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Nitrogen fixation is a chemical process by which molecular nitrogen (), which has a strong triple covalent bond, is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. The nitrogen in air is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or diazotrophy is an important microbe-mediated process that converts dinitrogen (N) gas to ammonia (NH) using the nitrogenase protein complex (Nif).
Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which include some explosives, pharmaceuticals, and dyes.
Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria, such as Azotobacter and Rhizobia, and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NO production by lightning.
All biological reactions involving the process of nitrogen fixation are catalyzed by enzymes called nitrogenases. These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium. | 1 | Biochemistry |
The association of the Vps35-Vps29-Vps26 complex with the cytosolic domains of cargo molecules on endosomal membranes initiates the activation of retrograde trafficking and cargo capture. The nucleation complex is formed through the interaction of VPS complex with GTP-activated Rab7 with clathrin, clathrin-adaptors and various binding proteins.
The SNX-BAR dimer enters the nucleation complex via direct binding or lateral movement on endosomal surface. The increased level of Retromer SNX-BARs causes a conformational switch to a curvature-inducing mode which initiates membrane tubule formation. Once the cargo carriers are matured, the carrier scission is then catalyzed by dynamin-II or EHD1, together with the mechanical forces generated by actin polymerization and motor activity.
The cargo carrier is transported to the TGN by motor proteins such as dynein. Tethering of the cargo carrier to the recipient compartment is thought to lead to the uncoating of the carrier, which is driven by ATP-hydrolysis and Rab7-GTP hydrolysis. Once released from the carrier, the Vps35-Vps29-Vps26 complex and the SNX-BAR dimers get recycled back onto the endosomal membranes. | 1 | Biochemistry |
In contrast to the diversity of 3D structures observed for glycoside hydrolases, glycosyltransferase have a much smaller range of structures. In fact, according to the Structural Classification of Proteins database, only three different folds have been observed for glycosyltransferases Very recently, a new glycosyltransferase fold was identified for the glycosyltransferases involved in the biosynthesis of the NAG-NAM polymer backbone of peptidoglycan. | 0 | Organic Chemistry |
Amidrazones are a class of chemical compounds formally derived from carboxylic acids. Amidrazones can exists in two tautomeric forms: hydrazide imides (RC(=NH)NHNH) and amide hydrazones (RC(NH)=NNH). | 0 | Organic Chemistry |
Cefradine is distributed in the form of capsules containing 250 mg or 500 mg, as a syrup containing 250 mg/5 ml, or in vials for injection containing 500 mg or 1 g.
It is not approved by the FDA for use in the United States. | 4 | Stereochemistry |
One of the properties of polytorotaxanes involves the photoelectronic response when introducing photoactive or electrionic-active units into the mechanically interlocked structures.
For examples, the polyrotaxane structures are capable of enhancing the fluorescence quenching molecules that grafting on the rings and the other molecules at the ends. Amplification of a fluorescence chemosensory can be achieved by using polyrotaxane structure, which enhances the energy migration in the polymer. It was found that a rapid migration of the hole-electron pair to the rotaxanes sites is followed by a rapid combination which leads to the enhancement of the energy migration. In addition, the conductivity of these polyrotaxanes was lower than the parent components.
Also conductive polyrotaxanes can be obtained by employing metal binding in the polyrotaxanes structure. For example, a polyrotaxane containing a conjugated backbone can be synthesized through metal template and electropolymerization. The metal ion binding is reversible when another metal with stronger binding ability is employed to remove the previous ion, which results in the "scaffolding effect reversibility". The free coordination sites and the organic matrix are able to be maintained by the labile scaffolding. | 6 | Supramolecular Chemistry |
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered, even down to near absolute zero, a superconductor has a characteristic critical temperature below which the resistance drops abruptly to zero. An electric current through a loop of superconducting wire can persist indefinitely with no power source.
The superconductivity phenomenon was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. Like ferromagnetism and atomic spectral lines, superconductivity is a phenomenon which can only be explained by quantum mechanics. It is characterized by the Meissner effect, the complete cancelation of the magnetic field in the interior of the superconductor during its transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above . Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. The cheaply available coolant liquid nitrogen boils at and thus the existence of superconductivity at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. | 7 | Physical Chemistry |
The word atropisomer (, , meaning "without turn") was coined in application to a theoretical concept by German biochemist Richard Kuhn for Karl Freudenbergs seminal Stereochemie' volume in 1933. Atropisomerism was first experimentally detected in a tetra substituted biphenyl, a diacid, by George Christie and James Kenner in 1922. Michinori Ōki further refined the definition of atropisomers taking into account the temperature-dependence associated with the interconversion of conformers, specifying that atropisomers interconvert with a half-life of at least 1000 seconds at a given temperature, corresponding to an energy barrier of 93 kJ mol (22 kcal mol ) at 300 K (27 °C). | 4 | Stereochemistry |
When a molecule of chlorophyll absorbs light it is promoted from its ground state to its first singlet excited state. The excited state then has three main fates. Either the energy is; 1. passed to another chlorophyll molecule by Förster resonance energy transfer (in this way excitation is gradually passed to the photochemical reaction centers (photosystem I and photosystem II) where energy is used in photosynthesis (called photochemical quenching)); or 2. the excited state can return to the ground state by emitting the energy as heat (called non-photochemical quenching); or 3. the excited state can return to the ground state by emitting a photon (fluorescence).
In higher plants, the absorption of light continues to increase as light intensity increases, while the capacity for photosynthesis tends to saturate. Therefore, there is the potential for the absorption of excess light energy by photosynthetic light harvesting systems. This excess excitation energy leads to an increase in the lifetime of singlet excited chlorophyll, increasing the chances of the formation of long-lived chlorophyll triplet states by inter-system crossing. Triplet chlorophyll is a potent photosensitiser of molecular oxygen forming singlet oxygen which can cause oxidative damage to the pigments, lipids and proteins of the photosynthetic thylakoid membrane. To counter this problem, one photoprotective mechanism is so-called non-photochemical quenching (NPQ), which relies upon the conversion and dissipation of the excess excitation energy into heat. NPQ involves conformational changes within the light harvesting proteins of photosystem (PS) II that bring about a change in pigment interactions causing the formation of energy traps. The conformational changes are stimulated by a combination of transmembrane proton gradient, the photosystem II subunit S (PsBs) and the enzymatic conversion of the carotenoid violaxanthin to zeaxanthin (the xanthophyll cycle).
Violaxanthin is a carotenoid downstream from chlorophyll a and b within the antenna of PS II and nearest to the special chlorophyll a located in the reaction center of the antenna. As light intensity increases, acidification of the thylakoid lumen takes place through the stimulation of carbonic anhydrase, which in turn converts bicarbonate (HCO) into carbon dioxide causing an influx of CO and inhibiting Rubisco oxygenase activity. This acidification also leads to the protonation of the PsBs subunit of PS II which catalyze the conversion of violaxanthin to zeaxanthin, and is involved in the alteration orientation of the photosystems at times of high light absorption to reduce the quantities of carbon dioxide created and start the non-photochemical quenching, along with the activation of enzyme violaxanthin de-epoxidase which eliminates an epoxide and forms an alkene on a six-member ring of violaxanthin giving rise to another carotenoid known as antheraxanthin. Violaxanthin contains two epoxides each bonded to a six-member ring and when both are eliminated by de-epoxidase the carotenoid zeaxanthin is formed. Only violaxanthin is able to transport a photon to the special chlorophyll a. Antheraxanthin and zeaxanthin dissipate the energy from the photon as heat preserving the integrity of photosystem II. This dissipation of energy as heat is one form of non-photochemical quenching. | 5 | Photochemistry |
Photoredox catalysis exploits [Ru(bpy)] as a sensitizer as a strategy for organic synthesis. Many analogues of [Ru(bpy)] are employed as well. These transformations exploit the redox properties of [Ru(bpy)]* and its reductively quenched derivative [Ru(bpy)]. | 5 | Photochemistry |
One ancient view of the origin of life, from Aristotle until the 19th century, is of spontaneous generation. This theory held that "lower" animals were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Brownes Pseudodoxia Epidemica'. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By the middle of the 19th century, spontaneous generation was considered disproven. | 9 | Geochemistry |
Oxidative addition and reductive elimination are invoked in many catalytic processes in homogeneous catalysis, e.g., hydrogenations, hydroformylations, hydrosilylations, etc. Cross-coupling reactions like the Suzuki coupling, Negishi coupling, and the Sonogashira coupling also proceed by oxidative addition. | 0 | Organic Chemistry |
As the pump and the probe beam must have the same exact frequency, the most convenient solution is for them to come from the same laser. The probe beam can be made of a reflection of the pump beam passed through neutral density filter to reduce its intensity. To fine-tune the frequency of the laser, a diode laser with a piezoelectric transducer that controls the cavity wavelength can be used. Due to photodiode noise, the laser frequency can be swept across the transition and the photodiode reading averaged over many sweeps.
In real atoms, there are sometimes more than two relevant transitions within the sample's Doppler profile (e.g. in alkali atoms with hyperfine interactions). This will generate the apparition of other dips in the absorption feature due to these new resonances in addition to crossover resonances. | 7 | Physical Chemistry |
In industrial processes, the reduction of metal oxides is often effected by a carbothermic reaction, using carbon as a reducing agent. Carbon is available cheaply as coal, which can be rendered to coke. When carbon reacts with oxygen it forms the gaseous oxides carbon monoxide and carbon dioxide, so the thermodynamics of its oxidation is different from that for metals: its oxidation has a more negative ΔG with the higher temperatures (above 700 °C). Carbon can thus serve as reducing agent. Using this property, reduction of metals may be performed as a double redox reaction at relatively low temperature. | 8 | Metallurgy |
Manganese is an essential biological element in all organisms. It is used in many enzymes and proteins. It is essential in plants. | 1 | Biochemistry |
The US states of Montana and Wisconsin, the Czech Republic, Hungary, have banned cyanide mining. The European Commission rejected a proposal for such a ban, noting that existing regulations (see below) provide adequate environmental and health protection. Several attempts to ban gold cyanidation in Romania were rejected by the Romanian Parliament. There are currently protests in Romania calling for a ban on the use of cyanide in mining (see 2013 Romanian protests against the Roșia Montană Project).
In the EU, industrial use of hazardous chemicals is controlled by the so-called Seveso II Directive (Directive 96/82/EC, which replaced the original Seveso Directive (82/501/EEC brought in after the 1976 dioxin disaster. "Free cyanide and any compound capable of releasing free cyanide in solution" are further controlled by being on List I of the Groundwater Directive (Directive 80/68/EEC) which bans any discharge of a size which might cause deterioration in the quality of the groundwater at the time or in the future. The Groundwater Directive was largely replaced in 2000 by the Water Framework Directive (2000/60/EC).
In response to the 2000 Baia Mare cyanide spill, the European Parliament and the Council adopted Directive 2006/21/EC on the management of waste from extractive industries. Article 13(6) requires "the concentration of weak acid dissociable cyanide in the pond is reduced to the lowest possible level using best available techniques", and at most all mines started after 1 May 2008 may not discharge waste containing over 10ppm WAD cyanide, mines built or permitted before that date are allowed no more than 50ppm initially, dropping to 25ppm in 2013 and 10ppm by 2018.
Under Article 14, companies must also put in place financial guarantees to ensure clean-up after the mine has finished. This in particular may affect smaller companies wanting to build gold mines in the EU, as they are less likely to have the financial strength to give these kinds of guarantees.
The industry has come up with a voluntary "Cyanide Code" that aims to reduce environmental impacts with third party audits of a company's cyanide management. | 8 | Metallurgy |
Modular Cloning, or MoClo, is an assembly method introduced in 2011 by Ernst Weber et al., whereby using Type IIS restriction sites, the user can ligate at least six DNA parts together into a backbone in a one-pot reaction. It is a method based on Golden Gate Assembly, where Type IIS restriction enzymes cleave outside of their recognition site to one side, allowing for removal of those restriction sites from the design. This helps eliminate excess base pairs, or scars, from forming between DNA Parts. However, in order to ligate together properly, MoClo utilizes a set of 4-base pair fusion sites, which remain between parts after ligation, forming 4-base pair scars between DNA parts in the final DNA sequence following ligation of two or more parts.
MoClo utilizes a parallel approach, where all constructs from tier-one(level 0 modules) have restriction sites for BpiI on both sides of the inserts. The vector(also known as "destination vector"), where genes will be added, has an outward-facing BsaI restriction site with a drop-out screening cassette. LacZ is a common screening cassette, where it is replaced by the multigene construct on the destination vector. Each tier-one construct and the vector have different overhangs on them yet complementary to the overhang of the next segment, and this determines the layout of the final multigene construct. Golden Gate Cloning usually starts with level 0 modules. However, if the level 0 module is too large, cloning will start from level -1 fragments, which have to be sequenced, to help cloning the large construct. If starting from level -1 fragments, the level 0 modules do not need to be sequenced again, whereas if starting from level 0 modules, the modules must be sequenced. | 1 | Biochemistry |
This puckering leads to a total of 38 distinct basic pyranose conformations: 2 chairs, 6 boats, 6 skew-boats, 12 half-chairs, and 12 envelopes.
These conformers can interconvert with one another; however, each form may have very different relative energy, so a significant barrier to interconversion may be present. The energy of these conformations can be calculated from quantum mechanics; an example of possible glucopyranose interconversions is given.
The conformations of the pyranose ring are superficially similar to that of the cyclohexane ring. However, the specific nomenclature of pyranoses includes reference to the ring oxygen, and the presence of hydroxyls on the ring have distinct effects on its conformational preference. There are also conformational and stereochemical effects specific to the pyranose ring. | 0 | Organic Chemistry |
The Chesapeake Bay watershed has been heavily impacted by natural forces such as erosion, tides, and a history of hurricanes and other storms. Along with environmental factors, the bay has been negatively impacted by humans since being settled in the 17th century, bringing with them problems like pollution, construction, and destruction of the environment. All of these circumstances have made it increasingly difficult for the MMAP to identify potential underwater archaeological sites. As sea levels rise and historically significant areas are sunk and covered in sediment, the MMAP relies on various pieces of equipment to locate these man-made anomalies but also ensure that the material being examined is kept intact. Using marine magnetometers (detects iron/absent space), side-scan sonar,(detects objects on sea floor), along with precise global positioning systems, Langley and the MMAP have been much more successful in locating submerged archaeological sites. After locating the site, Langley and her team have a strict process in order to preserve the site and its contents, allowing more accurate and thorough research to be conducted. The remains of nearly every site have been submerged in saltwater for sometimes centuries, the integrity of shipwrecks and other materials are fragile and careful precaution must be used when working with them. Taking photos and videos, creating maps, and constructing models are all a part of the process of preserving remains. Susan Langley notes herself, “If you have only ten percent of a ship’s hull, you can reconstruct the ship. Construction techniques can tell us about the people who built the vessels, artifacts can tell us about the people who profited from the ship’s trade, and eco-facts—evidence of insect infestation and organic remains, such as seeds, that are preserved in anaerobic, muddy environments—can tell us about the climate and season when a ship sank." Still, the MMAP makes it a point to publish their data and information once a site is officially identified; however, the details of the location are left out to sway would-be looters, who have plagued marine archaeologists for decades. | 2 | Environmental Chemistry |
If oxygen is present, then following glycolysis, the two pyruvate molecules are brought into the mitochondrion itself to go through the Krebs cycle. In this cycle, the pyruvate molecules from glycolysis are further broken down to harness the remaining energy. Each pyruvate goes through a series of reactions that converts it to acetyl coenzyme A. From here, only the acetyl group participates in the Krebs cycle—in which it goes through a series of redox reactions, catalyzed by enzymes, to further harness the energy from the acetyl group. The energy from the acetyl group, in the form of electrons, is used to reduce NAD+ and FAD to NADH and FADH, respectively. NADH and FADH contain the stored energy harnessed from the initial glucose molecule and is used in the electron transport chain where the bulk of the ATP is produced. | 1 | Biochemistry |
Certain M┄H−C interactions are not classified as agostic but are described by the term anagostic. Anagostic interactions are more electrostatic in character. In terms of structures of anagostic interactions, the M┄H distances and M┄H−C angles fall into the ranges 2.3–2.9 Å and 110°–170°, respectively. | 0 | Organic Chemistry |
CCL7 is expressed in many types of cells, including stromal cells, keratinocytes, airway smooth muscle cells, parenchymal cells, fibroblasts and leukocytes and also in tumor cells.
CCL7 mainly acts as a chemoattractant for several leukocytes, including monocytes, eosinophils, basophils, dendritic cells (DCs), neutrophils, NK cells and activated T lymphocytes. Thus, chemotactic factor CCL7 recruits leukocytes to infected tissues to mediate the immune response. Furthermore, CCL7 has an influence to diapedesis and extravasation of leukocytes. The positive effect of CCL7 is mainly observed in monocyte mobilization from bone marrow to blood circulation and in the recruitment of monocytes to sites of inflammation. It was also reported, that CCL7 can also induce neutrophil migration to the inflammatory site by increasing intracellular Ca2+ flux, which is more typical for the CXC chemokine family members.
The speed of immune responses varies depending on the type of the cells. In epithelial cells, fibroblasts, and endothelial cells the response is immediate after the stimulation by proinflammatory cytokines as IL-1β and TNFα. In T lymphocytes the expression of CCL7 occurs after 3–5 days after the stimulation.
CCL7 has been shown to interact with MMP2 by binding CCR2 receptor. | 1 | Biochemistry |
Animal-derived foods that are high in fat and protein are generally AGE-rich and are prone to further AGE formation during cooking. However, only low molecular weight AGEs are absorbed through diet, and vegetarians have been found to have higher concentrations of overall AGEs compared to non-vegetarians. Therefore, it is unclear whether dietary AGEs contribute to disease and aging, or whether only endogenous AGEs (those produced in the body) matter. This does not free diet from potentially negatively influencing AGE, but potentially implies that dietary AGE may deserve less attention than other aspects of diet that lead to elevated blood sugar levels and formation of AGEs. | 1 | Biochemistry |
Villy Sundström (born February 6, 1949) is a Swedish physical chemist known for his work in ultrafast science and molecular photochemistry using time-resolved laser and X-ray spectroscopy techniques. | 5 | Photochemistry |
A cartilage-derived angiogenesis inhibitor is an angiogenesis inhibitor produced from cartilage. Examples include the peptide troponin I and chondromodulin I.
The antiangiogenic effect may be an inhibition of basement membrane degradation.
These inhibitory agents prevent vascular invasion, which is the proliferation of tumor cells in the blood or lymph vessels. They are usually highly expressed in cartilage and within chondrocytes. Their genetic transcription increases upon the expansion of cartilaginous regions.
Recent studies on Troponin I hypothesize that this protein performs its anti-proliferation effect on endothelial cells via interactions with a bFGF receptor. Neighboring studies on other anti-angiogenic factors are evolving, however, the general mechanism of action is still unknown today. | 1 | Biochemistry |
Viral RNA modifications play important roles in interactions with the immune system of host cells. The mA modification of viral RNAs allows for the viruses to escape recognition by the retinoic acid inducible gene-I receptor (RIG-I), in the type 1 IFN response, a crucial pathway of innate immunity. 5' N-7methylguanisone capping and 2-O-methylation also play vital roles for the viral infections. The cap structures help viral RNA to blend in among modified cellular mRNA and avoid triggering immune response systems. | 1 | Biochemistry |
The near-instantaneous quenching of the metal causes the metal to have a near-amorphous crystalline structure, which is very uncharacteristic of a typical crystal. This structure is very similar to liquids, and the only difference between liquids and amorphous solids is the high viscosity of the solid. Solids in general have a crystalline structure instead of an amorphous structure because the crystalline structure has a stronger binding energy. The way a solid can have the irregular spacing between its atoms is when a liquid is cooled below its melting temperature. The reason for this is the molecules do not have enough time to rearrange themselves in a crystalline structure, and therefore stay in the liquid-like structure. | 8 | Metallurgy |
Diffusion hardening is performed by completely surrounding a metal part with the element to be diffused into it in either the solid, liquid, or gas phase depending on the type of diffusion process being performed. The concentration of the diffusing element surrounding the part must be higher than the concentration of the element inside the part, or diffusion will not occur. The metal and the surrounding element must then be heated to a temperature sufficiently high for diffusion to occur. In the case of pack carburizing, the temperature must be 900 °C and the part must be allowed to sit for 12 to 72 hours for the correct amount of diffusion to occur. | 8 | Metallurgy |
A caged protein is a protein that is activated in the presence of a stimulating light source. In most cases, photo-uncaging is the technique revealing the active region of a compound by the process of photolysis of the shielding molecule (‘cage’). However, uncaging the protein requires an appropriate wavelength, intensity, and timing of the light. Achieving this is possible due to the fact that the optical fiber may be modified to deliver specific amounts of light. In addition, short bursts of stimulation allow results similar to the physiological norm. The steps of photostimulation are time independent in that protein delivery and light activation can be done at different times. This is because the two steps are dependent on each other for activation of the protein.
Some proteins are innately photosensitive and function in the presence of light. Proteins known as opsins form the crux of the photosensitive proteins. These proteins are often found in the eye. In addition, many of these proteins function as ion channels and receptors. One example is when a certain wavelength of light is put onto certain channels, the blockage in the pore is relieved and allows ion transduction.
To uncage molecules, a photolysis system is required to cleave the covalent bond. An example system can consist of a light source (generally a laser or a lamp), a controller for the amount of light that enters, a guide for the light, and a delivery system. Often, the design function in such a way that a medium is met between the diffusing light that may cause additional, unwanted photolysis and light attenuation; both being significant problems with a photolysis system. | 1 | Biochemistry |
Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.
Polysaccharides containing sulfate groups can be isolated from algae or obtained by chemical modification.
Polysaccharides are major classes of biomolecules. They are long chains of carbohydrate molecules, composed of several smaller monosaccharides. These complex bio-macromolecules functions as an important source of energy in animal cell and form a structural component of a plant cell. It can be a homopolysaccharide or a heteropolysaccharide depending upon the type of the monosaccharides.
Polysaccharides can be a straight chain of monosaccharides known as linear polysaccharides, or it can be branched known as a branched polysaccharide. | 0 | Organic Chemistry |
The identification of a factor as limiting is possible only in distinction to one or more other factors that are non-limiting. Disciplines differ in their use of the term as to whether they allow the simultaneous existence of more than one limiting factor which (may then be called "co-limiting"), but they all require the existence of at least one non-limiting factor when the terms are used. There are several different possible scenarios of limitation when more than one factor is present. The first scenario, called single limitation occurs when only one factor, the one with maximum demand, limits the System. Serial co-limitation is when one factor has no direct limiting effects on the system, but must be present to increase the limitation of a second factor. A third scenario, independent limitation, occurs when two factors both have limiting effects on the system but work through different mechanisms. Another scenario, synergistic limitation, occurs when both factors contribute to the same limitation mechanism, but in different ways.
In 1905 Frederick Blackman articulated the role of limiting factors as follows: "When a process is conditioned as to its rapidity by several separate factors the rate of the process is limited by the pace of the slowest factor." In terms of the magnitude of a function, he wrote, "When the magnitude of a function is limited by one of a set of possible factors, increase of that factor, and of that one alone, will be found to bring about an increase of the magnitude of the function." | 7 | Physical Chemistry |
Both agents have similar symptoms and method of action to other nerve agents that act on cholinesterase, and treatment remains the same. However, the window for effectively treating second generation V series seizures is shorter, as they rapidly denature the acetylcholinesterase protein in a similar manner to soman, making treatment with the standard nerve gas antidote pralidoxime ineffective unless it is given very soon after exposure. Pre-treatment with pyridostigmine prior to exposure, and treatment with other drugs such as atropine and diazepam after exposure, will reduce symptoms of nerve agent toxicity but may not be sufficient to prevent death if a large dose of nerve agent has been absorbed. In addition to the standard seizures, some of the second generation V series agents are known to cause comas. | 1 | Biochemistry |
In organic synthesis, VCl is used for the oxidative coupling of phenols. For example, it converts phenol into a mixture of 4,4-, 2,4-, and 2,2'-biphenols:
:2 CHOH + 2 VCl → HOCH–CHOH + 2 VCl + 2 HCl | 0 | Organic Chemistry |
In water-borne coatings, an aqueous polymer dispersion creates a film on the substrate once the solvent has evaporated. Surface functionalization of the polymer particles is a key component of a coating formulation allowing control over such properties as dispersion, film formation temperature, and the coating rheology. Dispersing aids often involve steric or electrostatic repulsion of the polymer particles, providing colloidal stability. The dispersing aids adsorb (as in a grafting onto scheme) onto latex particles giving them functionality. The association of other additives, such as thickeners shown in the schematic to the right, with adsorbed polymer material give rise to complex rheological behavior and excellent control over a coating's flow properties. | 7 | Physical Chemistry |
Synthetic crude oil, also known as syncrude, is the output from a bitumen upgrader facility used in connection with oil sand production in Canada. Bituminous sands are mined using enormous (100-ton capacity) power shovels and loaded into even larger (400-ton capacity) dump trucks for movement to an upgrading facility. The process used to extract the bitumen from the sand is a hot water process originally developed by Dr. Karl Clark of the University of Alberta during the 1920s. After extraction from the sand, the bitumen is fed into a bitumen upgrader which converts it into a light crude oil equivalent. This synthetic substance is fluid enough to be transferred through conventional oil pipelines and can be fed into conventional oil refineries without any further treatment. By 2015 Canadian bitumen upgraders were producing over per day of synthetic crude oil, of which 75% was exported to oil refineries in the United States.
In Alberta, five bitumen upgraders produce synthetic crude oil and a variety of other products: The Suncor Energy upgrader near Fort McMurray, Alberta produces synthetic crude oil plus diesel fuel; the Syncrude Canada, Canadian Natural Resources, and Nexen upgraders near Fort McMurray produce synthetic crude oil; and the Shell Scotford Upgrader near Edmonton produces synthetic crude oil plus an intermediate feedstock for the nearby Shell Oil Refinery. A sixth upgrader, under construction in 2015 near Redwater, Alberta, will upgrade half of its crude bitumen directly to diesel fuel, with the remainder of the output being sold as feedstock to nearby oil refineries and petrochemical plants. | 7 | Physical Chemistry |
Dexketoprofen belongs to a class of medicines called NSAIDs. It works by blocking the action of a substance in the body called cyclo-oxygenase, which is involved in the production of chemicals in the body called prostaglandins. Prostaglandins are produced in response to injury or certain diseases and would otherwise go on to cause swelling, inflammation and pain. By blocking cyclo-oxygenase, dexketoprofen prevents the production of prostaglandins and therefore reduces inflammation and pain. Along with peripheral analgesic action, it possesses central analgesic action. | 4 | Stereochemistry |
The bond dipole moment uses the idea of electric dipole moment to measure the polarity of a chemical bond within a molecule. It occurs whenever there is a separation of positive and negative charges.
The bond dipole μ is given by:
The bond dipole is modeled as δ — δ with a distance d between the partial charges δ and δ. It is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional for electric dipole moment vectors.
Chemists often draw the vector pointing from plus to minus. This vector can be physically interpreted as the movement undergone by electrons when the two atoms are placed a distance d apart and allowed to interact, the electrons will move from their free state positions to be localised more around the more electronegative atom.
The SI unit for electric dipole moment is the coulomb–meter. This is too large to be practical on the molecular scale.
Bond dipole moments are commonly measured in debyes, represented by the symbol D, which is obtained by measuring the charge in units of 10 statcoulomb and the distance d in Angstroms. Based on the conversion factor of
10 statcoulomb being 0.208 units of elementary charge, so 1.0 debye results from an electron and a proton separated by 0.208 Å. A useful conversion factor is 1 D = 3.335 64 C m.
For diatomic molecules there is only one (single or multiple) bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D. At one extreme, a symmetrical molecule such as chlorine, , has zero dipole moment, while near the other extreme, gas phase potassium bromide, KBr, which is highly ionic, has a dipole moment of 10.41 D.
For polyatomic molecules, there is more than one bond. The total molecular dipole moment may be approximated as the vector sum of the individual bond dipole moments. Often bond dipoles are obtained by the reverse process: a known total dipole of a molecule can be decomposed into bond dipoles. This is done to transfer bond dipole moments to molecules that have the same bonds, but for which the total dipole moment is not yet known. The vector sum of the transferred bond dipoles gives an estimate for the total (unknown) dipole of the molecule. | 7 | Physical Chemistry |
In SI units, number density is measured in m, although cm is often used. However, these units are not quite practical when dealing with atoms or molecules of gases, liquids or solids at room temperature and atmospheric pressure, because the resulting numbers are extremely large (on the order of 10). Using the number density of an ideal gas at and as a yardstick: is often introduced as a unit of number density, for any substances at any conditions (not necessarily limited to an ideal gas at and ). | 3 | Analytical Chemistry |
Thermal hydraulics (also called thermohydraulics) is the study of hydraulic flow in thermal fluids. The area can be mainly divided into three parts: thermodynamics, fluid mechanics, and heat transfer, but they are often closely linked to each other. A common example is steam generation in power plants and the associated energy transfer to mechanical motion and the change of states of the water while undergoing this process. Thermal-hydraulic analysis can determine important parameters for reactor design such as plant efficiency and coolability of the system.
The common adjectives are "thermohydraulic", "thermal-hydraulic" and "thermalhydraulic". | 7 | Physical Chemistry |
Although the new generation of mTOR inhibitors hold great promise for anticancer therapy and are rapidly moving into clinical trials, there are many important issues that determine their success in the clinic. First of all predictable biomarkers for benefit of these inhibitors are not available. It appears that genetic determinants predispose cancer cells to be sensitive or resistant to these compounds. Tumors that depend on PI3K/mTOR pathway should respond to these agents but it is unclear if compounds are effective in cancers with distinct genetic lesions.
Inhibition of mTOR is a promising strategy for treatment of number of cancers. Limited clinical activity of selective mTORC1 agents have made them unlikely to have impact in cancer treatment. The development of competitive ATP-catalytic inhibitors have the ability to block both mTORC1 and mTORC2. | 1 | Biochemistry |
This reactivity is exploited in the regiospecific preparation of many di- and tri-substituted aromatic compounds. The approach exploits the meta-directing effect of the sulfonic acid group. 2-Chlorotoluene for example can be prepared by chlorination of p-toluenesulfonic acid, followed by hydrolysis. The method is also useful for the preparation of 2,6-dinitroaniline and 2-bromophenol via phenol-2,4-disulfonic acid. | 0 | Organic Chemistry |
There are two major pathways of deficiency of CoQ in humans: reduced biosynthesis, and increased use by the body. Biosynthesis is the major source of CoQ. Biosynthesis requires at least 15 genes, and mutations in any of them can cause CoQ deficiency. CoQ levels also may be affected by other genetic defects (such as mutations of mitochondrial DNA, ETFDH, APTX, FXN, and BRAF, genes that are not directly related to the CoQ biosynthetic process). Some of these, such as mutations in COQ6, can lead to serious diseases such as steroid-resistant nephrotic syndrome with sensorineural deafness. | 1 | Biochemistry |
Biogenic silica (bSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. For example, microscopic particles of silica called phytoliths can be found in grasses and other plants.
Silica is an amorphous metalloid oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising carbonate and phosphate, which occur in nature as crystalline iono-covalent solids (e.g. salts) whose precipitation is dictated by solubility equilibria. Chemically, bSi is hydrated silica (SiO·nHO), which is essential to many plants and animals.
Diatoms in both fresh and salt water extract dissolved silica from the water to use as a component of their cell walls. Likewise, some holoplanktonic protozoa (Radiolaria), some sponges, and some plants (leaf phytoliths) use silicon as a structural material. Silicon is known to be required by chicks and rats for growth and skeletal development. Silicon is in human connective tissues, bones, teeth, skin, eyes, glands and organs. | 1 | Biochemistry |
Rowley entered politics in 1981, where he unsuccessfully contested the Tobago West seat in the general election of that year. To date he has the distinction of being the only People's National Movement candidate to have contested a seat in a General Election in both Tobago and Trinidad. He first served in Parliament as an Opposition Senator from 1987 to 1990 (3rd Parliament). Subsequently, he was appointed as Minister of Agriculture, Land and Marine Resources (4th Parliament), Minister of Planning and Development and Minister of Housing (as cabinet reshuffled) (8th Parliament) and Minister of Trade and Industry (9th Parliament) until he was fired by then Prime Minister Patrick Manning. | 9 | Geochemistry |
The ELISpot and FluoroSpot assays can be used in many research fields: vaccine development, cancer, allergies, monocytes/macrophages/dendritic cells characterization, apolipoproteins analysis, and veterinary research. With the ELISpot, you can study antigen-specific cytokine responses, antibody specific secreting cells, tumor antigens, granzyme B and Perforin release by T cells, vaccine efficacy, epitope mapping, cytotoxic T-cell activity, detection of IL-4, IL-5, and IL-13, vaccine-induced antibody responses, antigen-specific memory B cells, and much more.
More specifically, the T-cell ELISpot assay is used to characterize T-cell subsets. This is because the assay can detect the production of cytokines IFN-y, IL-2, TNF-alpha, IL-4, IL-5, and IL-13. The first three cytokines are produced by Th1 cells, while the last three are produced by Th2 cells. Measuring T-cell responses through cytokine production also makes it possible to study vaccine efficacy.
With T-cell FluoroSpot, you can monitor tumor-infiltrating lymphocytes. You can also analyze the IFN-y cytokine and granzyme B secretion in order to assess cytotoxic T-cell responses. Both of these are used for cancer research.
With B-cell FluoroSpot, vaccine efficacy can also be observed by quantifying the secretion of IgG, IgA, and IgM before and after a vaccination. This analysis of multiple immunoglobulins is made possible because of the fluorescence method used in the FluoroSpot. | 1 | Biochemistry |
Z-HIT, also denoted as ZHIT, Z-HIT relationship , is a bidirectional mathematical tranformation, connecting the two parts of a complex function, - i.e. its modulus and its phase. Z-HIT relations are somewhat similar to the Kramers–Kronig relations, where the real part can be computed from the imaginary part (or vice versa). In contrast to the Kramers–Kronig relations, in the Z-HIT the impedance modulus is computed from the course of the phase angle (or vice versa). The main practical advantage of Z-HIT relationships over Kramers–Kronig relationships is, that the Z-HIT integration limits do not require any extrapolation: instead, an intergration over the experimentally available frequency range provides accurate data.
More specifically, the angular frequency (ω) boundaries for computing one component of the complex function from the other one using the Kramers-Kronig relations, are ω=0 and ω=∞; these boundaries require extrapolation procedures of the measured impedance spectra. Concerning the ZHIT however, the computing of the course of the impedance modulus from the course of the phase shift can be performed within the measured frequency range, without the need of extrapolation. This avoids complications which may arise from the fact that impedance spectra can only be measured in a limited frequency range. Therefore, the Z-HIT-algorithm allows for verification of the stationarity of the measured test object as well as calculating the impedance values using the phase data. The latter property becomes important when drift effects are present in the impedance spectra which had to be detected or even removed when analysing and/or interpreting the spectra.
Z-HIT relations find use in Dielectric spectroscopy and in Electrochemical Impedance Spectroscopy. | 7 | Physical Chemistry |
Competitive antagonists bind to receptors at the same binding site (active site) as the endogenous ligand or agonist, but without activating the receptor. Agonists and antagonists "compete" for the same binding site on the receptor. Once bound, an antagonist will block agonist binding. Sufficient concentrations of an antagonist will displace the agonist from the binding sites, resulting in a lower frequency of receptor activation. The level of activity of the receptor will be determined by the relative affinity of each molecule for the site and their relative concentrations. High concentrations of a competitive agonist will increase the proportion of receptors that the agonist occupies, higher concentrations of the antagonist will be required to obtain the same degree of binding site occupancy. In functional assays using competitive antagonists, a parallel rightward shift of agonist dose–response curves with no alteration of the maximal response is observed.
Competitive antagonists are used to prevent the activity of drugs, and to reverse the effects of drugs that have already been consumed. Naloxone (also known as Narcan) is used to reverse opioid overdose caused by drugs such as heroin or morphine. Similarly, Ro15-4513 is an antidote to alcohol and flumazenil is an antidote to benzodiazepines.
Competitive antagonists are sub-classified as reversible (surmountable) or irreversible (insurmountable) competitive antagonists, depending on how they interact with their receptor protein targets. Reversible antagonists, which bind via noncovalent intermolecular forces, will eventually dissociate from the receptor, freeing the receptor to be bound again. Irreversible antagonists bind via covalent intermolecular forces. Because there is not enough free energy to break covalent bonds in the local environment, the bond is essentially "permanent", meaning the receptor-antagonist complex will never dissociate. The receptor will thereby remain permanently antagonized until it is ubiquitinated and thus destroyed. | 1 | Biochemistry |
Vapor pressure is a measurement of how readily a condensed phase forms a vapor at a given temperature. A substance enclosed in a sealed vessel initially at vacuum (no air inside) will quickly fill any empty space with vapor. After the system reaches equilibrium and the rate of evaporation matches the rate of condensation, the vapor pressure can be measured. Increasing the temperature increases the amount of vapor that is formed and thus the vapor pressure. In a mixture, each substance contributes to the overall vapor pressure of the mixture, with more volatile compounds making a larger contribution. | 7 | Physical Chemistry |
Pesticide use raises a number of environmental concerns. Over 98% of sprayed insecticides and 95% of herbicides reach a destination other than their target species, including non-target species, air, water and soil. Pesticide drift occurs when pesticides suspended in the air as particles are carried by wind to other areas, potentially contaminating them. Pesticides are one of the causes of water pollution, and some pesticides were persistent organic pollutants (now banned), which contribute to soil and flower (pollen, nectar) contamination. Furthermore, pesticide use can adversely impact neighboring agricultural activity, as pests themselves drift to and harm nearby crops that have no pesticide used on them.
In addition, pesticide use reduces biodiversity, contributes to pollinator decline, destroys habitat (especially for birds), and threatens endangered species. Pests can develop a resistance to the pesticide (pesticide resistance), necessitating a new pesticide. Alternatively a greater dose of the pesticide can be used to counteract the resistance, although this will cause a worsening of the ambient pollution problem.
The Stockholm Convention on Persistent Organic Pollutants, banned all persistent pesticides, in particular DDT and other organochlorine pesticides, which were stable and lipophilic, and thus able to bioaccumulate in the body and the food chain. and which spread throughout the planet. Persistent pesticides are no longer used for agriculture, and will not be approved by the authorities. Because the half life in soil is long (for DDT 2–15 years) residues can still be detected in humans at levels 5 to 10 times lower than found in the 1970s.
Pesticides now have to be degradable in the environment. Such degradation of pesticides is due to both innate chemical properties of the compounds and environmental processes or conditions. For example, the presence of halogens within a chemical structure often slows down degradation in an aerobic environment. Adsorption to soil may retard pesticide movement, but also may reduce bioavailability to microbial degraders.
Pesticide contamination in the environment can be monitored through bioindicators such as bee pollinators. | 2 | Environmental Chemistry |
Chemokine receptors associate with G-proteins to transmit cell signals following ligand binding. Activation of G proteins, by chemokine receptors, causes the subsequent activation of an enzyme known as phospholipase C (PLC). PLC cleaves a molecule called phosphatidylinositol (4,5)-bisphosphate (PIP2) into two second messenger molecules known as Inositol triphosphate (IP3) and diacylglycerol (DAG) that trigger intracellular signaling events; DAG activates another enzyme called protein kinase C (PKC), and IP3 triggers the release of calcium from intracellular stores. These events promote many signaling cascades (such as the MAP kinase pathway) that generate responses like chemotaxis, degranulation, release of superoxide anions and changes in the avidity of cell adhesion molecules called integrins within the cell harbouring the chemokine receptor. | 1 | Biochemistry |
It was set up to allow interdisciplinary communication between environmental scientists around the world. It was founded in 1979 in North America. | 2 | Environmental Chemistry |
Intron-mediated enhancement (IME) is the ability of an intron sequence to enhance the expression of a gene containing that intron. In particular, the intron must be present in the transcribed region of the gene for enhancement to occur, differentiating IME from the action of typical transcriptional enhancers. Descriptions of this phenomenon were first published in cultured maize cells in 1987, and the term "intron-mediated enhancement" was subsequently coined in 1990. A number of publications have demonstrated that this phenomenon is conserved across eukaryotes, including humans, mice, Arabidopsis, rice, and C. elegans. However, the mechanism(s) by which IME works are still not completely understood.
When testing to see whether any given intron enhances the expression of a gene, it is typical to compare the expression of two constructs, one containing the intron and one without it, and to express the difference between the two results as a "fold increase" in enhancement. Further experiments can specifically point to IME as the cause of expression enhancement - one of the most common is to move the intron upstream of the transcription start site, removing it from the transcript. If the intron can no longer enhance expression, then inclusion of the intron in the transcript is important, and the intron probably causes IME.
Not all introns enhance gene expression, but those that do can enhance expression between 2– and >1,000–fold relative to an intronless control. In Arabidopsis and other plant species, the [http://korflab.ucdavis.edu/cgi-bin/IMEter_2014/web-imeter2.1.pl IMEter] has been developed to calculate the likelihood that an intron sequence will enhance gene expression. It does this by calculating a score based on the patterns of nucleotide sequences within the target sequence. The position of an intron within the transcript is also important - the closer an intron is to the start (5' end) of a transcript, the greater its enhancement of gene expression. | 1 | Biochemistry |
Partial agonists are any chemical that can bind to a receptor without eliciting the maximum downstream response as compared to the response from a full agonist. A given partial agonist's affinity for a given receptor is also irrelevant to the consequent effect. An example is buprenorphine, a partial opioid receptor agonist used to treat opioid addictions by directly substituting for them without the same strength of effect. | 1 | Biochemistry |
Phylogenetic analyses performed by Zhang et al. (2020) suggest that the luciferses of the Lampyridae, Rhagopthalmidae, and Phenogodidae families diverged from the Elateridae family 205 Mya. According to phylogenetic data, the emergences of these two luciferases appeared even before the families could diverge– indicating their analogous nature due phenotypic convergences. | 1 | Biochemistry |
An immobilized enzyme is an enzyme, with restricted mobility, attached to an inert, insoluble material—such as calcium alginate (produced by reacting a mixture of sodium alginate solution and enzyme solution with calcium chloride). This can provide increased resistance to changes in conditions such as pH or temperature. It also lets enzymes be held in place throughout the reaction, following which they are easily separated from the products and may be used again - a far more efficient process and so is widely used in industry for enzyme catalysed reactions. An alternative to enzyme immobilization is whole cell immobilization. Immobilized enzymes are easily to be handled, simply separated from their products, and can be reused.
Enzymes are bio-catalysts which play an essential role in the enhancement of chemical reactions in cells without being persistently modified, wasted, nor resulting in the loss of equilibrium of chemical reactions. Although the characteristics of enzymes are extremely unique, their utility in the industry is limited due to the lack of re-usability, stability, and high-cost of production. | 4 | Stereochemistry |
Replication timing domains have been shown to be associated with TADs as their boundary is co localized with the boundaries of TADs that are located at either sides of compartments. Insulated neighborhoods, DNA loops formed by CTCF/cohesin-bound regions, are proposed to functionally underlie TADs. | 1 | Biochemistry |
Functional magnetic resonance spectroscopy of the brain (fMRS) uses magnetic resonance imaging (MRI) to study brain metabolism during brain activation. The data generated by fMRS usually shows spectra of resonances, instead of a brain image, as with MRI. The area under peaks in the spectrum represents relative concentrations of metabolites.
fMRS is based on the same principles as in vivo magnetic resonance spectroscopy (MRS). However, while conventional MRS records a single spectrum of metabolites from a region of interest, a key interest of fMRS is to detect multiple spectra and study metabolite concentration dynamics during brain function. Therefore, it is sometimes referred to as dynamic MRS, event-related MRS or time-resolved MRS. A novel variant of fMRS is functional diffusion-weighted spectroscopy (fDWS) which measures diffusion properties of brain metabolites upon brain activation.
Unlike in vivo MRS which is intensively used in clinical settings, fMRS is used primarily as a research tool, both in a clinical context, for example, to study metabolite dynamics in patients with epilepsy, migraine and dyslexia, and to study healthy brains. fMRS can be used to study metabolism dynamics also in other parts of the body, for example, in muscles and heart; however, brain studies have been far more popular.
The main goals of fMRS studies are to contribute to the understanding of energy metabolism in the brain, and to test and improve data acquisition and quantification techniques to ensure and enhance validity and reliability of fMRS studies. | 7 | Physical Chemistry |
Electrostatic interaction: In an aqueous environment, the oppositely charged groups in amino acid side chains within the active site and substrates attract each other, which is termed electrostatic interaction. For example, when a carboxylic acid (R-COOH) dissociates into RCOO and H ions, COO will attract positively charged groups such as protonated guanidine side chain of arginine.
Hydrogen bond: A hydrogen bond is a specific type of dipole-dipole interaction between a partially positive hydrogen atom and a partially negative electron donor that contain a pair of electrons such as oxygen, fluorine and nitrogen. The strength of hydrogen bond depends on the chemical nature and geometric arrangement of each group.
Van der Waals force: Van der Waals force is formed between oppositely charged groups due to transient uneven electron distribution in each group. If all electrons are concentrated at one pole of the group this end will be negative, while the other end will be positive. Although the individual force is weak, as the total number of interactions between the active site and substrate is massive the sum of them will be significant.
Hydrophobic interaction: Non-polar hydrophobic groups tend to aggregate together in the aqueous environment and try to leave from polar solvent. These hydrophobic groups usually have long carbon chain and do not react with water molecules. When dissolving in water a protein molecule will curl up into a ball-like shape, leaving hydrophilic groups in outside while hydrophobic groups are deeply buried within the centre. | 1 | Biochemistry |
The regulation of modafinil as a doping agent has been controversial in the sporting world, with high-profile cases attracting press coverage since several prominent American athletes tested positive for the substance. Some athletes who used modafinil protested that the drug was not on the prohibited list at the time of their offenses. However, the World Anti-Doping Agency (WADA) maintains that modafinil was related to already-banned substances. The Agency added modafinil to its list of prohibited substances on August 3, 2004, ten days before the start of the 2004 Summer Olympics.
Several athletes (such as sprinter Kelli White in 2003, cyclist David Clinger and basketball player Diana Taurasi in 2010, and rower Timothy Grant in 2015) were accused of using modafinil as a performance-enhancing doping agent. Taurasi and another player—Monique Coker, tested at the same lab—were later cleared. Kelli White, who tested positive after her 100m victory at the 2003 World Championships in Paris, was stripped of her gold medals. She claimed that she used modafinil to treat narcolepsy, but the International Association of Athletics Federations (IAAF) ruled that modafinil was a performance-enhancing drug.
The BALCO scandal brought to light an unsubstantiated (but widely published) account of Major League Baseballs all-time leading home-run hitter Barry Bonds supplemental chemical regimen that included modafinil in addition to anabolic steroids and human growth hormone. | 4 | Stereochemistry |
The most common effect is fatigue or somnolence, particularly in older adults, although patients with pre-existing daytime sleepiness and fatigue may experience paradoxical improvement of these symptoms.
Escitalopram has not been shown to affect serial reaction time, logical reasoning, serial subtraction, multitask, or Mackworth Clock task performance. | 4 | Stereochemistry |
Pregnancy rate may be defined in various ways. In the United States, SART and the Centers for Disease Control (and appearing in the table in the Success Rates section above) include statistics on positive pregnancy test and clinical pregnancy rate.
The 2019 summary compiled by the SART the following data for non-donor eggs (first embryo transfer) in the United States:
In 2006, Canadian clinics reported an average pregnancy rate of 35%. A French study estimated that 66% of patients starting IVF treatment finally succeed in having a child (40% during the IVF treatment at the centre and 26% after IVF discontinuation). Achievement of having a child after IVF discontinuation was mainly due to adoption (46%) or spontaneous pregnancy (42%). | 1 | Biochemistry |
Starting with the sulfate and using ammonium hydroxide as the base, depending on the conditions, one obtains the 9-hydrate, the 6-hydrate, or the 4-hydrate of hexol sulfate. These salts form dark brownish-violet or black tabular crystals, with low solubility in water. When treated with concentrated hydrochloric acid, hexol sulfate converts to cis-diaquotetramminecobalt(III) sulfate. In boiling dilute sulfuric acid, hexol sulfate further degrades with evolution of oxygen and nitrogen. | 4 | Stereochemistry |
The first and dominant class of titanium-based catalysts (and some vanadium-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses:
* catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol% (LLDPE resins), and
* catalysts suitable for the synthesis of isotactic 1-alkenes.
The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.
Commercial catalysts are supported by being bound to a solid with a high surface area. Both TiCl and TiCl give active catalysts. The support in the majority of the catalysts is MgCl. A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is microporous spheres of amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the titanium compounds and MgCl are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as Al(CH).
All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with TiCl as the active ingredient and MgCl as a support. Another component of all such catalysts is an organic modifier, usually an ester of an aromatic diacid or a diether. The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts. These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers. | 7 | Physical Chemistry |
In 1995, Smith and his co-workers reported the second total synthesis of (-)-discodermolide. Smith adapted the triply convergent strategy of the Schreiber synthesis. In Smith's strategy, all three fragments shared a common precursor, which was the product of a highly efficient 50g scale five-step conversion from 3-hydroxy-2-methylpropionate with 59% yield. An Evans syn aldol reaction was utilized to obtain the desired stereochemical outcome.
The synthesis of fragment A, alkyl iodide, applied the Evans' acyl oxazolidinone method. After hydroxyl protection and reductive removal of the chiral auxiliary, an iodination was carried out to afford fragment A. The preparation of fragment B began with TBS protection and DIBAL reduction. The resulting aldehyde can be converted to desired Z-trisubstituted vinyl halide with 6:1 selectivity. The key feature of the synthesis of fragment C was the addition of the anion derived from dithiane to benzyl glycidyl ether.
Palladium(0)-mediated crosscoupling of vinyl iodide with the organozinc derivative of alkyl iodide afford product in 66% yield. After a two-step conversion to the corresponding phosphonium salt, Wittig union of phosphonium salt with aldehyde, fragment C, proceeded in 76% yield and good selectivity. The last feature of this synthesis was the titanium-mediated installation of the diene. The smith synthesis of (-)-discodermolide has an overall yield of 2.0% with a longest linear sequence of 29 steps and 42 total steps. | 0 | Organic Chemistry |
The mechanisms of tardigrade desiccation protection were originally thought to result from high levels of the sugar trehalose. Trehalose is used by organisms like yeast to avoid desiccation in dry environments by working with heat shock proteins to keep desiccation-sensitive proteins in solution. However, while tardigrades can accumulate small levels of trehalose, the levels are insufficient to provide protection from extreme conditions. Other molecules which help certain organisms avoid cellular desiccation include late embryogenesis abundant proteins, which provide protection to embryonic cotton seeds. Certain proteins actually responsible for the tardigrade's hardiness, including the cytoplasmic and secreted abundant heat soluble proteins, were discovered when searching for late embryogenesis abundant proteins in tardigrades.
One strategy used by the tardigrade to survive in dry environments is anhydrobiosis. Anhydrobiosis is a process in which an organism can lose nearly all of its water and enter an ametabolic state. | 1 | Biochemistry |
Nanotechnology miniaturizes normal objects, in this case, a guitar. It can be used to create tiny cameras, scales, and covert listening devices. An example of this is smart dust, which can be either a camera or a listening device smaller than a grain of sand. A nanometer is one-billionth of a meter. For comparison, a human hair is about 200,000 nanometers thick. The nano guitar is about as long as one-twentieth of the diameter of a human hair, 10 micrometers or 10,000 nanometers long. Each of the six strings is 50 nanometers wide. The entire guitar is the size of an average red blood cell. The guitar is carved from a grain of crystalline silicon by scanning a laser over a film called a resist. This technique is known as electron-beam lithography.
The guitar strings can be made to vibrate by tiny lasers using an atomic force microscope, in the same way, a guitar player might use a plectrum. The strings vibrate at around 40 000 000 Hz, roughly 15 octaves higher than a normal guitar, which can typically reach up to 1318.510 Hz. Even if its sound were amplified, it could not be detected by the human ear. | 6 | Supramolecular Chemistry |
The Chapman rearrangement is the thermal conversion of aryl N‐arylbenzimidates to the corresponding amides, via intramolecular migration of an aryl group from oxygen to nitrogen. It is named after Arthur William Chapman, who first described it, and is conceptually similar to the Newman–Kwart rearrangement. | 0 | Organic Chemistry |
The Scatchard equation is an equation used in molecular biology to calculate the affinity and number of binding sites of a receptor for a ligand. It is named after the American chemist George Scatchard. | 1 | Biochemistry |
Carbodiimides such as dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) are frequently used for amide bond formation. The reaction proceeds via the formation of a highly reactive O-acylisourea. This reactive intermediate is attacked by the peptide N-terminal amine, forming a peptide bond. Formation of the O-acylisourea proceeds fastest in non-polar solvents such as dichloromethane.
DIC is particularly useful for SPPS since as a liquid it is easily dispensed, and the urea byproduct is easily washed away. Conversely, the related carbodiimide 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) is often used for solution-phase peptide couplings as its urea byproduct can be removed by washing during aqueous work-up.
Carbodiimide activation opens the possibility for racemization of the activated amino acid. Racemization can be circumvented with racemization suppressing additives such as the triazoles 1-hydroxy-benzotriazole (HOBt), and 1-hydroxy-7-aza-benzotriazole (HOAt). These reagents attack the O-acylisourea intermediate to form an active ester, which subsequently reacts with the peptide to form the desired peptide bond. Ethyl cyanohydroxyiminoacetate (Oxyma), an additive for carbodiimide coupling, acts as an alternative to HOAt. | 1 | Biochemistry |
Phosphine imides can be isolated as intermediates in the Staudinger reaction and have also been prepared by the action of hydroxylamine-O-sulfonic acid on phosphines, proceeding via a p-aminophosphonium salt. | 0 | Organic Chemistry |
Electron excitation is the movement of an electron to a higher energy state. This can either be done by photoexcitation (PE), where the original electron absorbs the photon and gains all the photon's energy or by electrical excitation (EE), where the original electron absorbs the energy of another, energetic electron. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to move electrons to a higher energy band. When an excited electron falls back to a lower energy state again, it is called electron relaxation. This can be done by radiation of a photon or giving the energy to a third spectator particle as well.
In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state. The excited state, in general, is in relation to the ground state, where the excited state is at a higher energy level than the ground state. | 7 | Physical Chemistry |
Sulfuryl fluoride (also spelled sulphuryl fluoride) is an inorganic compound with the formula SOF. It is an easily condensed gas and has properties more similar to sulfur hexafluoride than sulfuryl chloride, being resistant to hydrolysis even up to 150 °C. It is neurotoxic and a potent greenhouse gas, but is widely used as a fumigant insecticide to control termites. | 2 | Environmental Chemistry |
When DNA is damaged such as due to a lesion, a complex signal transduction pathway is activated which is responsible for recognizing the damage and instigating the cell's response for repair. Compared to the other lesion repair mechanisms, DDR is the highest level of repair and is employed for the most complex lesions. DDR consists of various pathways, the most common of which are the DDR kinase signaling cascades. These are controlled by phosphatidylinositol 3-kinase-related kinases (PIKK), and range from DNA-dependent protein kinase (DNA-PKcs) and ataxia telangiectasia-mutated (ATM) most involved in repairing DSBs to the more versatile Rad3-related (ATR). ATR is crucial to human cell viability, while ATM mutations cause the severe disorder ataxia-telangiectasia leading to neurodegeneration, cancer, and immunodeficiency. These three DDR kinases all recognize damage via protein-protein interactions which localize the kinases to the areas of damage. Next, further protein-protein interactions and posttranslational modifications (PTMs) complete the kinase activation, and a series of phosphorylation events takes place. DDR kinases perform repair regulation at three levels - via PTMs, at the level of chromatin, and at the level of the nucleus. | 1 | Biochemistry |
The DNA molecule of interest must be incorporated into a hairpin, and attached to a magnetic bead on one end and to an immobile glass surface on the other end. The hairpin is attached to the glass surface via a digoxigenin-antidigoxigenin bond. The magnetic bead is attached to the opposite end via biotin-streptavidin interaction. Such DNA hairpin setup can be made in two ways:
:1) In the case of double-stranded DNA molecules (for whole genome sequencing, or targeted sequencing), the DNA fragment is ligated to a DNA loop at one end and a DNA fork structure, labeled with biotin and digoxigenin at the two ends.
:2) For RNA-seq, the mRNA can be trapped on a poly-T-coated bead, where reverse transcription reaction is performed on the bead to generate a cDNA hairpin. | 1 | Biochemistry |
Capping is a three-step process that utilizes the enzymes RNA triphosphatase, guanylyltransferase, and methyltransferase. Through a series of three steps, the cap is added to the first nucleotides 5 hydroxyl group of the growing mRNA strand while transcription is still occurring. First, RNA 5 triphosphatase hydrolyzes the 5 triphosphate group to make diphosphate-RNA. Then, the addition of GMP by guanylyltransferase produces the guanosine cap. Last, RNA methyltransferase transfers a methyl group to the guanosine cap to yield 7-methylguanosine cap that is attached to the 5' end of the transcript. These three enzymes, collectively called the capping enzymes, are only able to catalyze their respective reactions when attached to RNA polymerase II, an enzyme necessary for the transcription of DNA into pre-mRNA. When this complex of RNA polymerase II and the capping enzymes is achieved, the capping enzymes are able to add the cap to the mRNA while it is produced by RNA polymerase II. | 1 | Biochemistry |
Catholic Bishops in Kenya are among those who have spread a conspiracy theory asserting that HCG forms part of a covert sterilization program, forcing denials from the Kenyan government.
In order to induce a stronger immune response, some versions of human chorionic gonadotropin-based anti-fertility vaccines were designed as conjugates of the β subunit of HCG covalently linked to tetanus toxoid. It was alleged that a non-conjugated tetanus vaccine used in developing countries was laced with a human chorionic gonadotropin-based anti-fertility drug and was distributed as a means of mass sterilization. This charge has been vigorously denied by the World Health Organization (WHO) and UNICEF. Others have argued that a hCG-laced vaccine could not possibly be used for sterilization, since the effects of the anti-fertility vaccines are reversible (requiring booster doses to maintain infertility) and a non-conjugated vaccine is likely to be ineffective. Finally, independent testing of the tetanus vaccine by Kenya's health authorities revealed no traces of the human chorionic gonadotropin hormone. | 1 | Biochemistry |
Permeability is a property of foundry sand with respect to how well the sand can vent, i.e. how well gases pass through the sand. And in other words, permeability is the property by which we can know the ability of material to transmit fluid/gases. The permeability is commonly tested to see if it is correct for the casting conditions. | 8 | Metallurgy |
RNA ladders composed of RNA molecular-weight size markers were initially developed by using the synthetic circle method to produce different-sized markers. This technique was improved upon by inventor Eric T. Kool to use circular DNA vectors as a method for producing RNA molecular-weight size markers. As referred to as the rolling circle method, the improvements of this technique stems from its efficiency in synthesizing RNA oligonucleotides. From the circular DNA template, single-stranded RNA varying in length from 4-1500 bp can be produced without the need for primers and by recycling nucleotide triphosphate. DNA can also be synthesized from the circular template, adding to this technique's versatility. In comparison to runoff transcription, the synthetic circle method produces RNA oligonucleotides without the runoff. In comparison to PCR, the synthetic circle method produces RNA oligonucleotides without the need for polymerase nor a thermal cycler. This method is also cost-efficient in its ability to synthesize grand amounts of product at a lower error rate than machine synthesizers. | 1 | Biochemistry |
Coherent Rabi oscillations may also be driven by two-photon transitions. In this case we consider a system with three atomic energy levels, , , and , where represents a so-called intermediate state with corresponding frequency , and an electromagnetic field with two frequency components:
Now, may be much greater than both and , or , as illustrated in the figure on the right.
A two-photon transition is not the same as excitation from the ground to intermediate state, and then out of the intermediate state to the excited state. Instead, the atom absorbs two photons simultaneously and is promoted directly between the initial and final states. There are two necessary conditions for this two-photon process (also known as a Raman process), to be the dominant model of the light-matter interaction:
In words, the sum of the frequencies of the two photons must be on resonance with the transition between the initial and final states, and the individual frequencies of the photons must be detuned from the intermediate state to initial and final state transitions. If the latter condition is not met and , the dominant process will be one governed by rate equations in which the intermediate state is populated and stimulated and Spontaneous emission events from that state prevent the possibility of driving coherent oscillations between the initial and final states.
We may derive the two-photon Rabi frequency by returning to the equations
which now describe excitation between the ground and intermediate states. We know we have the solution
where is the generalized Rabi frequency for the transition from the initial to intermediate state. Similarly for the intermediate to final state transition we have the equations
Now we plug into the above equation for
Such that, upon solving this equation, we find the coefficient to be proportional to:
This is the effective or two-photon Rabi frequency. It is the product of the individual Rabi frequencies for the and transitions, divided by the detuning from the intermediate state . | 7 | Physical Chemistry |
Triphenylphosphine dichloride, (CH)PCl, is a chlorinating agent widely used in organic chemistry. Applications include the conversion of alcohols and ethers to alkyl chlorides, the cleavage of epoxides to vicinal dichlorides and the chlorination of carboxylic acids to acyl chlorides. | 0 | Organic Chemistry |
Psychrophiles or cryophiles (adj. psychrophilic or cryophilic) are extremophilic organisms that are capable of growth and reproduction in low temperatures, ranging from to . They are found in places that are permanently cold, such as the polar regions and the deep sea. They can be contrasted with thermophiles, which are organisms that thrive at unusually high temperatures, and mesophiles at intermediate temperatures. Psychrophile is Greek for cold-loving, .
Many such organisms are bacteria or archaea, but some eukaryotes such as lichens, snow algae, phytoplankton, fungi, and wingless midges, are also classified as psychrophiles. | 1 | Biochemistry |
Based on structural features and the phylogeny of the essential protein ORF2p, LINEs can be separated into six main groups, referred to as R2, RanI, L1, RTE, I and Jockey. These groups can further be subdivided into at least 28 clades.
In plant genomes, so far only LINEs of the L1 and RTE clade have been reported. Whereas L1 elements diversify into several subclades, RTE-type LINEs are highly conserved, often constituting a single family.
In fungi, Tad, L1, CRE, Deceiver and Inkcap-like elements have been identified, with Tad-like elements appearing exclusively in fungal genomes.
All LINEs encode a least one protein, ORF2, which contains an RT and an endonuclease (EN) domain, either an N-terminal APE or a C-terminal RLE or rarely both. A ribonuclease H domain is occasionally present. Except for the evolutionary ancient R2 and RTE superfamilies, LINEs usually encode for another protein named ORF1, which may contain an Gag-knuckle, a L1-like RRM (), and/or an esterase. LINE elements are relatively rare compared to LTR-retrotransposons in plants, fungi or insects, but are dominant in vertebrates and especially in mammals, where they represent around 20% of the genome. | 1 | Biochemistry |
Most conventional polymers such as polyethylene are electrical insulators, but the development of polymers containing π-conjugated bonds has led to a wealth of polymer-based semiconductors, such as polythiophenes. This has led to many applications in the field of organic electronics. | 7 | Physical Chemistry |
PEA is generally considered safe, and without adverse drug reactions (ADRs) or drug interactions. A 2016 study assessing safety claims in sixteen clinical trials, six case reports/pilot studies and a meta‐analysis of PEA as an analgesic, concluded that for treatment periods up to 49 days, clinical data argued against serious ADRs at an incidence of 1/200 or greater. A 2016 pooled meta-analysis involving twelve studies found that no serious ADRs were registered and/or reported. No data on interactions with PEA have been reported. Based on its mechanism, PEA may be considered likely to interact with other PPAR-α agonists used to treat high triglycerides; this remains unconfirmed. | 1 | Biochemistry |
Carbohydrate-deficient transferrin (CDT, also known as desialotransferrin or asialotransferrin) is a laboratory test used to help detect heavy ethanol consumption. | 1 | Biochemistry |
Peer-reviewed test methods have been published by government agencies and private research organizations. Approved published methods must be used when testing to demonstrate compliance with regulatory requirements. | 2 | Environmental Chemistry |
He was the Eastman Kodak Companys Senior Research Associate from 1986 to 2004, the Director of Technology for IPValue at Xerox from 2004 to 2007, and is currently the CEO of Idealurgy, a knowledge company, which currently serves the needs of Fortune 500 corporations, International Industry Consortia, and major international research universities. Paz-Pujalts patents are seminal to the formation of Frintz.com, an industry changing advertising medium. | 7 | Physical Chemistry |
An Indium gallium nitride (InxGa1-xN) photocatalyst achieved a solar-to-hydrogen efficiency of 9.2% from pure water and concentrated sunlight. The effiency is due to the synergistic effects of promoting hydrogen–oxygen evolution and inhibiting recombination by operating at an optimal reaction temperature (~70 degrees C), powered by harvesting previously wasted infrared light. An STH efficiency of about 7% was realized from tap water and seawater and efficiency of 6.2% in a larger-scale system with a solar light capacity of 257 watts. | 5 | Photochemistry |
Selectivity in nucleophilic additions to chiral aldehydes is often explained by the Felkin–Anh model (see figure). The nucleophile approaches the carbon of the carbonyl group at the Burgi-Dunitz angle. At this trajectory, attack from the bottom face is disfavored due to steric bulk of the adjacent, large, functional group.
The polar Felkin–Anh model is applied in the scenario where X is an electronegative group. The polar Felkin–Anh model postulates that the observed stereochemistry arises due to hyperconjugative stabilization arising from the anti-periplanar interaction between the C-X antibonding σ* orbital and the forming bond.
Improving Felkin–Anh selectivity for organometal additions to aldehydes can be achieved by using organo-aluminum nucleophiles instead of the corresponding Grignard or organolithium nucleophiles. Claude Spino and co-workers have demonstrated significant stereoselectivity improvements upon switching from vinylgrignard to vinylalane reagents with a number of chiral aldehydes. | 4 | Stereochemistry |
Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose (NaCMC) and gelatin. They are normally combined with some type of sealant, i.e. polyurethane to stick to the skin. | 7 | Physical Chemistry |
The limit of detection (LOD or LoD) is the lowest signal, or the lowest corresponding quantity to be determined (or extracted) from the signal, that can be observed with a sufficient degree of confidence or statistical significance. However, the exact threshold (level of decision) used to decide when a signal significantly emerges above the continuously fluctuating background noise remains arbitrary and is a matter of policy and often of debate among scientists, statisticians and regulators depending on the stakes in different fields. | 3 | Analytical Chemistry |
Polyelectrolytes have many applications, mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they can be used to destabilize a colloidal suspension and to initiate flocculation (precipitation). They can also be used to impart a surface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used as thickeners, emulsifiers, conditioners, clarifying agents, and even drag reducers. They are used in water treatment and for oil recovery. Many soaps, shampoos, and cosmetics incorporate polyelectrolytes. Furthermore, they are added to many foods and to concrete mixtures (superplasticizer). Some of the polyelectrolytes that appear on food labels are pectin, carrageenan, alginates, and carboxymethyl cellulose. All but the last are of natural origin. Finally, they are used in various materials, including cement.
Because some of them are water-soluble, they are also investigated for biochemical and medical applications. There is currently much research on using biocompatible polyelectrolytes for implant coatings, controlled drug release, and other applications. Thus, recently, the biocompatible and biodegradable macroporous material composed of polyelectrolyte complex was described, where the material exhibited excellent proliferation of mammalian cells and muscle like soft actuators. | 7 | Physical Chemistry |
The half-Heusler compounds have distinctive properties and high tunability which makes the class very promising as thermoelectric materials. A study has predicted that there can be as many as 481 stable half-Heusler compounds using high-throughput ab initio calculation combine with machine learning techniques. The particular half-Heusler compounds of interest as thermoelectric materials (space group ) are the semiconducting ternary compounds with a general formula XYZ where X is a more electropositive transition metal (such as Ti or Zr), Y is a less electropositive transition metal (such Ni or Co), and Z is heavy main group element (such as Sn or Sb). This flexible range of element selection allows many different combinations to form a half-Heusler phase and enables a diverse range of material properties.
Half-Heusler thermoelectric materials have distinct advantages over many other thermoelectric materials; low toxicity, inexpensive element, robust mechanical properties, and high thermal stability make half-Heusler thermoelectrics an excellent option for mid-high temperature application. However, the high thermal conductivity, which is intrinsic to highly symmetric HH structure, has made HH thermoelectric generally less efficient than other classes of TE materials. Many studies have focused on improving HH thermoelectric by reducing the lattice thermal conductivity and zT > 1 has been repeatedly recorded. | 8 | Metallurgy |
*1986 – Weissberger-Williams lectureship Award
*1987 – Korean Chemical Society Award
*1989 – Chemical Society of Japan Award
*1995 – DowElanco lectureship Award
*2000 – The H. C. Brown Lecture Award
*2003 – Japan Academy Prize
*2009 – Paul Karrer Gold Medal
*2009 – Special Member of Royal Society of Chemistry (RSC)
*2010 – Nobel Prize in Chemistry
*2010 – Order of Culture
*2010 – Person of Cultural Merit
*2011 – Member of the Japan Academy
*2011 – honored on a stamp issued by Republic of the Congo
*2016 – Honorary chair professorship, National Cheng Kung University | 0 | Organic Chemistry |
In polymer science, the polymer chain or simply backbone of a polymer is the main chain of a polymer. Polymers are often classified according to the elements in the main chains. The character of the backbone, i.e. its flexibility, determines the properties of the polymer (such as the glass transition temperature). For example, in polysiloxanes (silicone), the backbone chain is very flexible, which results in a very low glass transition temperature of . The polymers with rigid backbones are prone to crystallization (e.g. polythiophenes) in thin films and in solution. Crystallization in its turn affects the optical properties of the polymers, its optical band gap and electronic levels. | 0 | Organic Chemistry |
Prior to World War I, work on synthetic organic chemistry in the United States had been quite limited, and most of the reagents used in laboratories had to be imported from Europe. When export stoppages and trade embargoes cut off this source, Clarence Derick, a professor of chemistry at University of Illinois at Urbana-Champaign, began an effort to synthesize these needed chemicals in industrial quantities in a university laboratory with the help of a few graduate students. This work was performed during the summer break and came to be known as the "summer prep". Students who worked in the laboratory were paid and received credit.
The basic procedures were often obtained from textbooks, and the procedures were sketchy. Reproducibility was important in summer preps, so students were required to keep meticulous record books. The procedures were finally collected and published for the first time in a four-pamphlet set called Organic Chemical Reagents, which quickly sold out. The publishers received submissions from other chemists, which spawned the idea for serial publication, and the first annual volume of Organic Syntheses was thus published in 1921. By then, chemists from other universities and industry were also contributing.
One example of much needed chemicals were dyes for sensitizing photographic film. Research efforts in this field led to the foundation of Eastman Kodak Organic Chemicals Division.
The summer preps contributed to the war effort in World War II but were discontinued in 1950 because by then an infrastructure of chemical companies with their own research had been established.
Until 1998, Organic Syntheses was published only as an annual printed volume. In that year, all past volumes were made available on an open access website and new articles are now published online as soon as they are accepted. | 0 | Organic Chemistry |
In 2020 a spectroscopic analysis was reported to show signs of phosphine in the atmosphere of Venus in quantities that could not be explained by known abiotic processes. Later re-analysis of this work showed interpolation errors had been made, and re-analysis of data with the fixed algorithm do not result in the detection of phosphine. The authors of the original study then claimed to detect it with a much lower concentration of 1 ppb. | 0 | Organic Chemistry |
Detailed water models predict the occurrence of water clusters, as configurations of water molecules whose total energy is a local minimum.
Of particular interest are the cyclic clusters (HO); these have been predicted to exist for n = 3 to 60.
Research shows experimental result of the size of water cluster with 20 water molecules of 0.822 nm Research uses of graph invariants for efficiently generating hydrogen bond topologies and predicting physical properties of water clusters and ice. The utility of graph invariants is confirmed by considering two water clusters, the (HO) cage and (HO) dodecahedron, which, associated with roughly the same oxygen atom arrangements in solid and liquid phases of water. At low temperatures, nearly 50% of water molecules are included in clusters.
With increasing cluster size the oxygen to oxygen distance is found to decrease which is attributed to so-called cooperative many-body interactions: due to a change in charge distribution the H-acceptor molecule becomes a better H-donor molecule with each expansion of the water assembly. Many isomeric forms seem to exist for the hexamer (HO): from ring, book, bag, cage, to prism shape with nearly identical energy. Two cage-like isomers exist for heptamers (HO), and octamers (HO) are found either cyclic or in the shape of a cube.
Other theoretical studies predict clusters with more complex three-dimensional structures. Examples include the fullerene-like cluster (HO), named the water buckyball, and the 280-water-molecule monster icosahedral network (with each water molecule coordinate to 4 others). The latter, which is 3 nm in diameter, consists of nested icosahedral shells with 280 and 100 molecules. There is also an augmented version with another shell of 320 molecules. There is increased stability with the addition of each shell. There are theoretical models of water clusters of more than 700 water molecules, but they have not been observed experimentally. | 7 | Physical Chemistry |
Peroxide fusion is used to prepare samples for inductively coupled plasma (ICP), atomic absorption (AA) analysis and wet chemistry. Sodium peroxide (NaO) is used to oxidize the sample that becomes soluble in a diluted acid solution. This method allows complete dissolution of numerous refractory compounds like chromite, magnetite, ilmenite, rutile, and even silicon, carbides, alloys, noble metals and materials with high sulfide contents.
Peroxide fusion can be performed either manually or with automated systems. The latter have the advantage of increasing productivity, improving safety, maintaining repeatable preparation conditions, and avoiding spattering as well as cross-contamination. | 3 | Analytical Chemistry |
Insulin is synthesized and secreted in the beta cells of the islets of Langerhans. Once insulin is synthesized, the beta cells are ready to release it in two different phases. As for the first phase, insulin release is triggered rapidly when the blood glucose level is increased. The second phase is a slow release of newly formed vesicles that are triggered regardless of the blood sugar level.
Glucose enters the beta cells and goes through glycolysis to form ATP that eventually causes depolarization of the beta cell membrane (as explained in Insulin secretion section of this article). The depolarization process causes voltage-controlled calcium channels (Ca2+) opening, allowing the calcium to flow into the cells. An increased calcium level activates phospholipase C, which cleaves the membrane phospholipid phosphatidylinositol 4,5-bisphosphate into Inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 binds to receptor proteins in the membrane of the endoplasmic reticulum (ER). This releases (Ca2+) from the ER via IP3 gated channels, and raises the cell concentration of calcium even more. The influx of Ca2+ ions causes the secretion of insulin stored in vesicles through the cell membrane. The process of insulin secretion is an example of a trigger mechanism in a signal transduction pathway because insulin is secreted after glucose enters the beta cell and that triggers several other processes in a chain reaction. | 1 | Biochemistry |
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