Heat reflux extraction
** Instant controlled pressure drop extraction (Détente instantanée contrôlée)
* Perstraction | 3 | Analytical Chemistry |
The Hansen solubility parameter (HSP) values are based on dispersion bonds (δD), polar bonds (δP) and hydrogen bonds (δH). These contain information about the inter-molecular interactions with other solvents and also with polymers, pigments, nanoparticles, etc. This allows for rational formulations knowing, for example, that there is a good HSP match between a solvent and a polymer. Rational substitutions can also be made for "good" solvents (effective at dissolving the solute) that are "bad" (expensive or hazardous to health or the environment). The following table shows that the intuitions from "non-polar", "polar aprotic" and "polar protic" are put numerically – the "polar" molecules have higher levels of δP and the protic solvents have higher levels of δH. Because numerical values are used, comparisons can be made rationally by comparing numbers. For example, acetonitrile is much more polar than acetone but exhibits slightly less hydrogen bonding.
If, for environmental or other reasons, a solvent or solvent blend is required to replace another of equivalent solvency, the substitution can be made on the basis of the Hansen solubility parameters of each. The values for mixtures are taken as the weighted averages of the values for the neat solvents. This can be calculated by trial-and-error, a spreadsheet of values, or HSP software. A 1:1 mixture of toluene and 1,4 dioxane has δD, δP and δH values of 17.8, 1.6 and 5.5, comparable to those of chloroform at 17.8, 3.1 and 5.7 respectively. Because of the health hazards associated with toluene itself, other mixtures of solvents may be found using a full HSP dataset. | 2 | Environmental Chemistry |
Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces. | 2 | Environmental Chemistry |
Calcium is an essential component of soil. When deposited in the form of lime, it cannot be used by plants. To combat this, carbon dioxide produced by plants reacts with water in the environment to produce carbonic acid. Carbonic acid is then able to dissolve limestone, enabling the release of calcium ions. This reaction is more readily available with smaller particles of limestone than it is with large pieces of rock due to the increased surface area. When lime is leached into soil, calcium levels inevitably increase, both stabilising pH and enabling calcium to mix with water to form Ca ions, thus making it soluble and accessible to plants to be absorbed and utilised by the root system. The calcium ions travel up the xylem of the plant alongside water to reach the leaves. The plant can utilise this calcium in the form of calcium pectate to stabilise cell walls and provide rigidity. Calcium is also used by plant enzymes to signal growth and coordinate life-promoting processes. Additionally, the release of calcium ions enables microorganisms to access phosphorus and other micro nutrients with greater ease, improving the soil ecosystem drastically thus indirectly promoting plant growth and nutrition.
Inevitable plant and animal death results in the return of calcium contained within the organism back into the soil to be utilised by other plants. Decomposing organisms break them down, returning the calcium back into the soil and enabling the cycling of calcium to continue. Additionally, these animals and plants are eaten by other animals, similarly continuing the cycle. It is however important to note the modern introduction of calcium into the soil by humans (through fertilisers and other horticultural products) has resulted in a higher concentration of calcium contained within soil. | 1 | Biochemistry |
In addition to storage, COF materials are exceptional at gas separation. For instance, COFs like imine-linked COF LZU1 and azine-linked COF ACOF-1 were used as a bilayer membrane for the selective separation of the following mixtures: H/CO, H/N, and H/CH. The COFs outperformed molecular sieves due to the inherent thermal and operational stability of the structures. It has also been shown that COFs inherently act as adsorbents, adhering to the gaseous molecules to enable storage and separation. | 6 | Supramolecular Chemistry |
For hydrophobic ligands (e.g. PIP2) in complex with a hydrophobic protein (e.g. lipid-gated ion channels) determining the affinity is complicated by non-specific hydrophobic interactions. Non-specific hydrophobic interactions can be overcome when the affinity of the ligand is high. For example, PIP2 binds with high affinity to PIP2 gated ion channels. | 1 | Biochemistry |
In organic chemistry, the von Baeyer nomenclature is a system for describing polycyclic (i.e. multi-ringed) hydrocarbons. The system was originally developed in 1900 by German chemist Adolf von Baeyer for bicyclic systems and in 1913 expanded by Eduard Buchner and Wilhelm Weigand for tricyclic systems. The system has been adopted and extended by the IUPAC as part of its nomenclature for organic chemistry. The modern version has been extended to cover more cases of compounds including an arbitrary number of cycles, heterocyclic compounds and unsaturated compounds. | 0 | Organic Chemistry |
Reciprocal silencing on the tissue level refers to the same pattern of silencing and expression of homeologous loci. However, in this case, the differences in silencing and expression occur between two types of tissue within the same individual, rather than in individuals of different populations. This is an example of neofunctionalization, a process where duplicated genes that were once at equivalent loci evolve to carry out two separate functions. Since different tissues require different genes to be expressed, reciprocal silencing can occur between tissues. Importantly, while the pattern of gene expression is the same as in the population case, the genetic means by which this pattern is achieved are very different. While silencing mutations are thought to be the main source of reciprocal silencing
at the population level, at the tissue level only epigenetic factors are in play, since expressible copies of both homeologous loci must exist in all cells in an individual if different tissues express different homeologs. | 1 | Biochemistry |
This term is also used in the field of gene synthesis—for example "40-base synthetic oligonucleotides are built into 500- to 800-bp synthons". | 0 | Organic Chemistry |
* BMS-986121: μ-PAM
* BMS-986122: μ-PAM
* Ignavine
* Oxytocin: μ-PAM
* δ-PAM (see reference)
* Cannabidiol
* Tetrahydrocannabinol
* Sodium (Na) | 1 | Biochemistry |
Materials with anisotropic structures, such as crystals (with less than cubic symmetry, for example martensitic phases) and many composites, will generally have different linear expansion coefficients in different directions. As a result, the total volumetric expansion is distributed unequally among the three axes. If the crystal symmetry is monoclinic or triclinic, even the angles between these axes are subject to thermal changes. In such cases it is necessary to treat the coefficient of thermal expansion as a tensor with up to six independent elements. A good way to determine the elements of the tensor is to study the expansion by x-ray powder diffraction. The thermal expansion coefficient tensor for the materials possessing cubic symmetry (for e.g. FCC, BCC) is isotropic. | 7 | Physical Chemistry |
Phenyl isothiocyanate is reacted with an uncharged N-terminal amino group, under mildly alkaline conditions, to form a cyclical phenylthiocarbamoyl derivative. Then, under acidic conditions, this derivative of the terminal amino acid is cleaved as a thiazolinone derivative. The thiazolinone amino acid is then selectively extracted into an organic solvent and treated with acid to form the more stable phenylthiohydantoin (PTH)- amino acid derivative that can be identified by using chromatography or electrophoresis. This procedure can then be repeated again to identify the next amino acid. A major drawback to this technique is that the peptides being sequenced in this manner cannot have more than 50 to 60 residues (and in practice, under 30). The peptide length is limited due to the cyclical derivatization not always going to completion. The derivatization problem can be resolved by cleaving large peptides into smaller peptides before proceeding with the reaction. It is able to accurately sequence up to 30 amino acids with modern machines capable of over 99% efficiency per amino acid. An advantage of the Edman degradation is that it only uses 10 - 100 pico-moles of peptide for the sequencing process. The Edman degradation reaction was automated in 1967 by Edman and Beggs to speed up the process and 100 automated devices were in use worldwide by 1973. | 0 | Organic Chemistry |
The original model for how the respiratory chain complexes are organized was that they diffuse freely and independently in the mitochondrial membrane. However, recent data suggest that the complexes might form higher-order structures called supercomplexes or "respirasomes". In this model, the various complexes exist as organized sets of interacting enzymes. These associations might allow channeling of substrates between the various enzyme complexes, increasing the rate and efficiency of electron transfer. Within such mammalian supercomplexes, some components would be present in higher amounts than others, with some data suggesting a ratio between complexes I/II/III/IV and the ATP synthase of approximately 1:1:3:7:4. However, the debate over this supercomplex hypothesis is not completely resolved, as some data do not appear to fit with this model. | 1 | Biochemistry |
In molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions. Two-component systems typically consist of a membrane-bound histidine kinase that senses a specific environmental stimulus, and a corresponding response regulator that mediates the cellular response, mostly through differential expression of target genes. Although two-component signaling systems are found in all domains of life, they are most common by far in bacteria, particularly in Gram-negative and cyanobacteria; both histidine kinases and response regulators are among the largest gene families in bacteria. They are much less common in archaea and eukaryotes; although they do appear in yeasts, filamentous fungi, and slime molds, and are common in plants, two-component systems have been described as "conspicuously absent" from animals. | 1 | Biochemistry |
Schwab was also known as a writer of physical chemistry and catalysis textbooks, with important works such as the Physico-chemical Foundations of Chemical Technology (, 1928) or Catalysis from the Standoint of Chemical Kinetics (, 1931), the English translation of which was a standard textbook on catalysis for decades. He was the editor of all 7 volumes of the international Handbook of Catalysis (1940–1960). | 7 | Physical Chemistry |
*An Excellent Nickel Boride Catalyst for the Cis-Selective Semihydrogenation of Acetylenes Tetrahedron (J. Choi and N. M. Yoon), Tetrahedron Lett. Vol.37 p. 1057 (1996)
*A New Coupling Reaction of Alkyl Iodides with Electron Deficient Alkenes Using Nickel Boride(cat.)- Borohydride Exchange Resin in Methanol (T. B. Sim, J. Choi, M. J. Joung and N. M. Yoon), J. Org. Chem. Vol.62 p. 2357 (1997)
*Sodium Diethyldialkynylaluminate A New Chemoselective Alkynylating Agent (J. H. Ahn, M. J. Joung, and N. M. Yoon) J. Org. Chem. Vol.60 p. 6173 (1995)
*Synthesis of Disulfides by Copper Catalyzed Disproportionation of Thiols (J. Choi and N. M. Yoon), J. Org. Chem. Vol.60 p. 3266 (1995)
*Monoisopinocampheylborane- A New Chiral Hydroborating Agent for Relatively Hindered (Trisubstituted) Olefins (H. C. Brown and N. M. Yoon), J. Am. Chem. Soc. Vol.99 p. 5514 (1977)
*Diisopinocampheylborane of high Optical Purity. Asymmetric Synthesis via Hydroboration with Essentially Complete Asymmetric Induction (H. C. Brown and N. M. Yoon), Israel J. Chem Vol.15 p. 12 (1976~1977)
*Lithium Trimethylethynylaluminate A New Chemoselective Ethynylating Agent (M. J. Joung, J. H. Ahn and N. M. Yoon), J. Org. Chem. Vol.61 p. 4472 (1996)
*The Rapido Reaction of Carboxylic Acids with Borane-Tetrahydrofuran. A Remarkably Convenient Procedure for the Selective Conversion of Carboxylic Acids to the Corresponding Alcohols in the Presence of Other Functional Groups (N. M. Yoon, C. S. Park, H. C. Brown, S. Krishnamurthy, and T. P. Stoky), J. Org. Chem. Vol.38 p. 2786 (1973) | 0 | Organic Chemistry |
A solution containing the analyte, A, in the presence of some conductive buffer. If an electrolytic potential is applied to the solution through a working electrode, then the measured current depends (in part) on the concentration of the analyte. Measurement of this current can be used to determine the concentration of the analyte directly; this is a form of amperometry. However, the difficulty is that the measured current depends on several other variables, and it is not always possible to control all of them adequately. This limits the precision of direct amperometry.
If the potential applied to the working electrode is sufficient to reduce the analyte, then the concentration of analyte close to the working electrode will decrease. More of the analyte will slowly diffuse into the volume of solution close to the working electrode, restoring the concentration. If the potential applied to the working electrode is great enough (an overpotential), then the concentration of analyte next to the working electrode will depend entirely on the rate of diffusion. In such a case, the current is said to be diffusion limited. As the analyte is reduced at the working electrode, the concentration of the analyte in the whole solution will very slowly decrease; this depends on the size of the working electrode compared to the volume of the solution.
What happens if some other species which reacts with the analyte (the titrant) is added? (For instance, chromate ions can be added to oxidize lead ions.) After a small quantity of the titrant (chromate) is added, the concentration of the analyte (lead) has decreased due to the reaction with chromate. The current from the reduction of lead ion at the working electrode will decrease. The addition is repeated, and the current decreases again. A plot of the current against volume of added titrant will be a straight line.
After enough titrant has been added to react completely with the analyte, the excess titrant may itself be reduced at the working electrode. Since this is a different species with different diffusion characteristics (and different half-reaction), the slope of current versus added titrant will have a different slope after the equivalence point. This change in slope marks the equivalence point, in the same way that, for instance, the sudden change in pH marks the equivalence point in an acid–base titration.
The electrode potential may also be chosen such that the titrant is reduced, but the analyte is not. In this case, the presence of excess titrant is easily detected by the increase in current above background (charging) current. | 3 | Analytical Chemistry |
A polyatomic ion (also known as a molecular ion) is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may not be used to refer to a polyatomic ion, depending on the definition used. The prefix poly- carries the meaning "many" in Greek, but even ions of two atoms are commonly described as polyatomic.
In older literature, a polyatomic ion may instead be referred to as a radical (or less commonly, as a radical group). In contemporary usage, the term radical refers to various free radicals, which are species that have an unpaired electron and need not be charged.
A simple example of a polyatomic ion is the hydroxide ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of −1; its chemical formula is . In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is .
Polyatomic ions often are useful in the context of acid–base chemistry and in the formation of salts.
Often, a polyatomic ion can be considered as the conjugate acid or base of a neutral molecule. For example, the conjugate base of sulfuric acid (HSO) is the polyatomic hydrogen sulfate anion (). The removal of another hydrogen ion produces the sulfate anion (). | 7 | Physical Chemistry |
Reactive transport models are constructed to understand the composition of natural waters; the origin of economic mineral deposits; the formation and dissolution of rocks and minerals in geologic formations in response to injection of industrial wastes, steam, or carbon dioxide; and the generation of acidic waters and leaching of metals from mine wastes. They are often relied upon to predict the migration of contaminant plumes; the mobility of radionuclides in waste repositories; and the biodegradation of chemicals in landfills. When applied to the study of contaminants in the environments, they are known as fate and transport models. | 9 | Geochemistry |
Mission data are provided to the public by the NASA Goddard Earth Science Data and Information Services Center (GES DISC). The Level 1B data product is the least processed and contains records for all collected soundings (about 74,000 soundings per orbit). The Level 2 product contains estimates of the column-averaged dry-air mole fractions of carbon dioxide, among other parameters such as surface albedo and aerosol content. The Level 3 product consists of global maps of carbon dioxide concentrations developed by OCO-2 scientists. | 2 | Environmental Chemistry |
Carbon monoxide bonds to transition metals using "synergistic pi* back-bonding". The M-C bonding has three components, giving rise to a partial triple bond. A sigma (σ) bond arises from overlap of the nonbonding (or weakly anti-bonding) sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. A pair of pi (π) bonds arises from overlap of filled d-orbitals on the metal with a pair of π*-antibonding orbitals projecting from the carbon atom of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (0 or +1) which makes the back-donation of electron density favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken the carbon–oxygen bond compared with free carbon monoxide, while the metal–carbon bond is strengthened. Because of the multiple bond character of the M–CO linkage, the distance between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal–alkyl bond. The M-CO and MC-O distance are sensitive to other ligands on the metal. Illustrative of these effects are the following data for Mo-C and C-O distances in Mo(CO) and Mo(CO)(4-methylpyridine): 2.06 vs 1.90 and 1.11 vs 1.18 Å.
Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. For compounds with doubly bridging CO ligands, denoted μ-CO or often just μ-CO, the bond stretching frequency ν is usually shifted by 100–200 cm to lower energy compared to the signatures of terminal CO, which are in the region 1800 cm. Bands for face capping (μ) CO ligands appear at even lower energies. In addition to symmetrical bridging modes, CO can be found to bridge asymmetrically or through donation from a metal d orbital to the π* orbital of CO. The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C–O bond. | 0 | Organic Chemistry |
The HNF6 subfamily members contain a cut-homeodomain (ONECUT) bind to DNA as monomers.
* HNF6α/OC-1/ONECUT1 ()
* HNF6β/OC-2/ONECUT2 () | 1 | Biochemistry |
Chloroplasts have their own genome, which encodes a number of thylakoid proteins. However, during the course of plastid evolution from their cyanobacterial endosymbiotic ancestors, extensive gene transfer from the chloroplast genome to the cell nucleus took place. This results in the four major thylakoid protein complexes being encoded in part by the chloroplast genome and in part by the nuclear genome. Plants have developed several mechanisms to co-regulate the expression of the different subunits encoded in the two different organelles to assure the proper stoichiometry and assembly of these protein complexes. For example, transcription of nuclear genes encoding parts of the photosynthetic apparatus is regulated by light. Biogenesis, stability and turnover of thylakoid protein complexes are regulated by phosphorylation via redox-sensitive kinases in the thylakoid membranes. The translation rate of chloroplast-encoded proteins is controlled by the presence or absence of assembly partners (control by epistasy of synthesis). This mechanism involves negative feedback through binding of excess protein to the 5' untranslated region of the chloroplast mRNA. Chloroplasts also need to balance the ratios of photosystem I and II for the electron transfer chain. The redox state of the electron carrier plastoquinone in the thylakoid membrane directly affects the transcription of chloroplast genes encoding proteins of the reaction centers of the photosystems, thus counteracting imbalances in the electron transfer chain. | 5 | Photochemistry |
Ampicillin can be administered by mouth, an intramuscular injection (shot) or by intravenous infusion. The oral form, available as capsules or oral suspensions, is not given as an initial treatment for severe infections, but rather as a follow-up to an IM or IV injection. For IV and IM injections, ampicillin is kept as a powder that must be reconstituted.
IV injections must be given slowly, as rapid IV injections can lead to convulsive seizures. | 4 | Stereochemistry |
Catabolite repression was extensively studied in Escherichia coli. E. coli grows faster on glucose than on any other carbon source. For example, if E. coli is placed on an agar plate containing only glucose and lactose, the bacteria will use glucose first and lactose second. When glucose is available in the environment, the synthesis of β-galactosidase is under repression due to the effect of catabolite repression caused by glucose. The catabolite repression in this case is achieved through the utilization of phosphotransferase system.
An important enzyme from the phosphotransferase system called Enzyme II A (EIIA) plays a central role in this mechanism. There are different catabolite-specific EIIA in a single cell, even though different bacterial groups have specificities to different sets of catabolites. In enteric bacteria one of the EIIA enzymes in their set is specific for glucose transport only. When glucose levels are high inside the bacteria, EIIA mostly exists in its unphosphorylated form. This leads to inhibition of adenylyl cyclase and lactose permease, therefore cAMP levels are low and lactose can not be transported inside the bacteria.
Once the glucose is all used up, the second preferred carbon source (i.e. lactose) has to be used by bacteria. Absence of glucose will "turn off" catabolite repression. When glucose levels are low, the phosphorylated form of EIIA accumulates and consequently activates the enzyme adenylyl cyclase, which will produce high levels of cAMP. cAMP binds to catabolite activator protein (CAP) and together they will bind to a promoter sequence on the lac operon. However, this is not enough for the lactose genes to be transcribed. Lactose must be present inside the cell to remove the lactose repressor from the operator sequence (transcriptional regulation). When these two conditions are satisfied, it means for the bacteria that glucose is absent and lactose is available. Next, bacteria start to transcribe the lac operon and produce β-galactosidase enzymes for lactose metabolism. The example above is a simplification of a complex process. Catabolite repression is considered to be a part of global control system and therefore it affects more genes rather than just lactose gene transcription. | 1 | Biochemistry |
Since the introduction of pertussis vaccines in the 1940s and 1950s, different genetic changes have been described surrounding the pertussis toxin. | 1 | Biochemistry |
In 1969, he left HP and joined Fairchild Semiconductor. He was the vice president and general manager of the Microwave & Optoelectronics division, from its inception in May 1969 up until November 1971. He continued his work on light-emitting diodes (LEDs), proposing they could be used for indicator lights and optical readers in 1971. He later left Fairchild in 1972. | 7 | Physical Chemistry |
The debate over the nature and classification of hypervalent molecules goes back to Gilbert N. Lewis and Irving Langmuir and the debate over the nature of the chemical bond in the 1920s. Lewis maintained the importance of the two-center two-electron (2c-2e) bond in describing hypervalence, thus using expanded octets to account for such molecules. Using the language of orbital hybridization, the bonds of molecules like PF and SF were said to be constructed from spd orbitals on the central atom. Langmuir, on the other hand, upheld the dominance of the octet rule and preferred the use of ionic bonds to account for hypervalence without violating the rule (e.g. " 2F" for SF).
In the late 1920s and 1930s, Sugden argued for the existence of a two-center one-electron (2c-1e) bond and thus rationalized bonding in hypervalent molecules without the need for expanded octets or ionic bond character; this was poorly accepted at the time. In the 1940s and 1950s, Rundle and Pimentel popularized the idea of the three-center four-electron bond, which is essentially the same concept which Sugden attempted to advance decades earlier; the three-center four-electron bond can be alternatively viewed as consisting of two collinear two-center one-electron bonds, with the remaining two nonbonding electrons localized to the ligands.
The attempt to actually prepare hypervalent organic molecules began with Hermann Staudinger and Georg Wittig in the first half of the twentieth century, who sought to challenge the extant valence theory and successfully prepare nitrogen and phosphorus-centered hypervalent molecules. The theoretical basis for hypervalency was not delineated until J.I. Musher's work in 1969.
In 1990, Magnusson published a seminal work definitively excluding the significance of d-orbital hybridization in the bonding of hypervalent compounds of second-row elements. This had long been a point of contention and confusion in describing these molecules using molecular orbital theory. Part of the confusion here originates from the fact that one must include d-functions in the basis sets used to describe these compounds (or else unreasonably high energies and distorted geometries result), and the contribution of the d-function to the molecular wavefunction is large. These facts were historically interpreted to mean that d-orbitals must be involved in bonding. However, Magnusson concludes in his work that d-orbital involvement is not implicated in hypervalency.
Nevertheless, a 2013 study showed that although the Pimentel ionic model best accounts for the bonding of hypervalent species, the energetic contribution of an expanded octet structure is also not null. In this modern valence bond theory study of the bonding of xenon difluoride, it was found that ionic structures account for about 81% of the overall wavefunction, of which 70% arises from ionic structures employing only the p orbital on xenon while 11% arises from ionic structures employing an hybrid on xenon. The contribution of a formally hypervalent structure employing an orbital of spd hybridization on xenon accounts for 11% of the wavefunction, with a diradical contribution making up the remaining 8%. The 11% spd contribution results in a net stabilization of the molecule by mol, a minor but significant fraction of the total energy of the total bond energy ( mol). Other studies have similarly found minor but non-negligible energetic contributions from expanded octet structures in SF (17%) and XeF (14%).
Despite the lack of chemical realism, the IUPAC recommends the drawing of expanded octet structures for functional groups like sulfones and phosphoranes, in order to avoid the drawing of a large number of formal charges or partial single bonds. | 4 | Stereochemistry |
Common adverse drug reactions (ADRs) associated with pseudoephedrine therapy include central nervous system stimulation, insomnia, nervousness, excitability, dizziness and anxiety. Infrequent ADRs include tachycardia or palpitations. Rarely, pseudoephedrine therapy may be associated with mydriasis (dilated pupils), hallucinations, arrhythmias, hypertension, seizures and ischemic colitis; as well as severe skin reactions known as recurrent pseudo-scarlatina, systemic contact dermatitis, and nonpigmenting fixed drug eruption. Pseudoephedrine, particularly when combined with other drugs including narcotics, may also play a role in the precipitation of episodes of paranoid psychosis. It has also been reported that pseudoephedrine, among other sympathomimetic agents, may be associated with the occurrence of stroke. | 4 | Stereochemistry |
miR-324-5p first appeared in literature in a paper published by John Kim et al. in early 2004 that identified 32 entirely new miRNAs from cultured rat cortical neurons using miRNA cloning and RNA analysis. The miRNA quickly gained traction in scientific literature, appearing in articles about the evolutionary conservation of microRNAs, HIV, cancer, and other topics within a few years. Today, the functions and roles of miR-324-5p are still not yet fully characterized. | 1 | Biochemistry |
To function, a modern computer needs three different capabilities: It must be able to store information, transmit information between components, and possess a basic system of logic. Prior to March 2013, scientists had successfully demonstrated the ability to store and transmit data using biological components made of proteins and DNA. Simple two-terminal logic gates had been demonstrated, but required multiple layers of inputs and thus were impractical due to scaling difficulties. | 1 | Biochemistry |
The alcoholysis of acyl halides (the alkoxy-dehalogenation) is believed to proceed via an S2 mechanism (Scheme 10). However, the mechanism can also be tetrahedral or S1 in highly polar solvents (while the S2 reaction involves a concerted reaction, the tetrahedral addition-elimination pathway involves a discernible intermediate).
Bases, such as pyridine or N,N-dimethylformamide, catalyze acylations. These reagents activate the acyl chloride via a nucleophilic catalysis mechanism. The amine attacks the carbonyl bond and presumably first forms a transient tetrahedral intermediate, then forms a quaternary acylammonium salt by the displacement of the leaving group. This quaternary acylammonium salt is more susceptible to attack by alcohols or other nucleophiles.
The use of two phases (aqueous for amine, organic for acyl chloride) is called the Schotten-Baumann reaction. This approach is used in the preparation of nylon via the so-called nylon rope trick. | 0 | Organic Chemistry |
When a ligand activates the G protein-coupled receptor, it induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP. The GTP (or GDP) is bound to the G subunit in the traditional view of heterotrimeric GPCR activation. This exchange triggers the dissociation of the G subunit (which is bound to GTP) from the G dimer and the receptor as a whole. However, models which suggest molecular rearrangement, reorganization, and pre-complexing of effector molecules are beginning to be accepted. Both G-GTP and G can then activate different signaling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein. | 1 | Biochemistry |
Glycogen is a branched biopolymer consisting of linear chains of glucose residues with an average chain length of approximately 8–12 glucose units and 2,000-60,000 residues per one molecule of glycogen.
Like amylopectin, glucose units are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next. Branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain.
Each glycogen is essentially a ball of glucose trees, with around 12 layers, centered on a glycogenin protein, with three kinds of glucose chains: A, B, and C. There is only one C-chain, attached to the glycogenin. This C-chain is formed by the self-glucosylation of the glycogenin, forming a short primer chain. From the C-chain grows out B-chains, and from B-chains branch out B- and A-chains. The B-chains have on average 2 branch points, while the A-chains are terminal, thus unbranched. On average, each chain has length 12, tightly constrained to be between 11 and 15. All A-chains reach the spherical surface of the glycogen.
Glycogen in muscle, liver, and fat cells is stored in a hydrated form, composed of three or four parts of water per part of glycogen associated with 0.45 millimoles (18 mg) of potassium per gram of glycogen.
Glucose is an osmotic molecule, and can have profound effects on osmotic pressure in high concentrations possibly leading to cell damage or death if stored in the cell without being modified. Glycogen is a non-osmotic molecule, so it can be used as a solution to storing glucose in the cell without disrupting osmotic pressure. | 1 | Biochemistry |
Joule performed his experiment with air at room temperature which was expanded from a pressure of about 22 bar. Air, under these conditions, is almost an ideal gas, but not quite. As a result the real temperature change will not be exactly zero. With our present knowledge of the thermodynamic properties of air we can calculate that the temperature of the air should drop by about 3 degrees Celsius when the volume is doubled under adiabatic conditions. However, due to the low heat capacity of the air and the high heat capacity of the strong copper containers and the water of the calorimeter, the observed temperature drop is much smaller, so Joule found that the temperature change was zero within his measuring accuracy. | 7 | Physical Chemistry |
As oxygen is fundamental for oxidative phosphorylation, a shortage in O level likely alters ATP production rates. However, proton motive force and ATP production can be maintained by intracellular acidosis. Cytosolic protons that have accumulated with ATP hydrolysis and lactic acidosis can freely diffuse across the mitochondrial outer-membrane and acidify the inter-membrane space, hence directly contributing to the proton motive force and ATP production. | 1 | Biochemistry |
The ZMapp cocktail was assessed by the World Health Organization for emergency use under the MEURI ethical protocol. The panel agreed that "the benefits of ZMapp outweigh its risks" while noting that it presented logistical challenges, particularly that of requiring a cold chain for distribution and storage. Four alternative therapies (remdesivir, the Regeneron product atoltivimab/maftivimab/odesivimab, favipiravir, and ansuvimab) were also considered for use, but they were at earlier stages of development.
In August 2019, the Democratic Republic of the Congo's national health authorities, the World Health Organization, and the National Institutes of Health announced that they would stop using ZMapp, along with all other Ebola treatments except atoltivimab/maftivimab/odesivimab and ansuvimab, in their ongoing clinical trials, citing the higher mortality rates of patients not treated with atoltivimab/maftivimab/odesivimab and ansuvimab.
In October 2020, the U.S. Food and Drug Administration (FDA) approved atoltivimab/maftivimab/odesivimab with an indication for the treatment of infection caused by Zaire ebolavirus. | 1 | Biochemistry |
In 1967, the supertanker Torrey Canyon leaked oil onto the English coastline. Alkylphenol surfactants were primarily used to break up the oil, but proved very toxic in the marine environment; all types of marine life were killed. This led to a reformulation of dispersants to be more environmentally sensitive. After the Torrey Canyon spill, new boat-spraying systems were developed. Later reformulations allowed more dispersant to be contained (at a higher concentration) to be aerosolized. | 2 | Environmental Chemistry |
Intermetallic compounds are generally brittle at room temperature and have high melting points. Cleavage or intergranular fracture modes are typical of intermetallics due to limited independent slip systems required for plastic deformation. However, there are some examples of intermetallics with ductile fracture modes such as Nb–15Al–40Ti. Other intermetallics can exhibit improved ductility by alloying with other elements to increase grain boundary cohesion. Alloying of other materials such as boron to improve grain boundary cohesion can improve ductility in many intermetallics. They often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing. They can also display desirable magnetic and chemical properties, due to their strong internal order and mixed (metallic and covalent/ionic) bonding, respectively. Intermetallics have given rise to various novel materials developments. Some examples include alnico and the hydrogen storage materials in nickel metal hydride batteries. NiAl, which is the hardening phase in the familiar nickel-base super alloys, and the various titanium aluminides have also attracted interest for turbine blade applications, while the latter is also used in very small quantities for grain refinement of titanium alloys. Silicides, inter-metallic involving silicon, are utilized as barrier and contact layers in microelectronics. | 8 | Metallurgy |
The study of action potentials has required the development of new experimental methods. The initial work, prior to 1955, was carried out primarily by Alan Lloyd Hodgkin and Andrew Fielding Huxley, who were, along John Carew Eccles, awarded the 1963 Nobel Prize in Physiology or Medicine for their contribution to the description of the ionic basis of nerve conduction. It focused on three goals: isolating signals from single neurons or axons, developing fast, sensitive electronics, and shrinking electrodes enough that the voltage inside a single cell could be recorded.
The first problem was solved by studying the giant axons found in the neurons of the squid (Loligo forbesii and Doryteuthis pealeii, at the time classified as Loligo pealeii). These axons are so large in diameter (roughly 1 mm, or 100-fold larger than a typical neuron) that they can be seen with the naked eye, making them easy to extract and manipulate. However, they are not representative of all excitable cells, and numerous other systems with action potentials have been studied.
The second problem was addressed with the crucial development of the voltage clamp, which permitted experimenters to study the ionic currents underlying an action potential in isolation, and eliminated a key source of electronic noise, the current I associated with the capacitance C of the membrane. Since the current equals C times the rate of change of the transmembrane voltage V, the solution was to design a circuit that kept V fixed (zero rate of change) regardless of the currents flowing across the membrane. Thus, the current required to keep V at a fixed value is a direct reflection of the current flowing through the membrane. Other electronic advances included the use of Faraday cages and electronics with high input impedance, so that the measurement itself did not affect the voltage being measured.
The third problem, that of obtaining electrodes small enough to record voltages within a single axon without perturbing it, was solved in 1949 with the invention of the glass micropipette electrode, which was quickly adopted by other researchers. Refinements of this method are able to produce electrode tips that are as fine as 100 Å (10 nm), which also confers high input impedance. Action potentials may also be recorded with small metal electrodes placed just next to a neuron, with neurochips containing EOSFETs, or optically with dyes that are sensitive to Ca or to voltage.
While glass micropipette electrodes measure the sum of the currents passing through many ion channels, studying the electrical properties of a single ion channel became possible in the 1970s with the development of the patch clamp by Erwin Neher and Bert Sakmann. For this discovery, they were awarded the Nobel Prize in Physiology or Medicine in 1991. Patch-clamping verified that ionic channels have discrete states of conductance, such as open, closed and inactivated.
Optical imaging technologies have been developed in recent years to measure action potentials, either via simultaneous multisite recordings or with ultra-spatial resolution. Using voltage-sensitive dyes, action potentials have been optically recorded from a tiny patch of cardiomyocyte membrane. | 7 | Physical Chemistry |
In typical groundwater or seawater, the measured total alkalinity is set equal to:
: A = [] + 2[] + [] + [OH] + 2[] + [] + [] − [H] − []
(Subscript T indicates the total concentration of the species in the solution as measured. This is opposed to the free concentration, which takes into account the significant amount of ion pair interactions that occur in seawater.)
Alkalinity can be measured by titrating a sample with a strong acid until all the buffering capacity of the aforementioned ions above the pH of bicarbonate or carbonate is consumed. This point is functionally set to pH 4.5. At this point, all the bases of interest have been protonated to the zero level species, hence they no longer cause alkalinity. In the carbonate system the bicarbonate ions [] and the carbonate ions [] have become converted to carbonic acid [HCO] at this pH. This pH is also called the CO equivalence point where the major component in water is dissolved CO which is converted to HCO in an aqueous solution. There are no strong acids or bases at this point. Therefore, the alkalinity is modeled and quantified with respect to the CO equivalence point. Because the alkalinity is measured with respect to the CO equivalence point, the dissolution of CO, although it adds acid and dissolved inorganic carbon, does not change the alkalinity. In natural conditions, the dissolution of basic rocks and addition of ammonia [NH] or organic amines leads to the addition of base to natural waters at the CO equivalence point. The dissolved base in water increases the pH and titrates an equivalent amount of CO to bicarbonate ion and carbonate ion. At equilibrium, the water contains a certain amount of alkalinity contributed by the concentration of weak acid anions. Conversely, the addition of acid converts weak acid anions to CO and continuous addition of strong acids can cause the alkalinity to become less than zero. For example, the following reactions take place during the addition of acid to a typical seawater solution:
: + H → B(OH) + HO
: OH + H → HO
: + 2 H →
: + H →
: [] + H → [Si(OH)]
It can be seen from the above protonation reactions that most bases consume one proton (H) to become a neutral species, thus increasing alkalinity by one per equivalent. however, will consume two protons before becoming a zero-level species (CO), thus it increases alkalinity by two per mole of . [H] and [] decrease alkalinity, as they act as sources of protons. They are often represented collectively as [H].
Alkalinity is typically reported as mg/L as CaCO. (The conjunction "as" is appropriate in this case because the alkalinity results from a mixture of ions but is reported "as if" all of this is due to CaCO.) This can be converted into milliequivalents per Liter (meq/L) by dividing by 50 (the approximate MW of CaCO divided by 2). | 9 | Geochemistry |
Many metals have several different crystal structures at the same composition, but most metals do not show this shape-memory effect. The special property that allows shape-memory alloys to revert to their original shape after heating is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms. A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in the way a parallelogram can be made out of a square by pushing on two opposing sides. At different temperatures, different structures are preferred and when the structure is cooled through the transition temperature, the martensitic structure forms from the austenitic phase. | 8 | Metallurgy |
N,-Diisopropylcarbodiimide is a carbodiimide used in peptide synthesis. As a liquid, it is easier to handle than the commonly used N,-dicyclohexylcarbodiimide, a waxy solid. In addition, N,-diisopropylurea, its byproduct in many chemical reactions, is soluble in most organic solvents, a property that facilitates work-up. | 1 | Biochemistry |
The reverse northern blot differs from both northern and Southern blot in that DNA is first immobilized on a blotting matrix and specific sequences are detected with labeled RNA probes. | 1 | Biochemistry |
A nanotextured surface (NTS) is a surface which is covered with nano-sized structures. Such surfaces have one dimension on the nanoscale, i.e., only the thickness of the surface of an object is between 0.1 and 100 nm. They are currently gaining popularity because of their special applications due to their unique physical properties. Nanotextured surfaces are in various forms like cones, columns, or fibers. These are water, ice, oil, and microorganism repellent that is superamphiphobic, anti-icing, and antifouling respectively and thus self-cleaning. They are simultaneously anti-reflective and transparent, hence they are termed smart surfaces.
In research published online October 21, 2013, in Advanced Materials, of a group of scientists at the U.S. Department of Energys Brookhaven National Laboratory (BNL), led by BNL physicist and lead author Antonio Checco, proposed that nanotexturing surfaces in the form of cones produces highly water-repellent surfaces. These nano-cone textures are superhydrophobic' or super-water-hating. | 7 | Physical Chemistry |
Polymers are susceptible to attack by atmospheric oxygen, especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusion and injection moulding involve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutch suddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a polypropylene insert within the aluminium tube of the device, and IR spectroscopy of the material showed that it had oxidised, possibly as a result of poor moulding.
Oxidation is usually relatively easy to detect, owing to the strong absorption by the carbonyl group in the spectrum of polyolefins. Polypropylene has a relatively simple spectrum, with few peaks at the carbonyl position (like polyethylene). Oxidation tends to start at tertiary carbon atoms because free radicals here are more stable, so last longer and are attacked by oxygen. The carbonyl group can be further oxidised to break the chain, so weakening the material by lowering the molecular weight, and cracks start to grow in the regions affected. | 7 | Physical Chemistry |
Once a nickel allergy is detected, the best treatment is avoidance of nickel-releasing items. The top 13 categories that contain nickel include beauty accessories, eyeglasses, money, cigarettes, clothes, kitchen and household, electronics and office equipment, metal utensils, aliment, jewelry, batteries, orthodontic and dental appliances, and medical equipment. Other than strict avoidance of items that release free nickel, there are other treatment options for reduction of exposure. The first step is to limit friction between skin and metallic items. Susceptible people may try to limit sweating while wearing nickel items, to reduce nickel release and thus decrease chances for developing sensitization or allergy. Another option is to shield electronics, metal devices, and tools with fabric, plastic, or acrylic coverings.
There are dimethylglyoxime test kits that can be very helpful to check for nickel release from items prior to purchasing. The American Contact Dermatitis Society find a provider resource can help identify clinicians with training in providing guidance lists of safe items. In addition to avoidance, healthcare providers may prescribe additional creams or medications to help relieve the skin reaction. | 1 | Biochemistry |
Enzymes incur catalysis by binding more strongly to transition states than substrates and products. At the catalytic binding site, several different interactions may act upon the substrate. These range from electric catalysis, acid and base catalysis, covalent catalysis, and metal ion catalysis. These interactions decrease the activation energy of a chemical reaction by providing favorable interactions to stabilize the high energy molecule. Enzyme binding allows for closer proximity and exclusion of substances irrelevant to the reaction. Side reactions are also discouraged by this specific binding.
Types of enzymes that can perform these actions include oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
For instance, the transferase hexokinase catalyzes the phosphorylation of glucose to make glucose-6-phosphate. Active site residues of hexokinase allow for stabilization of the glucose molecule in the active site and spur the onset of an alternative pathway of favorable interactions, decreasing the activation energy. | 1 | Biochemistry |
The tricarboxylic acid cycle (TCA) and glutaminolysis can also be targeted for cancer treatment, since they are essential for the survival and proliferation of cancer cells. Ivosidenib and Enasidenib, two FDA-approved cancer treatments, can arrest the TCA cycle of cancer cells by inhibiting isocitrate dehydrogenase-1 (IDH1) and isocitrate dehydrogenase-2 (IDH2), respectively. Ivosidenib is specific to acute myeloid leukemia (AML) and cholangiocarcinoma, whereas Enasidenib is specific to just acute myeloid leukemia (AML).
In a clinical trial consisting of 185 adult patients with cholangiocarcinoma and an IDH-1 mutation, there was a statistically significant improvement (p<0.0001; HR: 0.37) in patients randomized to Ivosidenib. Still, some of the adverse side effects in these patients included fatigue, nausea, diarrhea, decreased appetite, ascites, and anemia. In a clinical trial consisting of 199 adult patients with AML and an IDH2 mutation, 23% of patients experienced complete response (CR) or complete response with partial hematologic recovery (CRh) lasting a median of 8.2 months while on Enasidenib. Of the 157 patients who required transfusion at the beginning of the trial, 34% no longer required transfusions during the 56-day time period on Enasidenib. Of the 42% of patients who did not require transfusions at the beginning of the trial, 76% still did not require a transfusion by the end of the trial. Side effects of Enasidenib included nausea, diarrhea, elevated bilirubin and most notably, differentiation syndrome.
Glutaminase (GLS), the enzyme responsible for converting glutamine to glutamate via hydrolytic deamidation during the first reaction of glutaminolysis, can also be targeted. In recent years, many small molecules, such as azaserine, acivicin, and CB-839 have been shown to inhibit glutaminase, thus reducing cancer cell viability and inducing apoptosis in cancer cells. Due to its effective antitumor ability in several cancer types such as ovarian, breast and lung cancers, CB-839 is the only GLS inhibitor currently undergoing clinical studies for FDA-approval. | 1 | Biochemistry |
The basis for the ISFET is the MOSFET. Dutch engineer Piet Bergveld, at the University of Twente studied the MOSFET and realized it could be adapted into a sensor for electrochemical and biological applications. This led to Bergveld's invention of the ISFET in 1970. He described the ISFET as "a special type of MOSFET with a gate at a certain distance". It was the earliest Biosensor FET (BioFET).
ISFET sensors could be implemented in integrated circuits based on CMOS (complementary MOS) technology. ISFET devices are widely used in biomedical applications, such as the detection of DNA hybridization, biomarker detection from blood, antibody detection, glucose measurement and pH sensing. The ISFET is also the basis for later BioFETs, such as the DNA field-effect transistor (DNAFET), used in genetic technology. | 7 | Physical Chemistry |
In aquatic environments (rivers, lakes and ocean), dissolved Si is utilized by diatoms, dictyochales, radiolarians and sponges to produce solid bSiO structures. The biomineralized silica has an amorphous structure and therefore its properties may vary among the different organisms. Biomineralization by diatoms induces the largest Si flux within the ocean, and thus it has a crucial role in the global Si cycle. During Si uptake by diatoms, there is an isotopic discrimination against the heavy isotope, forming Si-depleted biogenic silica minerals. As a result, the remaining dissolved Si in the surrounding water is Si-enriched. Since diatoms rely on sunlight for photosynthesis, they inhabit in surface waters, and thus the surface water of the ocean are typically Si-enriched. Although there is less available data on the isotopic fractionation during biomineralization by radiolarians, it has been suggested that radiolarians also discriminate against the heavy isotope (Si), and that the magnitude of isotopic fractionation is of a similar range as biomineralization by diatoms. Sponges also show an isotopic preference for Si over Si, but the magnitude of their isotopic fractionation is often larger (For quantitative comparation, see Figure 2). | 9 | Geochemistry |
The use of synthetic oligonucleotides as specific probes for genetic sequence variations was pioneered by R. Bruce Wallace, working at the City of Hope National Medical Center in Duarte, California. In 1979 Wallace and his coworkers reported the use of ASO probes to detect variations in a single-stranded bacterial virus, and later applied the technique to cloned human genes. In 1983 and 1985 Wallace's lab reported the detection of the mutation for sickle cell anemia in samples of whole genomic DNA, although this application was hampered by the small amount of label that could be carried by the ASO.
Fortunately PCR, a method to greatly amplify a specific segment of DNA, was also reported in 1985. In less than a year PCR had been paired with ASO analysis. This combination solved the problem of ASO labeling, since the amount of target DNA could be amplified over a million-fold. Also, the specificity of the PCR process itself could be added to that of the ASO probes, greatly reducing the problem of spurious binding of the ASO to non-target sequences. The combination was specific enough that it could be used in a simple Dot blot, avoiding the laborious and inefficient Southern blot method. | 1 | Biochemistry |
In some complexes, the arene binds through only two or four carbons, η and η bonding, respectively. In these cases, the arene is no longer planar. Because the arene is dearomatized, the uncoordinated carbon centers display enhanced reactivity. A well studied example is [Ru(η-CMe)(η-CMe)], formed by the reduction of [Ru(η-CMe)]. An example of an [Os(η-CH)(NH))]. | 0 | Organic Chemistry |
In the case of multiple charged projectile ions a particular form of electronic sputtering can take place that has been termed potential sputtering. In these cases the potential energy stored in multiply charged ions (i.e., the energy necessary to produce an ion of this charge state from its neutral atom) is liberated when the ions recombine during impact on a solid surface (formation of hollow atoms). This sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can already take place at ion impact energies well below the physical sputtering threshold. Potential sputtering has only been observed for certain target species and requires a minimum potential energy. | 7 | Physical Chemistry |
Delivery of DDRNAI DNA constructs is simplified by the existence of several clinically approved and well-characterized gene therapy vectors developed for the purpose. Delivery is a major challenge for RNAi-based therapeutics with new modifications and reagents continually being developed to optimize target cell delivery. Two broad strategies to facilitate the delivery of DNA constructs to the desired cells are available: these use either viral vectors or one of several classes of transfection reagents.
In vivo delivery of DDRNAI constructs has been demonstrated using a range of vectors and reagents with different routes of administration (ROA).
DDRNAI constructs have also been successfully delivered into host cells ex vivo, and then transplanted back into the host.
For example, in phase I clinical trial at the City of Hope National Medical Center, California, US, four HIV-positive patients with non-Hodgkin's lymphoma were successfully treated with autologous hematopoietic progenitor cells pre-transduced ex vivo with DDRNAI constructs using lentiviral vectors. This construct was designed to express three therapeutic RNAs, one of which was a shRNA, thereby combating HIV replication in three different ways:
*shRNA, that silences the tat and rev genes of the HIV genome
*CCR5 ribozyme, inhibiting viral cell entry
*TAR decoy RNA, inhibiting initiation of viral transcription.
Ongoing expression of the shRNA has been confirmed in T cells, monocytes, and B cells more than one year after transplantation. | 1 | Biochemistry |
Many calculational studies have been used to compare the feasibility of different proposed geometries. Using the quantum semi-empirical method of MINDO/3, researchers were not able to conclude which geometry of the 2-norbornyl cation was most energetically favorable. However, the classical structure was found to be the only potential minimum for the alkyl-substituted 2-methyl-2-norbornyl cation. Additional calculations using Extended Hückel Theory for Molecular Orbitals were found to favor the non-classical geometry of the cation with reflectional symmetry. | 7 | Physical Chemistry |
Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide structure. The most studied varieties are
* carbonate green rust – GR(): [()] · [·2].
* chloride green rust – GR(): [()] · [·n].
* sulfate green rust – GR(): [()] · [·2].
Other varieties reported in the literature are bromide , fluoride , iodide , nitrate , and selenate.
Green rust was first recognized as a corrosion crust on iron and steel surfaces. It occurs in nature as the mineral fougerite. | 8 | Metallurgy |
In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that leave it unchanged. In three dimensions, space groups are classified into 219 distinct types, or 230 types if chiral copies are considered distinct. Space groups are discrete cocompact groups of isometries of an oriented Euclidean space in any number of dimensions. In dimensions other than 3, they are sometimes called Bieberbach groups.
In crystallography, space groups are also called the crystallographic or Fedorov groups, and represent a description of the symmetry of the crystal. A definitive source regarding 3-dimensional space groups is the International Tables for Crystallography . | 4 | Stereochemistry |
This theoretical paradox is directed at the then-mainstream strand of belief in a classical view of a sempiternal universe, whereby its matter is postulated as everlasting and having always been recognisably the universe. Heat death paradox is born of a paradigm resulting from fundamental ideas about the cosmos. It is necessary to change the paradigm to resolve the paradox.
The paradox was based upon the rigid mechanical point of view of the second law of thermodynamics postulated by Rudolf Clausius and Lord Kelvin, according to which heat can only be transferred from a warmer to a colder object. It notes: if the universe were eternal, as claimed classically, it should already be cold and isotropic (its objects should have the same temperature, and the distribution of matter or radiation should be even). Kelvin compared the universe to a clock that runs slower and slower, constantly dissipating energy in impalpable heat, although he was unsure whether it would stop for ever (reach thermodynamic equilibrium). According to this model, the existence of usable energy, which can be used to perform work and produce entropy, means that the clock has not stopped - since a conversion of heat in mechanical energy (which Kelvin called a rejuvenating universe scenario) is not contemplated.
According to the laws of thermodynamics, any hot object transfers heat to its cooler surroundings, until everything is at the same temperature. For two objects at the same temperature as much heat flows from one body as flows from the other, and the net effect is no change. If the universe were infinitely old, there must have been enough time for the stars to cool and warm their surroundings. Everywhere should therefore be at the same temperature and there should either be no stars, or everything should be as hot as stars. The universe should thus achieve, or asymptotically tend to, thermodynamic equilibrium, which corresponds to a state where no thermodynamic free energy is left, and therefore no further work is possible: this is the heat death of the universe, as predicted by Lord Kelvin in 1852. The average temperature of the cosmos should also asymptotically tend to Kelvin Zero, and it is possible that a maximum entropy state will be reached. | 7 | Physical Chemistry |
Even once the DNA duplex structure was solved, it was initially an open question whether additional DNA structures were needed to explain its overall topology. there were initially questions about how it might affect DNA replication. In 1963, autoradiographs of the E. coli chromosome demonstrated that it was a single circular molecule that is replicated at a pair of replication forks at which both new DNA strands are being synthesized. The two daughter chromosomes after replication would therefore be topologically linked. The separation of the two linked daughter DNA strands during replication either required DNA to have a net-zero helical twist, or for the strands to be cut, crossed, and rejoined. It was this apparent contradictions that early non-helical models attempted to address until the discovery of topoisomerases in 1970 resolved the problem.
In the 1960s and 1970s, a number of structures were hypothesised that would give a net-zero helical twist over the length of the DNA, either by being fully straight throughout or by alternating right-handed and left-handed helical twists. For example, in 1969, a linear tetramer structure was hypothesised, and in 1976, a structure with alternating sections of right-handed and left-handed helix was independently proposed by two different groups. The alternating twists model was initially presented with the helicity changing every half turn, but later long stretches of each helical direction were later proposed. However, these models suffered from a lack of experimental support. Under torsional stress, a Z-DNA structure can form with opposite twist to B-form DNA, but this is rare within the cellular environment. The discovery of topoisomerases and gyrases, enzymes that can change the linking number of circular nucleic acids and thus "unwind" and "rewind" the replicating bacterial chromosome, solved the topological objections to the B-form DNA helical structure. Indeed, in the absence of these topology-altering enzymes, small circular viral and plasmid DNA are inseparable supporting structure whose strands are topologically locked together.
Non-helical DNA proposals have therefore dropped from mainstream science. | 1 | Biochemistry |
Mitogens act primarily by influencing a set of proteins which are involved in the restriction of progression through the cell cycle. The G1 checkpoint is controlled most directly by mitogens: further cell cycle progression does not need mitogens to continue. The point where mitogens are no longer needed to move the cell cycle forward is called the "restriction point" and depends on cyclins to be passed. One of the most important of these is TP53, a gene which produces a family of proteins known as p53. It, combined with the Ras pathway, downregulate cyclin D1, a cyclin-dependent kinase, if they are not stimulated by the presence of mitogens. In the presence of mitogens, sufficient cyclin D1 can be produced. This process cascades onwards, producing other cyclins which stimulate the cell sufficiently to allow cell division. While animals produce internal signals that can drive the cell cycle forward, external mitogens can cause it to progress without these signals. | 1 | Biochemistry |
If two substituents on an atom are geometric isomers of each other, the Z-isomer has higher priority than the E-isomer. A stereoisomer that contains two higher priority groups on the same face of the double bond (cis) is classified as "Z." The stereoisomer with two higher priority groups on opposite sides of a carbon-carbon double bond (trans) is classified as "E." | 4 | Stereochemistry |
Compounds that contain the cyanate functional group, −O−C≡N, are known as cyanates or cyanate esters. Aryl cyanates such are phenyl cyanate, can be formed by a reaction of phenol with cyanogen chloride, ClCN, in the presence of a base.
Organic compounds that contain the isocyanate functional group −N=C=O are known as isocyanates. It is conventional in organic chemistry to write isocyanates with two double bonds, which accords with a simplistic valence bond theory of the bonding. In nucleophilic substitution reactions cyanate usually forms an isocyanate. Isocyanates are widely used in the manufacture of polyurethane products and pesticides; methyl isocyanate, used to make pesticides, was a major factor in the Bhopal disaster. | 0 | Organic Chemistry |
The van der Waals binding energy can be analyzed by another simple physical picture: modeling the motion of an electron around its nucleus by a three-dimensional simple harmonic oscillator with a potential energy V:
where m and ω are the mass and vibrational frequency of the electron, respectively.
As this atom approaches the surface of a metal and forms adsorption, this potential energy V will be modified due to the image charges by additional potential terms which are quadratic in the displacements:
: (from the Taylor expansion above.)
Assuming
the potential is well approximated as
where
If one assumes that the electron is in the ground state, then the van der Waals binding energy is essentially the change of the zero-point energy:
This expression also shows the nature of the Z dependence of the van der Waals interaction.
Furthermore, by introducing the atomic polarizability,
the van der Waals potential can be further simplified:
where
is the van der Waals constant which is related to the atomic polarizability.
Also, by expressing the fourth-order correction in the Taylor expansion above as (aCZ) / (Z), where a is some constant, we can define Z as the position of the dynamical image plane and obtain
The origin of Z comes from the spilling of the electron wavefunction out of the surface. As a result, the position of image plane representing the reference for the space coordinate is different from the substrate surface itself and modified by Z.
Table 1 shows the jellium model calculation for van der Waals constant C and dynamical image plane Z of rare gas atoms on various metal surfaces. The increasing of C from He to Xe for all metal substrates is caused by the larger atomic polarizability of the heavier rare gas atoms. For the position of the dynamical image plane, it decreases with increasing dielectric function and is typically on the order of 0.2 Å. | 7 | Physical Chemistry |
There have been reports of large magnetoelectric coupling at room-temperature in type-I multiferroics such as in the "diluted" magnetic perovskite (PbZrTiO)–(PbFeTaO) (PZTFT) in certain Aurivillius phases. Here, strong ME coupling has been observed on a microscopic scale using PFM under magnetic field among other techniques. Organic-inorganic hybrid multiferroics have been reported in the family of metal-formate perovskites, as well as molecular multiferroics such as [(CH)NH][Ni(HCOO)], with elastic strain-mediated coupling between the order parameters. | 7 | Physical Chemistry |
The criterion for aromaticity has evolved as new developments and insights continue to contribute to our understanding of these remarkably stable organic molecules. The required characteristics of these molecules has thus remained the subject of some controversy. Classically, aromatic compounds were defined as planar molecules that possess a cyclically delocalized system of (4n+2)π electrons, satisfying Huckel's rule. Most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability relative to predictions based on localized resonance structures. Succinctly, three important features seem to characterize aromatic compounds:
#molecular structure (i.e. coplanarity: all contributing atoms in the same plane)
#molecular energetics (i.e. increased thermodynamic stability)
#spectroscopic and magnetic properties (i.e. magnetic field induced ring current)
A number of exceptions to these conventional rules exist, however. Many molecules, including Möbius 4nπ electron species, pericyclic transition states, molecules in which delocalized electrons circulate in the ring plane or through σ (rather than π) bonds, many transition-metal sandwich molecules, and others have been deemed aromatic though they somehow deviate from the conventional parameters for aromaticity.
Consequently, the criterion for homoaromatic delocalization remains similarly ambiguous and somewhat controversial. The homotropylium cation, (CH), though not the first example of a homoaromatic compound ever discovered, has proven to be the most studied of the compounds classified as homoaromatic, and is therefore often considered the classic example of homoaromaticity. By the mid-1980s, there were more than 40 reported substituted derivatives of the homotropylium cation, reflecting the importance of this ion in formulating our understanding of homoaromatic compounds. | 7 | Physical Chemistry |
Biomaterials can be derived either from nature or synthesized in the laboratory using a variety of chemical approaches utilizing metallic components, polymers, ceramics or composite materials. They are often used and/or adapted for a medical application, and thus comprise the whole or part of a living structure or biomedical device which performs, augments, or replaces a natural function. Such functions may be relatively passive, like being used for a heart valve, or maybe bioactive with a more interactive functionality such as hydroxy-apatite coated hip implants. Biomaterials are also commonly used in dental applications, surgery, and drug delivery. For example, a construct with impregnated pharmaceutical products can be placed into the body, which permits the prolonged release of a drug over an extended period of time. A biomaterial may also be an autograft, allograft or xenograft used as a transplant material. | 1 | Biochemistry |
The gene is located on the short arm of chromosome 2 (2p13.2) on the plus (Watson) strand. It is 224,161 bases in length organised into 23 exons. The encoded protein has 4,167 amino acids and molecular weight of 460,937 Da. Three isoforms are known. The protein itself has a large tandem-repeat domain comprising 34 imperfect repetitions of 47 amino acids. Mutations associated with disease are usually found in exons 8, 10 and 16.
The gene is expressed in fetal tissues including the aorta, brain, eye, kidney, liver, lung, olfactory bulb, pancreas, skeletal muscle, spleen and testis. The protein is found in the cytoplasm, centrosome, cell projections and cilium basal body. During mitosis it localizes to both spindle poles. | 1 | Biochemistry |
There are some main areas of focus that can be looked at for the future of SVA to keep improving and being innovational in technology. Firstly, the chambers in which SVA takes place should continue to be improved on to allow precision of the process, as well as, reproducibility of the same structure on each attempt. The focus on these chambers and the components that make it precise have been a hypothetical thought process of what parameters affect reproducibility. It is imperative to continue to improve the amount of control over the annealing through being able to control all factors, such as humidity and temperature. The point of being meticulous in defining such parameters is for the possibility of multiple labs reproducing a certain compound to the same effect.
Next off, SVA with the improvement of the apparatus in which the process takes place, in situ studies, through X-ray and neutron scattering methods, can give more highly accurate images of the swollen and dried states of the BCP. Using methods such as also ellipsometry and interferometry can lead to discoveries about the thickness of the polymers in different states and nanostructure orientation, which will help to learn more about the equilibrium structure and the kinetics of developing a specified morphology. It is important here as well to be able to define small molecule additions to different parts of the block polymer at different points of the annealing and evaporation as to accurately be able to precisely know how the moieties will create certain orientations and directionality in structure. The final area moving forward is simply the implementation of the created block polymers in new intended applications and technology, beyond lab study and characterization of the method. It is important to go beyond creating the nanostructures and move into seeing the utility of the structures in an application, which will help reveal practical shortcomings of the created polymers and reveal areas of where to improve in parts of the structure, such as film integrity and attachment strength of the amorphous chains. Going beyond these simple surface imaging will allow us to realize and face some of the dangers and hindrances to functionality, such as the toxicity of working with organic solvents or the issues with dewetting the swollen state of the BCP. | 7 | Physical Chemistry |
The appearance of oxhide ingots in the archaeological record corresponds with the beginning of the bulk copper trade in the Mediterranean—approximately 1600 BC. The earliest oxhide ingots found come from Crete and date to the Late Minoan IB, approximately 1500 BC to 1450 BC. The latest oxhide ingots date to approximately 1000 BC, and were found on Sardinia. The copper trade was largely maritime: the principal sites where oxhide ingots are found are at sea, on the coast, and on islands. | 8 | Metallurgy |
Copper was the first metal in common use for tools and containers since it is one of the few metals available in non-oxidized form, not requiring the smelting of an ore. Copper is easily softened by heating and then cooling (it does not harden by quenching, e.g., quenching in cool water). In this annealed state it may then be hammered, stretched and otherwise formed, progressing toward the desired final shape but becoming harder and less ductile as work progresses. If work continues beyond a certain hardness the metal will tend to fracture when worked and so it may be re-annealed periodically as shaping continues. Annealing is stopped when the workpiece is near its final desired shape, and so the final product will have a desired strength and hardness. The technique of repoussé exploits these properties of copper, enabling the construction of durable jewelry articles and sculptures (such as the Statue of Liberty). | 8 | Metallurgy |
Sandwich assays are generally used for larger analytes because they tend to have multiple binding sites. As the sample migrates through the assay it first encounters a conjugate, which is an antibody specific to the target analyte labelled with a visual tag, usually colloidal gold. The antibodies bind to the target analyte within the sample and migrate together until they reach the test line. The test line also contains immobilized antibodies specific to the target analyte, which bind to the migrated analyte bound conjugate molecules. The test line then presents a visual change due to the concentrated visual tag, hence confirming the presence of the target molecules. The majority of sandwich assays also have a control line which will appear whether or not the target analyte is present to ensure proper function of the lateral flow pad.
The rapid, low-cost sandwich-based assay is commonly used for home pregnancy tests which detect human chorionic gonadotropin, hCG, in the urine of pregnant women. | 1 | Biochemistry |
In 1931, together with Henri Devaux, Pockels received the Laura R. Leonard Prize from the . In the following year, the Braunschweig University of Technology granted her an honorary doctorate, being the first woman to receive such an award.
Using an improved version of this slide trough, American chemist Irving Langmuir made additional discoveries on the properties of surface molecules, which earned him a Nobel Prize in chemistry in 1932. Pockels' device is a direct antecedent of the Langmuir–Blodgett trough, developed later by Langmuir and physicist Katharine Blodgett.
Since 1993, the Technical University Braunschweig has awarded the Agnes Pockels Medal to people who have advanced the Technical University Braunschweig, emphasizing those who have promoted teaching and research especially women.
The Agnes Pockels Laboratory was established at the Technical University Braunschweig in 2002. Its purpose is to foster chemical education, aid chemistry teachers, and encourage young children in the pursuit of the natural sciences. It emphasizes children under the age of 10, especially girls. In this laboratory, children experience learning-by-doing, much like a craft, without theoretical knowledge or advanced instrumentation.
Lord Rayleigh kept the correspondence he received from Pockels and her brother Friedrich Pockels. Pockels maintained a diary. These documents and photographs of the Pockels family were retained in the library of the Royal Society of Chemistry at Burlington House. On the occasion of Pockels' 70th birthday in 1932, German colloid chemist Wolfgang Ostwald published a biography of Pockels. The Ostwald article includes a list of her publication and a summary of several of them. It also includes autobiographical passages from Pockels. Her sister-in-law Elisabeth (the wife of Friedrich Pockels) also published a biography of Pockels, emphasizing her personal life. | 7 | Physical Chemistry |
Greenhouse gases are infrared active, meaning that they absorb and emit infrared radiation in the same long wavelength range as what is emitted by the Earth's surface, clouds and atmosphere.
99% of the Earths dry atmosphere (excluding water vapor) is made up of nitrogen () (78%) and oxygen () (21%). Because their molecules contain two atoms of the same element, they have no asymmetry in the distribution of their electrical charges, and so are almost totally unaffected by infrared thermal radiation, with only an extremely minor effect from collision-induced absorption. A further 0.9% of the atmosphere is made up by argon (Ar), which is monatomic, and so completely transparent to thermal radiation. On the other hand, carbon dioxide (0.04%), methane, nitrous oxide and even less abundant trace gases account for less than 0.1% of Earths atmosphere, but because their molecules contain atoms of different elements, there is an asymmetry in electric charge distribution which allows molecular vibrations to interact with electromagnetic radiation. This makes them infrared active, and so their presence causes greenhouse effect. | 2 | Environmental Chemistry |
Ring-closing Metathesis has shown utility in the synthesis of 5-30 membered rings, polycycles, and heterocycles containing atoms such as N, O, S, P, and even Si. Due to the functional group tolerance of modern RCM reactions, the synthesis of structurally complex compounds containing a range of functional groups such as epoxides, ketones, alcohols, ethers, amines, amides, and many others can be achieved more easily than previous methods. Oxygen and nitrogen heterocycles dominate due to their abundance in natural products and pharmaceuticals. Some examples are shown below (the red alkene indicates C-C bond formed through RCM).
In addition to terminal alkenes, tri- and tetrasubstituted alkenes have been used in RCM reactions to afford substituted cyclic olefin products. Ring-closing metathesis has also been used to cyclize rings containing an alkyne to produce a new terminal alkene, or even undergo a second cyclization to form bicycles. This type of reaction is more formally known as enyne ring-closing metathesis. | 0 | Organic Chemistry |
Attosecond physics typically deals with non-relativistic bounded particles and employs electromagnetic fields with a moderately high intensity ( W/cm).
This fact allows to set up a discussion in a non-relativistic and semi-classical quantum mechanics environment for light-matter interaction. | 7 | Physical Chemistry |
Titanium boride (TiB) is intentionally added to the melt for grain refinement to improve mechanical properties.
Phosphorus is added to the melt hypereutectic alloys for modification of the silicon phase for better mechanical properties. This creates AlP inclusions.
Boron treatment inclusions ( (Ti, V)B ) form when boron is added to the melt to increase conductivity by precipitating vanadium and titanium. | 8 | Metallurgy |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.