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Deuterium is one of only five stable nuclides with an odd number of protons and an odd number of neutrons. (H, Li, B, N, Ta; the long-lived radionuclides K, V, La, Lu also occur naturally.) Most odd-odd nuclei are unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects. Deuterium, however, benefits from having its proton and neutron coupled to a spin-1 state, which gives a stronger nuclear attraction; the corresponding spin-1 state does not exist in the two-neutron or two-proton system, due to the Pauli exclusion principle which would require one or the other identical particle with the same spin to have some other different quantum number, such as orbital angular momentum. But orbital angular momentum of either particle gives a lower binding energy for the system, primarily due to increasing distance of the particles in the steep gradient of the nuclear force. In both cases, this causes the diproton and dineutron nucleus to be unstable.
The proton and neutron making up deuterium can be dissociated through neutral current interactions with neutrinos. The cross section for this interaction is comparatively large, and deuterium was successfully used as a neutrino target in the Sudbury Neutrino Observatory experiment.
Diatomic deuterium (D or H) has ortho and para nuclear spin isomers like diatomic hydrogen, but with differences in the number and population of spin states and rotational levels, which occur because the deuteron is a boson with nuclear spin equal to one. | 9 | Geochemistry |
Divergent evolution of active site residues is slow, due to strong chemical constraints. Nevertheless, some protease superfamilies have evolved from one nucleophile to another. This can be inferred when a superfamily (with the same fold) contains families that use different nucleophiles. Such nucleophile switches have occurred several times during evolutionary history, however the mechanisms by which this happen are still unclear.
Within protease superfamilies that contain a mixture of nucleophiles (e.g. the PA clan), families are designated by their catalytic nucleophile (C=cysteine proteases, S=serine proteases). | 1 | Biochemistry |
In IUPAC nomenclature of chemistry, a pendant group (sometimes spelled pendent) or side group is a group of atoms attached to a backbone chain of a long molecule, usually a polymer. Pendant groups are different from pendant chains, as they are neither oligomeric nor polymeric.
For example, the phenyl groups are the pendant groups on a polystyrene chain.
Large, bulky pendant groups such as adamantyl usually raise the glass transition temperature () of a polymer by preventing the chains from sliding past each other easily. Short alkyl pendant groups may lower the by a lubricant effect. | 0 | Organic Chemistry |
Bird populations are one of many indicators of herbicide damage.Most observed effects are due not to toxicity, but to habitat changes and the decreases in abundance of species on which birds rely for food or shelter. Herbicide use in silviculture, used to favor certain types of growth following clearcutting, can cause significant drops in bird populations. Even when herbicides which have low toxicity to birds are used, they decrease the abundance of many types of vegetation on which the birds rely. Herbicide use in agriculture in the UK has been linked to a decline in seed-eating bird species which rely on the weeds killed by the herbicides. Heavy use of herbicides in neotropical agricultural areas has been one of many factors implicated in limiting the usefulness of such agricultural land for wintering migratory birds. | 2 | Environmental Chemistry |
Quinones and their derivatives are the basis of many organic redox systems. In one study, 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) and 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) were employed as cathodes, and conventional Pb/PbSO was the anolyte in a hybrid acid AORFB. Quinones accept two units of electrical charge, compared with one in conventional catholyte, implying that such a battery could store twice as much energy in a given volume.
Another quinone 9,10-Anthraquinone-2,7-disulfonic acid (AQDS), has been evaluated. AQDS undergoes rapid, reversible two-electron/two-proton reduction on a glassy carbon electrode in sulfuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the / redox couple, yields a peak galvanic power density exceeding 6,000 W/m at 13,000 A/m. Cycling showed > 99% storage capacity retention per cycle. Volumetric energy density was over 20 Wh/L. Anthraquinone-2-sulfonic acid and anthraquinone-2,6-disulfonic acid on the negative side and 1,2-dihydrobenzoquinone- 3,5-disulfonic acid on the positive side avoids the use of hazardous Br. The battery was claimed to last for 1,000 cycles without degradation. While this system appears robust, it has a low cell voltage (ca. 0.55V) and a low energy density (< 4Wh/L).
Hydrobromic acid used as an electrolyte has been replaced with a less toxic alkaline solution (1M KOH) and ferrocyanide. The higher pH is less corrosive, allowing the use of inexpensive polymer tanks. The increased electrical resistance in the membrane was compensated increased voltage. The cell voltage was 1.2V. The cells efficiency exceeded 99%, while round-trip efficiency measured 84%. The battery offered an expected lifetime of at least 1,000 cycles. Its theoretic energy density was 19Wh/L. Ferrocyanides chemical stability in high pH KOH solution without forming Fe(OH) or Fe(OH) needs to be verified before scale-up.
Integrating both anolyte and catholyte in the same molecule has been examined. Such bifunctional analytes or combi-molecules allow the same material to be used in both tanks. In one tank it is an electron donor, while in the other it is an electron recipient. This has relevant advantages such as diminishing the effect of crossover. Thus, quinone diaminoanthraquinone and indigo-based molecules as well as TEMPO/phenazine combining molecules are potential electrolytes for the development of symmetric redox-flow batteries (SRFB).
Another approach adopted a Blatter radical as the donor/recipient. It endured 275 charge and discharge cycles in tests, although it was not water-soluble. | 7 | Physical Chemistry |
Smelting and melting of copper and its alloys such as leaded bronze was done in crucibles similar to those of the Roman period which have thinner walls and flat bases to sit within the furnaces. The technology for this type of smelting started to change at the end of the Medieval period with the introduction of new tempering material for the ceramic crucibles. Some of these copper alloy crucibles were used in the making of bells. Bell foundry crucibles had to be larger at about 60 cm. These later medieval crucibles were a more mass-produced product.
The cementation process, which was lost from the end of the Roman to the early Medieval period, continued in the same way with brass. Brass production increased during the medieval period due to a better understanding of the technology behind it. Furthermore, the process for carrying out cementation for brass did not change greatly until the 19th century.
However, during this period a vast and highly important technological innovation happened using the cementation process, the production of crucible steel. Steel production using iron and carbon works similarly to brass, with the iron metal being mixed with carbon to produce steel. The first examples of cementation steel are wootz steel from India, where the crucibles were filled with good quality low-carbon wrought iron and carbon in the form of organics such as leaves, wood, etc. However, no charcoal was used within the crucible. These early crucibles would only produce a small amount of steel as they would have to be broken once the process has finished.
By the late Medieval period, steel production had moved from India to modern-day Uzbekistan where new materials were being used in the production of steel crucibles, for example, Mullite crucibles were introduced. These were sandy clay crucibles which had been formed around a fabric tube. These crucibles were used in the same way as other cementation vessels but with a hole in the top of the vessel to allow pressure to escape. | 3 | Analytical Chemistry |
The poly(A) tail contains binding sites for poly(A) binding proteins (PABPs). These proteins cooperate with other factors to affect the export, stability, decay, and translation of an mRNA. PABPs bound to the poly(A) tail may also interact with proteins, such as translation initiation factors, that are bound to the 5' cap of the mRNA. This interaction causes circularization of the transcript, which subsequently promotes translation initiation. Furthermore, it allows for efficient translation by causing recycling of ribosomes. While the presence of a poly(A) tail usually aids in triggering translation, the absence or removal of one often leads to exonuclease-mediated degradation of the mRNA. Polyadenylation itself is regulated by sequences within the 3′-UTR of the transcript. These sequences include cytoplasmic polyadenylation elements (CPEs), which are uridine-rich sequences that contribute to both polyadenylation activation and repression. CPE-binding protein (CPEB) binds to CPEs in conjunction with a variety of other proteins in order to elicit different responses. | 1 | Biochemistry |
Calcite changes to calcite-II and calcite-III at pressures of 1.5, and 2.2 GPa. Siderite undergoes a chemical change at 10 GPa at 1800K to form FeO. Dolomite decomposes 7GPa and below 1000 °C to yield aragonite and magnesite. However, there are forms of iron containing dolomite stable at higher pressures and temperatures. Over 130 GPa aragonite undergoes a transformation to a SP tetrahedrally connected carbon, in a covalent network in a C222 structure. Magnesite can survive 80 GPa, but with more than 100 GPa (as at a depth of 1800 km it changes to forms with three-member rings of CO tetrahedra (CO). If iron is present in this mineral, at these pressures it will convert to magnetite and diamond. Melted carbonates with SP carbon are predicted to be very viscous.
Some minerals that contain both silicate and carbonate exist, spurrite and tilleyite. But high-pressure forms have not been studied. There have been attempts to make silicon carbonate. Six coordinated silicates mixed with carbonate should not exist on Earth, but may exist on more massive planets. | 9 | Geochemistry |
In the presence of strong electrostatic fields it is predicted that virtual particles become separated from the vacuum state and form real matter. The fact that electromagnetic radiation can be transformed into matter and vice versa leads to fundamentally new features in quantum electrodynamics. One of the most important consequences is that, even in the vacuum, the Maxwell equations have to be exchanged by more complicated formulas. In general, it will be not possible to separate processes in the vacuum from the processes involving matter since electromagnetic fields can create matter if the field fluctuations are strong enough. This leads to highly complex nonlinear interaction - gravity will have an effect on the light at the same time the light has an effect on gravity. These effects were first predicted by Werner Heisenberg and Hans Heinrich Euler in 1936 and independently the same year by Victor Weisskopf who stated: "The physical properties of the vacuum originate in the "zero-point energy" of matter, which also depends on absent particles through the external field strengths and therefore contributes an additional term to the purely Maxwellian field energy". Thus strong magnetic fields vary the energy contained in the vacuum. The scale above which the electromagnetic field is expected to become nonlinear is known as the Schwinger limit. At this point the vacuum has all the properties of a birefringent medium, thus in principle a rotation of the polarization frame (the Faraday effect) can be observed in empty space.
Both Einstein's theory of special and general relativity state that light should pass freely through a vacuum without being altered, a principle known as Lorentz invariance. Yet, in theory, large nonlinear self-interaction of light due to quantum fluctuations should lead to this principle being measurably violated if the interactions are strong enough. Nearly all theories of quantum gravity predict that Lorentz invariance is not an exact symmetry of nature. It is predicted the speed at which light travels through the vacuum depends on its direction, polarization and the local strength of the magnetic field. There have been a number of inconclusive results which claim to show evidence of a Lorentz violation by finding a rotation of the polarization plane of light coming from distant galaxies. The first concrete evidence for vacuum birefringence was published in 2017 when a team of astronomers looked at the light coming from the star RX J1856.5-3754, the closest discovered neutron star to Earth.
Roberto Mignani at the National Institute for Astrophysics in Milan who led the team of astronomers has commented that "When Einstein came up with the theory of general relativity 100 years ago, he had no idea that it would be used for navigational systems. The consequences of this discovery probably will also have to be realised on a longer timescale." The team found that visible light from the star had undergone linear polarisation of around 16%. If the birefringence had been caused by light passing through interstellar gas or plasma, the effect should have been no more than 1%. Definitive proof would require repeating the observation at other wavelengths and on other neutron stars. At X-ray wavelengths the polarization from the quantum fluctuations should be near 100%. Although no telescope currently exists that can make such measurements, there are several proposed X-ray telescopes that may soon be able to verify the result conclusively such as Chinas Hard X-ray Modulation Telescope (HXMT) and NASAs Imaging X-ray Polarimetry Explorer (IXPE). | 7 | Physical Chemistry |
The Condor Dense Medium Separator (DMS) is a multi-stage, high efficiency media separation machine for mineral processing operations at the rougher and scavenger stage. It is typically used in a pre-concentration duty prior to processing or milling to reject barren material. The unit is manufactured with either two or three stages of separation depending on the media with one or two valuable densities resulting, while the unit can produce up to four products from one dense medium vessel altogether. The Condor DMS can take a larger feed particle size compared to a DMS cyclone of the same diameter and capacity, and is capable of handling higher sinks or floats loading without affecting performance. The valuable dense material (or sinks) can be combined or separated at the final stage and is then pumped onto the next process in the circuit. Sepro Mineral Systems Corp. supplies customizable DMS Plants for a wide variety of application requirements. Sepro's standard two product (concentrate, tailings) DMS Plant utilizes a two-stage Condor Separator and single density medium circuit, while the three product (concentrate, middlings, tailings) DMS Plant utilizes a three-stage Condor Separator and two medium circuits at high and low density. | 8 | Metallurgy |
The Born series can also be written for other scattering quantities like the T-matrix which is closely related to the scattering amplitude.
Iterating Lippmann-Schwinger equation for the T-matrix we get
For the T-matrix stands only for retarded Greens operator . The standing wave Greens operator would give the K-matrix instead. | 7 | Physical Chemistry |
In 1996—early in her career at UMN—Ashe was the first author on a paper published in Science, entitled "Correlative memory deficits, Aβ elevation, and amyloid plaques in transgenic mice", describing a mouse model of Alzheimers disease, which furthered her rising star as a scientist; the mice are used in research around the world, and students and scientists "come from all over the world to work with her", according to the Star Tribune. In 2006, three of her research papers made a list of the eighteen papers that had contributed the most to Alzheimers research.
Ashe is a co-author on a 2006 paper published in Nature, entitled "A specific amyloid-β protein assembly in the brain impairs memory". The paper describes the Aβ*56 oligomer (known as amyloid beta star 56 and Aβ*56) correlating with memory loss in mice prior to the appearance of amyloid plaques. According to a Science article, in 2022 the paper was the fifth-highest cited paper in Alzheimers research, with approximately 2,300 other articles citing it. The Guardian says the paper was "highly influential" and calls it "one of the most cited pieces of Alzheimers disease research in the last two decades", writing that it has "dominated the field" of research. The Daily Telegraph states that the "seminal research paper" led to increased drug research funding worldwide. The paper was discussed at the Alzheimer Research Forum as a "star is born".
In 2015, Ashe was a co-author on a paper entitled "Quaternary structure defines a large class of amyloid-beta oligomers neutralized by sequestration", which defines two forms of Aβ based on quaternary structure, type 1 and type 2, that have different effects on memory function in mice. Type 1 is dispersed in the brain and associated with impaired memory. Type 2 is entrapped in amyloid plaques and does not impair memory. In 2020, she published a review summarizing this work, entitled "The biogenesis and biology of amyloid β oligomers in the brain". | 1 | Biochemistry |
The parent metallacyclopentane has the formula LM(CH). Such compounds are intermediates in the metal catalysed dimerization, trimerization, and tetramerization of ethylene to give but-1-ene, hex-1-ene and oct-1-ene, respectively. Metallacyclopentanes are invoked as intermediates in the evolution of heterogeneous alkene metathesis catalysts from ethylene and metal oxides. Metallacyclopentane intermediates are proposed to isomerize to metallacyclobutanes, which then eliminate propylene giving the alkylidene. | 0 | Organic Chemistry |
A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlated with disease-causing alleles, such as the overactivity of oncogenes which can cause cancer.
A gene is defined as "a hereditary unit of DNA that is required to produce a functional product". Regulatory elements include:
* Promoter region
* TATA box
* Polyadenylation sequences
* Enhancers
These elements work in combination with the open reading frame to create a functional product. This product may be transcribed and be functional as RNA or is translated from mRNA to a protein to be functional in the cell. | 1 | Biochemistry |
As with other procedures predicated on biochemical alteration followed by sequencing, the development of high-throughput sequencing has removed the limitations requiring prior knowledge of sites of interest and primer design. The method causes a lot of RNA degradation, so it is necessary to start with a large amount of sample, or use effective normalisation techniques to account for amplification biases. One final limitation is that, for CMC labelling of pseudouridine to be specific, it is not complete, and therefore nor is it quantitative. A new reactant that could achieve a higher sensitivity with specificity would be beneficial. | 1 | Biochemistry |
Tetrabutyltin is used as an antifouling paint for ships, for the prevention of slimes in industrial recirculating water systems, for combating freshwater snails that cause bilharzia, as a wood and textile preservative, and as a disinfectant. Tricyclohexyltin hydroxide is used as an acaricide. Triphenyltin hydroxide and triphenyltin acetate are used as fungicides. | 1 | Biochemistry |
Biosynthesis by the transsulfuration pathway starts with aspartic acid. Relevant enzymes include aspartokinase, aspartate-semialdehyde dehydrogenase, homoserine dehydrogenase, homoserine O-transsuccinylase, cystathionine-γ-synthase, Cystathionine-β-lyase (in mammals, this step is performed by homocysteine methyltransferase or betaine—homocysteine S-methyltransferase.)
Methionine biosynthesis is subject to tight regulation. The repressor protein MetJ, in cooperation with the corepressor protein S-adenosyl-methionine, mediates the repression of methionine's biosynthesis. The regulator MetR is required for MetE and MetH gene expression and functions as a transactivator of transcription for these genes. MetR transcriptional activity is regulated by homocystein, which is the metabolic precursor of methionine. It is also known that vitamin B12 can repress MetE gene expression, which is mediated by the MetH holoenzyme. | 1 | Biochemistry |
Using ostracod crustaceans as palaeoclimate proxies has been well established for the Quaternary. Not only their indicator species, but also the trace element and stable isotope geochemistry of their shells have been documented as evidence of past climate fluctuations. | 9 | Geochemistry |
Somatic tissue can be stored in vitro for short periods of time. This is done in a light and temperature controlled environment that regulates the growth of cells. As an ex situ conservation technique tissue culture is primary used for clonal propagation of vegetative tissue or immature seeds. This allows for the proliferation of clonal plants from a relatively small amount of parent tissue. | 1 | Biochemistry |
One important property of carbon in organic chemistry is that it can form certain compounds, the individual molecules of which are capable of attaching themselves to one another, thereby forming a chain or a network. The process is called polymerization and the chains or networks polymers, while the source compound is a monomer. Two main groups of polymers exist: those artificially manufactured are referred to as industrial polymers or synthetic polymers (plastics) and those naturally occurring as biopolymers.
Monomers can have various chemical substituents, or functional groups, which can affect the chemical properties of organic compounds, such as solubility and chemical reactivity, as well as the physical properties, such as hardness, density, mechanical or tensile strength, abrasion resistance, heat resistance, transparency, color, etc.. In proteins, these differences give the polymer the ability to adopt a biologically active conformation in preference to others (see self-assembly).
People have been using natural organic polymers for centuries in the form of waxes and shellac, which is classified as a thermoplastic polymer. A plant polymer named cellulose provided the tensile strength for natural fibers and ropes, and by the early 19th century natural rubber was in widespread use. Polymers are the raw materials (the resins) used to make what are commonly called plastics. Plastics are the final product, created after one or more polymers or additives have been added to a resin during processing, which is then shaped into a final form. Polymers that have been around, and that are in current widespread use, include carbon-based polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylons, polyesters, acrylics, polyurethane, and polycarbonates, and silicon-based silicones. Plastics are generally classified as "commodity", "specialty" and "engineering" plastics. | 7 | Physical Chemistry |
In ecological terms, because of its important role in biological systems, phosphate is a highly sought after resource. Once used, it is often a limiting nutrient in environments, and its availability may govern the rate of growth of organisms. This is generally true of freshwater environments, whereas nitrogen is more often the limiting nutrient in marine (seawater) environments. Addition of high levels of phosphate to environments and to micro-environments in which it is typically rare can have significant ecological consequences. For example, blooms in the populations of some organisms at the expense of others, and the collapse of populations deprived of resources such as oxygen (see eutrophication) can occur. In the context of pollution, phosphates are one component of total dissolved solids, a major indicator of water quality, but not all phosphorus is in a molecular form that algae can break down and consume.
Calcium hydroxyapatite and calcite precipitates can be found around bacteria in alluvial topsoil. As clay minerals promote biomineralization, the presence of bacteria and clay minerals resulted in calcium hydroxyapatite and calcite precipitates.
Phosphate deposits can contain significant amounts of naturally occurring heavy metals. Mining operations processing phosphate rock can leave tailings piles containing elevated levels of cadmium, lead, nickel, copper, chromium, and uranium. Unless carefully managed, these waste products can leach heavy metals into groundwater or nearby estuaries. Uptake of these substances by plants and marine life can lead to concentration of toxic heavy metals in food products. | 0 | Organic Chemistry |
Various techniques are used in the investigative process of metallurgical failure analysis.
* Macroscopic examination: camera, stereoscope
* Microscopic examination: light microscopy, electron microscopy, x-ray microscopy, metallographic etching
* Mechanical testing: hardness testing, tensile testing, Charpy impact testing
* Chemical testing: microprobe analysis, energy dispersive spectroscopy
Non-destructive testing: Non-destructive testing is a test method that allows certain physical properties of metal to be examined without taking the samples completely out of service. NDT is generally used to detect failures in components before the component fails catastrophically.
Destructive testing: Destructive testing involves removing a metal component from service and sectioning the component for analysis. Destructive testing gives the failure analyst the ability to conduct the analysis in a laboratory setting and perform tests on the material that will ultimately destroy the component. | 8 | Metallurgy |
Rapid amplification of cDNA ends (RACE) is a technique used in molecular biology to obtain the full length sequence of an RNA transcript found within a cell. RACE results in the production of a cDNA copy of the RNA sequence of interest, produced through reverse transcription, followed by PCR amplification of the cDNA copies (see RT-PCR). The amplified cDNA copies are then sequenced and, if long enough, should map to a unique genomic region. RACE is commonly followed up by cloning before sequencing of what was originally individual RNA molecules. A more high-throughput alternative which is useful for identification of novel transcript structures, is to sequence the RACE-products by next generation sequencing technologies. | 1 | Biochemistry |
Thure E. Cerling received his Bachelor of Science degree in geology and chemistry from Iowa State University, in Ames, Iowa, in 1972, and, in 1973, his Master of Science in geology from Iowa State. In 1977 he was awarded a Ph.D. in geology from the University of California at Berkeley. From 1977 to 1979 he worked as a research scientist at Oak Ridge National Laboratory and, from 1979 he has been a member of the University of Utah's faculty. | 9 | Geochemistry |
Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms. With the chemical formula H(O)CCH(OH)CHOPO</sup>, this anion is a monophosphate ester of glyceraldehyde. | 5 | Photochemistry |
A dangling side chain is a hydrocarbon chain side chains that branch off of the backbone of the polymer. Attached to the side chain are polar functional groups. The side chains "dangle" across the surface of the hydrogel, allowing it to interact with other functional groups and form new bonds. The ideal side chain would be long and flexible so it could reach across the surface to react, but short enough to minimize steric hindrance and collapse from the hydrophobic effect. The side chains need to keep both the hydrophobic and hydrophilic effects in balance. In a study performed by the University of California San Diego to compare healing ability, hydrogels of varying side chain lengths with similar crosslinking contents were compared and the results showed that healing ability of the hydrogels depends nonmonotonically on the side chain length. With shorter side chain lengths, there is limited reach of the carboxyl group which decreases the mediation of the hydrogen bonds across the interface. As the chain increases in length, the reach of the carboxyl group becomes more flexible and the hydrogen bonds can mediated. However, when a side chain length is too long, the interruption between the interaction of the carboxyl and amide groups that help to mediate the hydrogen bonds. It can also accumulate and collapse the hydrogel and prevent the healing from occurring. | 7 | Physical Chemistry |
In the episode "Encyclopedia Galactica" of his TV series Cosmos: A Personal Voyage, Carl Sagan speculates that some intelligent extraterrestrial beings might have a genetic code based on polyaromatic sulfonyl halides instead of DNA. | 0 | Organic Chemistry |
This chemogenetic technique can be used for remote manipulation of cells, in particular excitable cells like neurons, both in vitro and in vivo with the administration of specific ligands. Similar techniques in this field include thermogenetics and optogenetics, the control of neurons with temperature or light, respectively.
Viral expression of DREADD proteins, both in-vivo enhancers and inhibitors of neuronal function, have been used to bidirectionally control behaviors in mice (e.g odor discrimination). Due to their ability to modulate neuronal activity, DREADDs are used as a tool to evaluate both the neuronal pathways and behaviors associated with drug-cues and drug addiction. | 1 | Biochemistry |
Polyolefins such as polyethylene and polypropylene are susceptible to photo-oxidation and around 70% of light stabilizers produced world-wide are used in their protection, despite them representing only around 50% of global plastic production. Aliphatic hydrocarbons can only adsorb high energy UV-rays with a wavelength below ~250 nm, however the Earth’s atmosphere and ozone layer screen out such rays, with the normal minimum wavelength being 280–290 nm.
The bulk of the polymer is therefore photo-inert and degradation is instead attributed to the presence of various impurities, which are introduced during the manufacturing or processing stages. These include hydroperoxide and carbonyl groups, as well as metal salts such as catalyst residues.
All of these species act as photoinitiators.
The organic hydroperoxide and carbonyl groups are able to absorb UV light above 290 nm whereupon they undergo photolysis to generate radicals. Metal impurities act as photocatalysts, although such reactions can be complex. It has also been suggested that polymer-O charge-transfer complexes are involved. Initiation generates radical-carbons on the polymer chain, sometimes called macroradicals (P•).
Chain initiation
Chain propagation
Chain branching
Termination
Classically the carbon-centred macroradicals (P•) rapidly react with oxygen to form hydroperoxyl radicals (POO•), which in turn abstract an H atom from the polymer chain to give a hydroperoxide (POOH) and a fresh macroradical. Hydroperoxides readily undergo photolysis to give an alkoxyl macroradical radical (PO•) and a hydroxyl radical (HO•), both of which may go on to form new polymer radicals via hydrogen abstraction. Non-classical alternatives to these steps have been proposed. The alkoxyl radical may also undergo beta scission, generating a acyl-ketone and macroradical. This is considered to be the main cause of chain breaking in polypropylene.
Secondary hydroperoxides can also undergo an intramolecular reaction to give a ketone group, although this is limited to polyethylene.
The ketones generated by these processes are themselves photo-active, although much more weakly. At ambient temperatures they undergo Type II Norrish reactions with chain scission. They may also absorb UV-energy, which they can then transfer to O, causing it to enter its highly reactive singlet state. Singlet oxygen is a potent oxidising agent can go on to form cause further degradation. | 5 | Photochemistry |
In 1707, Abraham Darby I patented a method of making cast iron pots. His pots were thinner and hence cheaper than those of his rivals. Needing a larger supply of pig iron he leased the blast furnace at Coalbrookdale in 1709. There, he made iron using coke, thus establishing the first successful business in Europe to do so. His products were all of cast iron, though his immediate successors attempted (with little commercial success) to fine this to bar iron.
Bar iron thus continued normally to be made with charcoal pig iron until the mid-1750s. In 1755 Abraham Darby II (with partners) opened a new coke-using furnace at Horsehay in Shropshire, and this was followed by others. These supplied coke pig iron to finery forges of the traditional kind for the production of bar iron. The reason for the delay remains controversial. | 8 | Metallurgy |
The following list describes a viewpoint on the interdisciplinary relationships between molecular biology and other related fields.
* Molecular biology is the study of the molecular underpinnings of the biological phenomena, focusing on molecular synthesis, modification, mechanisms and interactions.
* Biochemistry is the study of the chemical substances and vital processes occurring in living organisms. Biochemists focus heavily on the role, function, and structure of biomolecules such as proteins, lipids, carbohydrates and nucleic acids.
* Genetics is the study of how genetic differences affect organisms. Genetics attempts to predict how mutations, individual genes and genetic interactions can affect the expression of a phenotype
While researchers practice techniques specific to molecular biology, it is common to combine these with methods from genetics and biochemistry. Much of molecular biology is quantitative, and recently a significant amount of work has been done using computer science techniques such as bioinformatics and computational biology. Molecular genetics, the study of gene structure and function, has been among the most prominent sub-fields of molecular biology since the early 2000s. Other branches of biology are informed by molecular biology, by either directly studying the interactions of molecules in their own right such as in cell biology and developmental biology, or indirectly, where molecular techniques are used to infer historical attributes of populations or species, as in fields in evolutionary biology such as population genetics and phylogenetics. There is also a long tradition of studying biomolecules "from the ground up", or molecularly, in biophysics. | 1 | Biochemistry |
The museum has several functional areas, with the main museum building covering over 13,000 square feet. The displays at the main museum building contains iron-industry content as well as a wide variety of archaeological artifacts. In addition, the museum also houses the Walter B. Jones Centre for Industrial Archaeology and includes an exhibit centre, the 1858 May Plantation Cotton Gin House, a 30-seat theatre, as well as a gift shop. The exhibition centre displays preserved Birmingham's iron-and-steel industrial artifacts over the period from 1930s to the 1960s. | 8 | Metallurgy |
In chemistry, vinylidenes are compounds with the functional group C=CH. An example is 1,1-dichloroethene (CCl=CH) commonly called vinylidene chloride. It and vinylidene fluoride are precursors to commercially useful polymers. | 0 | Organic Chemistry |
In organic synthesis, industrial and otherwise, the nitro group is a versatile functional group. A mixture of nitric and sulfuric acids introduces a nitro substituent onto various aromatic compounds by electrophilic aromatic substitution. Many explosives, such as TNT, are prepared this way:
Either concentrated sulfuric acid or oleum absorbs the excess water.
The nitro group can be reduced to give an amine group, allowing synthesis of aniline compounds from various nitrobenzenes: | 3 | Analytical Chemistry |
In terms of bonding, isocyanates are closely related to carbon dioxide (CO) and carbodiimides (C(NR)). The C−N=C=O unit that defines isocyanates is planar, and the N=C=O linkage is nearly linear. In phenyl isocyanate, the C=N and C=O distances are respectively 1.195 and 1.173 Å. The C−N=C angle is 134.9° and the N=C=O angle is 173.1°. | 0 | Organic Chemistry |
Non-classical congenital adrenal hyperplasia caused by 21-hydroxylase deficiency (NCCAH) is a milder and late-onset congenital adrenal hyperplasia. Its prevalence rate in different ethnic groups varies from 1 in to 1 in . Some people affected by the condition have no relevant signs and symptoms, while others experience symptoms of hyperandrogenism.
Women with NCCAH usually have normal female genitalia at birth. In later life, the signs and symptoms of the condition may include acne, hirsutism, male-pattern baldness, irregular menstruation, and infertility.
Fewer studies have been published about males with NCCAH comparing to those about females, because males are generally asymptomatic. Males, however, may present with acne and early balding.
While symptoms are usually diagnosed after puberty, children may present with premature adrenarche. | 1 | Biochemistry |
Dissociation of pyrophosphoric acid generates four anions, , where the charge k ranges from 1 to 4. The last one is pyrophosphate . The pyrophosphates are mostly water-soluble.
Likewise, tripolyphosphoric acid yields at least five anions , where k ranges from 1 to 5, including tripolyphosphate . Tetrapolyphosphoric acid yields at least six anions, including tetrapolyphosphate , and so on. Note that each extra phosphoric unit adds one extra P atom, three extra O atoms, and either one extra hydrogen atom or an extra negative charge.
Branched polyphosphoric acids give similarly branched polyphosphate anions. The simplest example of this is triphosphono phosphate and its partially dissociated versions.
The general formula for such (non-cyclic) polyphosphate anions, linear or branched, is , where the charge k may vary from 1 to . Generally in an aqueous solution, the degree or percentage of dissociation depends on the pH of the solution. | 0 | Organic Chemistry |
1,1,1-Trichloroethane was one of the volatile organochlorides that have been tried as alternatives to chloroform in anaesthesia.
In the 1880s, it was found to be a safe and strong substitute for chloroform but its production was expensive and difficult.
In 1880, 1,1,1-Trichloroethane was suggested as an anaesthetic. It was first referred to as "methyl-chloroform" in the same year. At the time, the narcotic effects of chloral hydrate were owed to a hypothetical metabolic pathway to chloroform in "alkaline blood". Trichloroethane was studied for its structural similarity to chloral and potential anaesthetic effects. However, trichloroethane did not exhibit any conversion to chloroform in laboratory experiments. The 1,1,2-trichloroethane isomer, which lacked a trichloromethyl group, exhibited anaesthetic effects even stronger than the 1,1,1 isomer. | 2 | Environmental Chemistry |
Proteins are normally hydrolysed slowly even without enzymes or bacteria, with a half life of 460 years, but can be preserved if they are desiccated, pickled or frozen. Being enclosed in bone also helps preservation. Over time the amino acids tend to racemize, and those with more functional groups are lost earlier. Protein still will degrade on the timescale of a million years. DNA degrades rapidly, lasting only about four years in water. Cellulose and chitin have a half life in water at 25° of about 4.7 million years. Enzymes can accelerate this by a factor of 10. About 10 tons of chiting are produced each year, but it is almost all degraded.
Lignin is only efficiently degraded by fungi, white rot, or brown rot. These require oxygen.
Lipids are hydrolysed to fatty acids over long time periods. Plant cuticle waxes are very difficult to degrade, and may survive over geological time periods. | 9 | Geochemistry |
In 2018, the atmospheric concentration of CFC-11 was noted by researchers to be declining more slowly than expected, and it subsequently emerged that it remains in widespread use as a blowing agent for polyurethane foam insulation in the construction industry of China. In 2021, researchers announced that emissions declined by 20,000 U.S. tons from 2018 to 2019, which mostly reversed the previous spike in emissions. In 2022, the European Commission announced an updated regulation that mandates the recovery and prevention of emissions of CFC-11 blowing agents from foam insulation in demolition waste, which is still emitted at significant scale. | 2 | Environmental Chemistry |
Palmitoylcarnitine contains the saturated fatty acid known as palmitic acid (C16:0) which is bound to the β-hydroxy group of the carnitine. The core carnitine structure, consisting of butanoate with a quaternary ammonium attached to C4 and hydroxy group at C3, is a common molecular backbone for the transfer of multiple long chain fatty acids in the TCA cycle. | 1 | Biochemistry |
The AFM is interfaced with an infrared spectrometer. For work using Fourier transform infrared spectroscopy (FTIR), the spectrometer is equipped with a conventional black body infrared source. A particular region of the sample may first be chosen on the basis of the image obtained using the AFM imaging mode of operation. Then, when material at this location absorbs the electromagnetic radiation, heat is generated, which diffuses, giving rise to a decaying temperature profile. The thermal probe then detects the photothermal response of this region of the sample. The resultant measured temperature fluctuations provide an interferogram that replaces the interferogram obtained by a conventional FTIR setup, e.g., by direct detection of the radiation transmitted by a sample. The temperature profile can be made sharp by modulating the excitation beam. This results in the generation of thermal waves whose diffusion length is inversely proportional to the root of the modulation frequency. An important advantage of the thermal approach is that it permits to obtain depth-sensitive subsurface information from surface measurement, thanks to the dependence of thermal diffusion length on modulation frequency. | 7 | Physical Chemistry |
Muscular activity requires ATP, which is provided by the breakdown of glycogen in the skeletal muscles. The breakdown of glycogen, known as glycogenolysis, releases glucose in the form of glucose 1-phosphate (G1P). The G1P is converted to G6P by phosphoglucomutase. G6P is readily fed into glycolysis, (or can go into the pentose phosphate pathway if G6P concentration is high) a process that provides ATP to the muscle cells as an energy source. During muscular activity, the store of ATP needs to be constantly replenished. When the supply of oxygen is sufficient, this energy comes from feeding pyruvate, one product of glycolysis, into the citric acid cycle, which ultimately generates ATP through oxygen-dependent oxidative phosphorylation.
When oxygen supply is insufficient, typically during intense muscular activity, energy must be released through anaerobic metabolism. Lactic acid fermentation converts pyruvate to lactate by lactate dehydrogenase. Most importantly, fermentation regenerates NAD, maintaining its concentration so additional glycolysis reactions can occur. The fermentation step oxidizes the NADH produced by glycolysis back to NAD, transferring two electrons from NADH to reduce pyruvate into lactate. (Refer to the main articles on glycolysis and fermentation for the details.)
Instead of accumulating inside the muscle cells, lactate produced by anaerobic fermentation is taken up by the liver. This initiates the other half of the Cori cycle. In the liver, gluconeogenesis occurs. From an intuitive perspective, gluconeogenesis reverses both glycolysis and fermentation by converting lactate first into pyruvate, and finally back to glucose. The glucose is then supplied to the muscles through the bloodstream; it is ready to be fed into further glycolysis reactions. If muscle activity has stopped, the glucose is used to replenish the supplies of glycogen through glycogenesis.
Overall, the glycolysis steps of the cycle produce 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis steps. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules in the Cori cycle shifts the metabolic burden from the muscles to the liver. | 1 | Biochemistry |
Simple permutation-based estimation is used to determine how likely a given RP value or better is observed in a random experiment.
# generate p permutations of k rank lists of length n.
# calculate the rank products of the n genes in the p permutations.
# count how many times the rank products of the genes in the permutations are smaller or equal to the observed rank product. Set c to this value.
# calculate the average expected value for the rank product by: .
# calculate the percentage of false positives as : where is the rank of gene g in a list of all n genes sorted by increasing . | 1 | Biochemistry |
Metalation (Alt. spelling: Metallation) is a chemical reaction that forms a bond to a metal. This reaction usually refers to the replacement of a halogen atom in an organic molecule with a metal atom, resulting in an organometallic compound. In the laboratory, metalation is commonly used to activate organic molecules during the formation of C—X bonds (where X is typically carbon, oxygen, or nitrogen), which are necessary for the synthesis of many organic molecules.
In synthesis, metallated reagents are typically involved in nucleophilic substitution, single-electron-transfer (SET), and redox chemistry with functional groups on other molecules (including but not limited to ketones, aldehydes and alkyl halides). Metallated molecules may also participate in acid-base chemistry, with one organometallic reagent deprotonating an organic molecule to create a new organometallic reagent.
The most common classes of metallated compounds are organolithium reagents and Grignard reagents. However, other organometallic compounds — such as organozinc compounds — also experience common use in both laboratory and industrial applications. | 0 | Organic Chemistry |
* Water disinfection/decontamination, a form of solar water disinfection (SODIS). Adsorbents attract organics such as tetrachloroethylene. Adsorbents are placed in packed beds for 18 hours. Spent adsorbents are placed in regeneration fluid, essentially removing organics still attached by passing hot water opposite to the flow of water during adsorption. The regeneration fluid passes through fixed beds of silica gel photocatalysts to remove and decompose remaining organics.
* self-sterilizing coatings (for application to food contact surfaces and in other environments where microbial pathogens spread by indirect contact).
*Magnetic nanoparticle oxidation of organic contaminants agitated using a magnetic field.
*Sterilization of surgical instruments and removal of fingerprints from electrical and optical components. | 5 | Photochemistry |
Carboxypeptidase B (, protaminase, pancreatic carboxypeptidase B, tissue carboxypeptidase B, peptidyl-L-lysine [L-arginine]hydrolase) is a carboxypeptidase that preferentially cleaves off basic amino acids arginine and lysine from the C-terminus of a peptide. This enzyme is secreted by the pancreas, and it travels to the small intestine, where it aids in protein digestion. Plasma carboxypeptidase B (carboxypeptidase B2) is responsible for converting the C5a protein into C5a des-Arg, with one less amino acid. | 1 | Biochemistry |
AutoAnalyzers are still used for a few clinical applications such as neonatal screening or Anti-D, but the majority of instruments are now used for industrial and environmental work. Standardized methods have been published by the ASTM (ASTM International), the US Environmental Protection Agency (EPA) as well as the International Organization for Standardization (ISO) for environmental analytes such as nitrite, nitrate, ammonia, cyanide, and phenol. Autoanalyzers are also commonly used in soil testing laboratories, fertilizer analysis, process control, seawater analysis, air contaminants, and tobacco leaf analysis. | 3 | Analytical Chemistry |
In biochemistry, differences between the isotopomers of biochemicals such as starches is of practical importance in archaeology. They offer clues to the diet of prehistoric humans that lived as long ago as paleolithic times. This is because naturally occurring carbon dioxide contains both C and C. Monocots, such as rice and oats, differ from dicots, such as potatoes and tree fruits, in the relative amounts of CO and CO that they incorporate into their tissues as products of photosynthesis. When tissues of such subjects are recovered, usually tooth or bone, the relative isotopic content can give useful indications of the main source of the staple foods of the subjects of the investigations. | 4 | Stereochemistry |
Toxic hotspots are locations where emissions from specific sources such as water or air pollution may expose local populations to elevated health risks, such as cancer. These emissions contribute to cumulative health risks of emissions from other sources nearby. Urban, highly populated areas around pollutant emitters such as old factories and waste storage sites are often toxic hotspots. | 2 | Environmental Chemistry |
In nature, pyruvate oxidase employs two cofactors thiamine pyrophosphate (ThDP) and Flavin adenine dinucleotide (FAD) to catalyze a conversion of pyruvate to acetyl phosphate. First, ThDP mediates a decarboxylation of pyruvate and generates an active aldehyde as a product. The aldehyde is then oxidized by FAD and is subsequently attacked by phosphate to yield acetyl phosphate.
Diederich and coworkers mimicked this system with a supramolecular catalyst based on cyclophane. The catalyst has thiazolium ion, a reactive part of ThDP and flavin, a bare-bones core of FAD, in close proximity and near the substrate binding site. The catalytic cycle is almost the same as that in nature, except the substrate is an aromatic aldehyde rather than pyruvate. First, the catalyst binds the substrate within its cyclophane ring. Then, it uses thiazolium ion to condense with the substrate generating an active aldehyde. This aldehyde is oxidized by flavin and then attacked by methanol to yield a methyl ester. | 6 | Supramolecular Chemistry |
A group of researchers at the École Polytechnique Fédérale de Lausanne (EPFL) has reportedly increased the thermostability of DSC by using amphiphilic ruthenium sensitizer in conjunction with quasi-solid-state gel electrolyte. The stability of the device matches that of a conventional inorganic silicon-based solar cell. The cell sustained heating for 1,000 h at 80 °C.
The group has previously prepared a ruthenium amphiphilic dye Z-907 (cis-Ru(Hdcbpy)(dnbpy)(NCS), where the ligand Hdcbpy is 4,4′-dicarboxylic acid-2,2′-bipyridine and dnbpy is 4,4′-dinonyl-2,2′-bipyridine) to increase dye tolerance to water in the electrolytes. In addition, the group also prepared a quasi-solid-state gel electrolyte with a 3-methoxypropionitrile (MPN)-based liquid electrolyte that was solidified by a photochemically stable fluorine polymer, polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP).
The use of the amphiphilic Z-907 dye in conjunction with the polymer gel electrolyte in DSC achieved an energy conversion efficiency of 6.1%. More importantly, the device was stable under thermal stress and soaking with light. The high conversion efficiency of the cell was sustained after heating for 1,000 h at 80 °C, maintaining 94% of its initial value. After
accelerated testing in a solar simulator for 1,000 h of light-soaking at 55 °C (100 mW cm) the efficiency had decreased by less than 5% for cells covered with an ultraviolet absorbing polymer film. These results are well within the limit for that of traditional inorganic silicon solar cells.
The enhanced performance may arise from a decrease in solvent permeation across the sealant due to the application of the polymer gel electrolyte. The polymer gel electrolyte is quasi-solid at room temperature, and becomes a viscous liquid (viscosity: 4.34 mPa·s) at 80 °C compared with the traditional liquid electrolyte (viscosity: 0.91 mPa·s). The much improved stabilities of the device under both thermal stress and soaking with light has never before been seen in DSCs, and they match the durability criteria applied to solar cells for outdoor use, which makes these devices viable for practical application. | 5 | Photochemistry |
High performance liquid chromatography (HPLC) and electron ionization mass spectrometry (EIMS) are two analytical techniques that, in principle, seem to be incompatible. However, because these two approaches share a great deal of applications in the analysis of suitable molecules, typically less than 1000 u, a large effort has been devoted by the scientific community to develop a reliable, easy-to-use, and flawless interface. The first successful and commercially available device to combine EI and HPLC was designed by Willoughby and Browner in 1984. It was based on the conversion of the solute into a beam of particles, after the formation of spray droplets and the elimination of the solvent vapors through a multi-stage momentum separator. Although its efficient interfacing mechanism and a unique trait, particle beam performance was sometimes inadequate to an increasing number of new, demanding applications and was quickly replaced by a family of atmospheric pressure ionization-based interfaces (API) when they became commercially available. However, the possibility to record an EI spectrum from an HPLC application remained a challenge for a long time. The first Direct-EI prototype was first presented in 2002 and proposed an innovative approach that improved interfacing performance compared to that of particle beam and opened new opportunities for LC-MS applications. | 3 | Analytical Chemistry |
Unlike in protein coding regions, where the assumption of sequence conservation of functionally homologous genes have been frequently proved, there is not a clear relationship of conservation between sequences and their functions for regulatory regions. The transcriptional promoters regions are under less stringent selection, then have a higher substitutions rates, allowing transcription factor binding sites to be replaced easily be new ones arising from random mutations. Notwithstanding the sequence changes, mainly the functions of regulatory sequences remain conserved.
In recents years with the increase of availability of genome sequences, phylogenetic footprinting open the possibility to identify cis-elements, and then study their evolution processes. In this sense, Raijman et al., Dermitzakis et al. have developed techniques for analyzing evolutionary processes in transcription factor regions in Saccharomyces species promoters and mammalian regulatory networks respectively.
The basis for many of these evolutionary changes in nature are probably related with events within the cis-regulatory regions involve in gene expression. The impact of variation in regulatory regions is important for disease risk due their impact in the gene expression level. Furthermore, perturbations in the binding properties of proteins encoded by regulatory genes have been linked with phenotypes effects such as, duplicated structures, homeotic transformations and novel morphologies. | 1 | Biochemistry |
Wetting theory: Wetting is the oldest and most prevalent theory of adhesion. The adhesive components in a liquid solution anchor themselves in irregularities on the substrate and eventually harden, providing sites on which to adhere. Surface tension effects restrict the movement of the adhesive along the surface of the substrate, and is related to the thermodynamic work of adhesion by Dupre's Equation. Measuring the affinity of the adhesive for the substrate is performed by determining the contact angle. Contact angles closer to zero indicate a more wettable interaction, and those interactions have a greater spreadability.
Adsorption theory: Adsorption is another widely accepted theory, where adhesion between the substrate and adhesive is due to primary and secondary bonding. The primary bonds are due to chemisorption, and result in comparatively long lasting covalent and non-covalent bonds. Among covalent bonds disulfide bonds are likely most important. Thiolated polymers – designated thiomers – are mucoadhesive polymers that can form disulfide bonds with cysteine-rich subdomains of mucus glycoproteins. Recently several new classes of polymers have been developed that are capable of forming covalent bonds with mucosal surfaces similarly to thiomers. These polymers have acryloyl, methacryloyl, maleimide, boronate and N‐hydroxy (sulfo) succinimide ester groups in their structure. Among non-covalent bonds likely ionic interactions such as interactions of mucoadhesive chitosans with the anionically charged mucus and Hydrogen bonding are most important. The secondary bonds include weak Van Der Waals forces, and interactions between hydrophobic substructure.
Diffusion theory: The mechanism for diffusion involves polymer and mucin chains from the adhesive penetrating the matrix of the substrate and forming a semipermanent bond. As the similarities between the adhesive and the substrate increase, so does the degree of mucoadhesion. The bond strength increases with the degree of penetration, increasing the adhesion strength. The penetration rate is determined by the diffusion coefficient, the degree of flexibility of the adsorbate chains, mobility and contact time. The diffusion mechanism itself is affected by the length of the molecular chains being implanted and cross-linking density, and is driven by a concentration gradient.
Electrostatic theory: is an electrostatic process involving the transfer of electrons across the interface between the substrate and adhesive. The net result is the formation of a double layer of charges that are attracted to each other due to balancing of the Fermi layers, and therefore cause adhesion. This theory only works given the assumption that the substrate and adhesive have different electrostatic surface characteristics.
Fracture theory: Fracture theory is the major mechanism by which to determine the mechanical strength of a particular mucoadhesive, and describes the force necessary to separate the two materials after mucoadhesion has occurred. Ultimate tensile strength is determined by the separating force and the total surface area of the adhesion, and failure generally occurs in one of the surfaces rather than at the interface. Since the fracture theory only deals with the separation force, the diffusion and penetration of polymers is not accounted for in this mechanism. | 1 | Biochemistry |
Like in vivo MRS, fMRS can probe different nuclei, such as hydrogen (H) and carbon (C). The H nucleus is the most sensitive and is most commonly used to measure metabolite concentrations and concentration dynamics, whereas C is best suited for characterizing fluxes and pathways of brain metabolism. The natural abundance of C in the brain is only about 1%; therefore, C fMRS studies usually involve the isotope enrichment via infusion or ingestion.
In the literature C fMRS is commonly referred to as functional C MRS or just C MRS. | 7 | Physical Chemistry |
Due to its ability to form derivatives through its nitrogen atom and the structural rigidity of its chirality, camphorsultam is often used in reactions as a chiral auxiliary in order to allow a reaction to proceed with very specific stereoselectivity. During the synthesis of Manzacidin B, camphorsultam is used in order to obtain the desired stereoselective product. During a Michael reaction, a Claisen rearrangement, or a cycloaddition reaction camphorsultam is able to confer a great deal of stereoselectivity. This allows for more control over the reactions and the creation of very specific desired products. Stereoselectivity can be further increased if substrates are equipped with two chiral auxiliaries, acting in a cooperative fashion.
Camphorsultam also has applications in determining a compound's absolute stereochemistry. For that reason, it is sometimes referred to as a "chiral probe". | 0 | Organic Chemistry |
The Meselson-Stahl experiment was a landmark experiment in molecular biology that provided evidence for the semiconservative replication of DNA. Conducted in 1958 by Matthew Meselson and Franklin Stahl, the experiment involved growing E. coli bacteria in a medium containing heavy isotope of nitrogen (N) for several generations. This caused all the newly synthesized bacterial DNA to be incorporated with the heavy isotope.
After allowing the bacteria to replicate in a medium containing normal nitrogen (N), samples were taken at various time points. These samples were then subjected to centrifugation in a density gradient, which separated the DNA molecules based on their density.
The results showed that after one generation of replication in the N medium, the DNA formed a band of intermediate density between that of pure N DNA and pure N DNA. This supported the semiconservative DNA replication proposed by Watson and Crick, where each strand of the parental DNA molecule serves as a template for the synthesis of a new complementary strand, resulting in two daughter DNA molecules, each consisting of one parental and one newly synthesized strand.
The Meselson-Stahl experiment provided compelling evidence for the semiconservative replication of DNA, which is fundamental to the understanding of genetics and molecular biology. | 1 | Biochemistry |
An ideal solution would follow Raoult's law, but most solutions deviate from ideality. Interactions between gas molecules are typically quite small, especially if the vapor pressures are low. However, the interactions in a liquid are very strong. For a solution to be ideal, the interactions between unlike molecules must be of the same magnitude as those between like molecules. This approximation is only true when the different species are almost chemically identical. One can see that from considering the Gibbs free energy change of mixing:
This is always negative, so mixing is spontaneous. However, the expression is, apart from a factor , equal to the entropy of mixing. This leaves no room at all for an enthalpy effect and implies that must be equal to zero, and this can only be true if the interactions between the molecules are indifferent.
It can be shown using the Gibbs–Duhem equation that if Raoult's law holds over the entire concentration range in a binary solution then, for the second component, the same must also hold.
If deviations from the ideal are not too large, Raoults law is still valid in a narrow concentration range when approaching for the majority phase (the solvent). The solute also shows a linear limiting law, but with a different coefficient. This relationship is known as Henrys law.
The presence of these limited linear regimes has been experimentally verified in a great number of cases, though large deviations occur in a variety of cases. Consequently, both its pedagogical value and utility have been questioned at the introductory college level. In a perfectly ideal system, where ideal liquid and ideal vapor are assumed, a very useful equation emerges if Raoults law is combined with Daltons Law:
where is the mole fraction of component in the solution, and is its mole fraction in the gas phase. This equation shows that, for an ideal solution where each pure component has a different vapor pressure, the gas phase is enriched in the component with the higher vapor pressure when pure, and the solution is enriched in the component with the lower pure vapor pressure. This phenomenon is the basis for distillation. | 7 | Physical Chemistry |
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*Mobile Phone Based Remote Monitoring and Control system For Individualized Healthcare. First AMA-IEEE Medical Technology on Individualized Healthcare, Washington DC, US.2010
*Connected Healthcare Solution Using Cell phone. ASME's 5th Frontiers in Biomedical Devices Conference & Exhibition, California, US.2010
*Automated Wireless System for Individuals Requiring Continuous Remote Care 6th World Congress on Biomechanics, in conjunction with 14th International Conference on Biomedical Engineering (ICBME), Singapore .2010
*Ankabut Users Meeting January 2012. | 7 | Physical Chemistry |
Once a refinement has been completed the results should be checked to verify that the chosen model is acceptable. generally speaking, a model is acceptable when the data are fitted within experimental error, but there is no single criterion to use to make the judgement. The following should be considered. | 7 | Physical Chemistry |
The determination of trace acids in organic matrices is a common analytical task assigned to titrimetry. Examples are Total Acid Number (TAN) in mineral and lubricating oils and Free Fatty Acids (FFA) in edible fats and oils. Automated potentiometric titration procedures have been granted standard method status, for example by ASTM for TAN and AOAC for FFA. The methodology is similar in both instances. The sample is dissolved in a suitable solvent mixture; say a hydrocarbon and an alcohol which also must contain a small amount of water. The water is intended to enhance the electrical conductivity of the solution. The trace acids are titrated with standard base in an alcohol. The sample environment is essentially hostile to the pH electrode used to sense the titration. The electrode must be taken out of service on a regular basis to rehydrate the glass sensing membrane, which is also in danger of fouling by the oily sample solution.
A recent thermometric titrimetric procedure for the determination of FFA developed by Cameiro et al. (2002) has been shown to be particularly amenable to automation. It is fast, highly precise, and results agree very well with those obtained by the official AOAC method. The temperature change for the titration of very weak acids such as oleic acid by 0.1 mol/L KOH in propan-2-ol is too small to yield an accurate endpoint. In this procedure, a small amount of paraformaldehyde as a fine powder is added to the titrand before the titration. At the endpoint, the first excess of hydroxyl ions catalyzes the depolymerization of paraformaldehyde. The reaction is strongly endothermic and yields a sharp inflection. The titration plot is illustrated in Figure 13. The speed of this titration coupled with its precision and accuracy makes it ideal for the analysis of FFA in biodiesel feedstocks and product. | 3 | Analytical Chemistry |
Of particular interest are DNA ligases. These molecules have demonstrated remarkable chemoselectivity in RNA branching reactions. Although each repeating unit in a RNA strand owns a free hydroxyl group, the DNA ligase takes just one of them as a branching starting point. This cannot be done with traditional organic chemistry. | 7 | Physical Chemistry |
The photopyroelectric refers to the technique of the optimal system which is mainly based on the imaginary system and the pyroelectric detector. | 7 | Physical Chemistry |
The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for some minerals in 1997. Where there was not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake (AI) was used instead. AIs are typically matched to actual average consumption, with the assumption that there appears to be a need, and that need is met by what people consume. The current AI for women 19 years and older is 3.0 mg/day (includes pregnancy and lactation). The AI for men is 4.0 mg/day. The AI for children ages 1–18 increases from 0.7 to 3.0 mg/day. The major known risk of fluoride deficiency appears to be an increased risk of bacteria-caused tooth cavities. As for safety, the IOM sets tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of fluoride the UL is 10 mg/day. Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs).
The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women ages 18 and older the AI is set at 2.9 mg/day (includes pregnancy and lactation). For men the value is 3.4 mg/day. For children ages 1–17 years the AIs increase with age from 0.6 to 3.2 mg/day. These AIs are comparable to the U.S. AIs. The EFSA reviewed safety evidence and set an adult UL at 7.0 mg/day (lower for children).
For U.S. food and dietary supplement labeling purposes the amount of a vitamin or mineral in a serving is expressed as a percent of Daily Value (%DV). Although there is information to set Adequate Intake, fluoride does not have a Daily Value and is not required to be shown on food labels. | 1 | Biochemistry |
There are 20 naturally occurring amino acids, however some of these share similar characteristics. For example, leucine and isoleucine are both aliphatic, branched hydrophobes. Similarly, aspartic acid and glutamic acid are both small, negatively charged residues.
Although there are many ways to classify amino acids, they are often sorted into six main classes on the basis of their structure and the general chemical characteristics of their side chains (R groups).
Physicochemical distances aim at quantifying the intra-class and inter-class dissimilarity between amino acids based on their measurable properties, and many such measures have been proposed in the literature. Owing to their simplicity, two of the most commonly used measures are the ones of Grantham (1974) and Miyata et al (1979). A conservative replacement is therefore an exchange between two amino acids separated by a small physicochemical distance. Conversely, a radical replacement is an exchange between two amino acids separated by a large physicochemical distance. | 1 | Biochemistry |
In 1933, Prelog married Kamila Vitek. The couple had a son Jan (born 1949).
An intellectual with a wide cultural background, Prelog was one of the 109 Nobel Prize winners who signed the peace appeal for Croatia in 1991.
Vladimir Prelog died in Zürich, at the age of 91. An urn containing Prelog's ashes was ceremoniously interred at the Mirogoj cemetery in Zagreb on 27 September 2001. In 2008, a memorial to Prelog was unveiled in Prague. | 4 | Stereochemistry |
In cases where an enolization is occurring around an allylic group (usually as part of a cyclic system), A strain can cause the reaction to be nearly impossible. In these situations, acid treatment would normally cause the alkene to become protonated, moving the double bond to the carboxylic group, changing it to a hydroxy group. The resulting allylic strain between the alcohol and the other group involved in the allylic system is so great that the reaction can not occur under normal thermodynamic conditions. This same enolization occurs much more rapidly under basic conditions, as the carboxylic group is retained in the transition state and allows the molecule to adopt a conformation that does not cause allylic strain. | 4 | Stereochemistry |
Orthogonal spectra to the dynamic dataset E are obtained with the Hilbert-transform:
where:
*Ψ is the 2D asynchronous spectrum
*ν en ν are two spectral channels
*y is the vector composed of the signal intensities in E in column ν
*n the number of signals in the original dataset
*N the Noda-Hilbert transform matrix
The values of N, N are determined as follows:
*0 if j = k
* if j ≠ k
where:
*j the row number
*k the column number | 7 | Physical Chemistry |
Although it is generally accepted that vanilla was domesticated in Mesoamerica and subsequently spread to the Old World in the 16th century, in 2019, researchers published a paper stating that vanillin residue had been discovered inside jars within a tomb in Israel dating to the 2nd millennium BCE, suggesting the possible cultivation of an unidentified, Old World-endemic Vanilla species in Canaan since the Middle Bronze Age. Traces of vanillin were also found in wine jars in Jerusalem, which were used by the Judahite elite before the city was destroyed in 586 BCE.
Vanilla beans, called tlilxochitl, were discovered and cultivated as a flavoring for beverages by native Mesoamerican peoples, most famously the Totonacs of modern-day Veracruz, Mexico. Since at least the early 15th century, the Aztecs used vanilla as a flavoring for chocolate in drinks called xocohotl.
Vanillin was first isolated as a relatively pure substance in 1858 by Théodore Nicolas Gobley, who obtained it by evaporating a vanilla extract to dryness and recrystallizing the resulting solids from hot water. In 1874, the German scientists Ferdinand Tiemann and Wilhelm Haarmann deduced its chemical structure, at the same time finding a synthesis for vanillin from coniferin, a glucoside of isoeugenol found in pine bark. Tiemann and Haarmann founded a company Haarmann and Reimer (now part of Symrise) and started the first industrial production of vanillin using their process (now known as the Reimer–Tiemann reaction) in Holzminden, Germany. In 1876, Karl Reimer synthesized vanillin (2) from guaiacol (1).
By the late 19th century, semisynthetic vanillin derived from the eugenol found in clove oil was commercially available.
Synthetic vanillin became significantly more available in the 1930s, when production from clove oil was supplanted by production from the lignin-containing waste produced by the sulfite pulping process for preparing wood pulp for the paper industry. By 1981, a single pulp and paper mill in Thorold, Ontario, supplied 60% of the world market for synthetic vanillin. However, subsequent developments in the wood pulp industry have made its lignin wastes less attractive as a raw material for vanillin synthesis. Today, approximately 15% of the world's production of vanillin is still made from lignin wastes, while approximately 85% synthesized in a two-step process from the petrochemical precursors guaiacol and glyoxylic acid.
Beginning in 2000, Rhodia began marketing biosynthetic vanillin prepared by the action of microorganisms on ferulic acid extracted from rice bran. At USD$700/kg, this product, sold under the trademarked name Rhovanil Natural, is not cost-competitive with petrochemical vanillin, which sells for around US$15/kg. However, unlike vanillin synthesized from lignin or guaiacol, it can be labeled as a natural flavoring. | 0 | Organic Chemistry |
Sometime in the medieval period, water power was applied to the bloomery process. It is possible that this was at the Cistercian Abbey of Clairvaux as early as 1135, but it was certainly in use in early 13th century France and Sweden. In England, the first clear documentary evidence for this is the accounts of a forge of the Bishop of Durham, near Bedburn in 1408, but that was certainly not the first such ironworks. In the Furness district of England, powered bloomeries were in use into the beginning of the 18th century, and near Garstang until about 1770.
The Catalan Forge was a variety of powered bloomery. Bloomeries with hot blast were used in upstate New York in the mid-19th century. | 8 | Metallurgy |
fMRS was developed as an extension of MRS in the early 1990s. Its potential as a research technology became obvious when it was applied to an important research problem where PET studies had been inconclusive, namely the mismatch between oxygen and glucose consumption during sustained visual stimulation. The H fMRS studies highlighted the important role of lactate in this process and significantly contributed to the research in brain energy metabolism during brain activation. It confirmed the hypothesis that lactate increases during sustained visual stimulation and allowed the generalization of findings based on visual stimulation to other types of stimulation, e.g., auditory stimulation, motor task and cognitive tasks.
H fMRS measurements were instrumental in achieving the current consensus among most researchers that lactate levels increase during the first minutes of intense brain activation. However, there are no consistent results about the magnitude of increase, and questions about the exact role of lactate in brain energy metabolism still remain unanswered and are the subject of continuing research.
C MRS is a special type of fMRS particularly suited for measuring important neurophysiological fluxes in vivo and in real time to assess metabolic activity both in healthy and diseased brains (e.g., in human tumor tissue ). These fluxes include TCA cycle, glutamate–glutamine cycle, glucose and oxygen consumption. C MRS can provide detailed quantitative information about glucose dynamics that can not be obtained with H fMRS, because of the low concentration of glucose in the brain and the spread of its resonances in several multiplets in the H MRS spectrum.
C MRSs have been crucial in recognizing that the awake nonstimulated (resting) human brain is highly active using 70%–80% of its energy for glucose oxidation to support signaling within cortical networks which is suggested to be necessary for consciousness. This finding has an important implication for the interpretation of BOLD fMRI data where this high baseline activity is generally ignored and response to the task is shown as independent of the baseline activity. C MRS studies indicate that this approach can misjudge and even completely miss the brain activity induced by the task.
C MRS findings together with other results from PET and fMRI studies have been combined in a model to explain the function of resting state activity called default mode network.
Another important benefit of C MRS is that it provides unique means for determining the time course of metabolite pools and measuring turnover rates of TCA and glutamate–glutamine cycles. As such, it has been proved to be important in aging research by revealing that mitochondrial metabolism is reduced with aging which may explain the decline in cognitive and sensory processes. | 7 | Physical Chemistry |
In the case of ATI the photoelectron peaks should appear at
where the integer n represents the minimal number of photons absorbed, and the integer s represents the number of additional photons absorbed. W is the ionization energy, and is the electron kinetic energy of the peak corresponding to s additional photons being absorbed. | 7 | Physical Chemistry |
The formation of an amide using a carbodiimide is a common reaction, but carries the risk of several side reactions. The acid 1 will react with the carbodiimide to produce the key intermediate: the O-acylisourea 2, which can be viewed as a carboxylic ester with an activated leaving group. The O-acylisourea will react with amines to give the desired amide 3 and urea 4.
The possible reactions of the O-acylisourea 2 produce both desired and undesired products. The O-acylisourea 2 can react with an additional carboxylic acid 1 to give an acid anhydride 5, which can react further to give the amide 3. The main undesired reaction pathway involves the rearrangement of the O-acylisourea 2 to the stable N-acylurea 6. The use of solvents with low dielectric constants such as dichloromethane or chloroform can minimize this side reaction. | 0 | Organic Chemistry |
Altszyler et al. (2017) have shown that these ultrasensitivity measures can be linked by the following equation:
where denoted the mean value of the variable x over the range [a,b]. | 1 | Biochemistry |
If both the equilibrium constant, and the standard enthalpy change, , for a reaction have been determined experimentally, the standard entropy change for the reaction is easily derived. Since and
To a first approximation the standard enthalpy change is independent of temperature. Using this approximation, definite integration of the van 't Hoff equation
gives
This equation can be used to calculate the value of log K at a temperature, T, knowing the value at temperature T.
The van t Hoff equation also shows that, for an exothermic reaction (), when temperature increases K decreases and when temperature decreases K increases, in accordance with Le Chateliers principle. The reverse applies when the reaction is endothermic.
When K has been determined at more than two temperatures, a straight line fitting procedure may be applied to a plot of against to obtain a value for . Error propagation theory can be used to show that, with this procedure, the error on the calculated value is much greater than the error on individual log K values. Consequently, K needs to be determined to high precision when using this method. For example, with a silver ion-selective electrode each log K value was determined with a precision of ca. 0.001 and the method was applied successfully.
Standard thermodynamic arguments can be used to show that, more generally, enthalpy will change with temperature.
where C is the heat capacity at constant pressure. | 7 | Physical Chemistry |
Usha Ranjan Ghatak (1931–2005) was an Indian synthetic organic chemist, stereochemist and the director of the Indian Association for the Cultivation of Science (IACS). He was known for his contributions in developing novel protocols of stereoselective synthesis of diterpenoids. He was an elected fellow of the Indian Academy of Sciences and the Indian National Science Academy. The Council of Scientific and Industrial Research, the apex agency of the Government of India for scientific research, awarded him the Shanti Swarup Bhatnagar Prize for Science and Technology, one of the highest Indian science awards, in 1974, for his contributions to chemical sciences. | 4 | Stereochemistry |
Knot theory which categorises chain entanglements. The usage of knot theory is limited to a small percentage of proteins as most of them are unknot. | 6 | Supramolecular Chemistry |
A classic example of this process is the quinine sulfate fluorescence, which can be quenched by the use of various halide salts. The excited molecule can de-excite by increasing the thermal energy of the surrounding solvated ions.
Several natural molecules perform a fast internal conversion. This ability to transform the excitation energy of photon into heat can be a crucial property for photoprotection by molecules such as melanin. Fast internal conversion reduces the excited state lifetime, and thereby prevents bimolecular reactions. Bimolecular electron transfer always produces a reactive chemical species, free radicals. Nucleic acids (precisely the single, free nucleotides, not those bound in a DNA/RNA strand) have an extremely short lifetime due to a fast internal conversion.
Both melanin and DNA have some of the fastest internal conversion rates.
In applications that make use of bimolecular electron transfer the internal conversion is undesirable. For example, it is advantageous to have a long-lived excited state in Grätzel cells (Dye-sensitized solar cells). | 7 | Physical Chemistry |
The three-level-amplifier is the template of a quantum device. It operates by employing a hot and cold bath
to maintain population inversion between two energy levels which is used to amplify light by stimulated emission
The ground state level (1-g) and the excited level (3-h) are coupled to a hot bath of temperature .
The energy gap is . When the population on the levels equilibrate
where is the Planck constant and is the Boltzmann constant.
The cold bath of temperature couples the ground (1-g) to an intermediate level (2-c)
with energy gap .
When levels 2-c and 1-g equilibrate then
The device operates as an amplifier when levels (3-h) and (2-c) are coupled to an external field of frequency .
For optimal resonance conditions .
The efficiency of the amplifier in converting heat to power is the ratio of work output to heat input:
Amplification of the field is possible only for positive gain (population inversion)
This is equivalent to .
Inserting this expression into the efficiency formula leads to:
where is the Carnot cycle efficiency.
Equality is obtained under a zero gain condition .
The relation between the quantum amplifier and the Carnot efficiency was first pointed out by Scovil and Schultz-DuBois:
Reversing the operation driving heat from the cold bath to the hot bath by consuming power constitutes a refrigerator.
The efficiency of the refrigerator defined as the coefficient of performance (COP) for the reversed device is: | 7 | Physical Chemistry |
The measure of the promoter activity has a broad meaning. The promoter activity could be measured for different situations or research questions, such as:
* estimation of the level of expression in comparison (relative) to some known value
* how fast a gene is expressed after induction
* the timing of expression relative to others genes
* the specific spatial location of expression
Methods to study promoter activity commonly are based in the expression of a reporter gene from the promoter of the gene of interest. Mutations and deletions are made in a promoter region, and their changes on couple expression of the reporter gene are measured.
The most important reporter genes are the fluorescence proteins as GFP. These reporters allow to measure promoter activation by increasing fluorescent signals, and deactivation by decrease in the rate of fluorescence. | 1 | Biochemistry |
Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal conversion to the ground state (non-radiative decay), thus harmlessly dissipating excess excitation energy as heat through molecular vibrations. NPQ occurs in almost all photosynthetic eukaryotes (algae and plants), and helps to regulate and protect photosynthesis in environments where light energy absorption exceeds the capacity for light utilization in photosynthesis. | 5 | Photochemistry |
Two limiting mechanisms, one operating under kinetic and the other thermodynamic control, have been identified for lateral lithiation reactions. The mechanisms of most lateral lithiations fall somewhere between these two limiting mechanisms, and the precise mechanism of a particular lithiation depends on two factors:
* The Lewis acidity of the organolithium reagent (RLi > LiNR)
* The Lewis basicity of the heteroatom substituent (N > O > S)
When both the Lewis acidity of the organolithium compound and the Lewis basicity of the substituent are high, as in lithiations of ortho-(dialkylamino)methyl toluenes with n-butyllithium in a non-coordinating solvent, coordination of the base to the heteroatom substituent takes place. Lithiation then occurs at the most kinetically accessible ortho benzylic position; ortho lithiation is slower in this case.
As either the Lewis acidity of the base or the coordinating ability of the substituent decrease, a mechanism involving purely inductive effects becomes more important. For instance, the lithiation of 1 with lithium di(isopropyl)amide (LDA) affords only the product of benzylic metalation 2; none of the ortho-lithiated product 3 is observed. This result is explained by a mechanism in which the amide substituent affects the acidity of the para benzylic position solely through inductive effects and coordination of the base is not operative. Deprotonation occurs to afford the most thermodynamically stable product.
In most cases, both mechanisms will lead to the same product, as the sites of kinetic and thermodynamic deprotonation will coincide. | 0 | Organic Chemistry |
The eye is most sensitive to damage by UV in the lower UV‑C band at 265–275 nm. Radiation of this wavelength is almost absent from sunlight at the surface of the Earth but is emitted by artificial sources such as the electrical arcs employed in arc welding. Unprotected exposure to these sources can cause "welder's flash" or "arc eye" (photokeratitis) and can lead to cataracts, pterygium and pinguecula formation. To a lesser extent, UV‑B in sunlight from 310 to 280 nm also causes photokeratitis ("snow blindness"), and the cornea, the lens, and the retina can be damaged.
Protective eyewear is beneficial to those exposed to ultraviolet radiation. Since light can reach the eyes from the sides, full-coverage eye protection is usually warranted if there is an increased risk of exposure, as in high-altitude mountaineering. Mountaineers are exposed to higher-than-ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice.
Ordinary, untreated eyeglasses give some protection. Most plastic lenses give more protection than glass lenses, because, as noted above, glass is transparent to UV‑A and the common acrylic plastic used for lenses is less so. Some plastic lens materials, such as polycarbonate, inherently block most UV. | 5 | Photochemistry |
Stirred tank, also called agitation leaching, involves contacting material, which has usually undergone size reduction and classification, with leach solution in agitated tanks. The agitation can enhance reaction kinetics by enhancing mass transfer. Tanks are often configured as reactors in series. | 8 | Metallurgy |
Applications of dendrimers typically involve conjugating other chemical species to the dendrimer surface that can function as detecting agents (such as a dye molecule), affinity ligands, targeting components, radioligands, imaging agents, or pharmaceutically active compounds. Dendrimers have very strong potential for these applications because their structure can lead to multivalent systems. In other words, one dendrimer molecule has hundreds of possible sites to couple to an active species. Researchers aimed to utilize the hydrophobic environments of the dendritic media to conduct photochemical reactions that generate the products that are synthetically challenged. Carboxylic acid and phenol-terminated water-soluble dendrimers were synthesized to establish their utility in drug delivery as well as conducting chemical reactions in their interiors. This might allow researchers to attach both targeting molecules and drug molecules to the same dendrimer, which could reduce negative side effects of medications on healthy cells.
Dendrimers can also be used as a solubilizing agent. Since their introduction in the mid-1980s, this novel class of dendrimer architecture has been a prime candidate for host–guest chemistry. Dendrimers with hydrophobic core and hydrophilic periphery have shown to exhibit micelle-like behavior and have container properties in solution. The use of dendrimers as unimolecular micelles was proposed by Newkome in 1985. This analogy highlighted the utility of dendrimers as solubilizing agents. The majority of drugs available in pharmaceutical industry are hydrophobic in nature and this property in particular creates major formulation problems. This drawback of drugs can be ameliorated by dendrimeric scaffolding, which can be used to encapsulate as well as to solubilize the drugs because of the capability of such scaffolds to participate in extensive hydrogen bonding with water. Dendrimer labs are trying to manipulate dendrimer's solubilizing trait, to explore dendrimers for drug delivery and to target specific carriers.
For dendrimers to be able to be used in pharmaceutical applications, they must surmount the required regulatory hurdles to reach market. One dendrimer scaffold designed to achieve this is the polyethoxyethylglycinamide (PEE-G) dendrimer. This dendrimer scaffold has been designed and shown to have high HPLC purity, stability, aqueous solubility and low inherent toxicity. | 6 | Supramolecular Chemistry |
The Boltzmann constant provides a mapping from the characteristic microscopic energy to the macroscopic temperature scale . In fundamental physics, this mapping is often simplified by using the natural units of setting to unity. This convention means that temperature and energy quantities have the same dimensions. In particular, the SI unit kelvin becomes superfluous, being defined in terms of joules as . With this convention, temperature is always given in units of energy, and the Boltzmann constant is not explicitly needed in formulas.
This convention simplifies many physical relationships and formulas. For example, the equipartition formula for the energy associated with each classical degree of freedom ( above) becomes
As another example, the definition of thermodynamic entropy coincides with the form of information entropy:
where is the probability of each microstate. | 7 | Physical Chemistry |
The modern understanding of heat is often partly attributed to Thompsons 1798 mechanical theory of heat (An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction'), postulating a mechanical equivalent of heat.
A collaboration between Nicolas Clément and Sadi Carnot (Reflections on the Motive Power of Fire) in the 1820s had some related thinking along similar lines. In 1842, Julius Robert Mayer frictionally generated heat in paper pulp and measured the temperature rise. In 1845, Joule published a paper entitled The Mechanical Equivalent of Heat, in which he specified a numerical value for the amount of mechanical work required to "produce a unit of heat", based on heat production by friction in the passage of electricity through a resistor and in the rotation of a paddle in a vat of water. The theory of classical thermodynamics matured in the 1850s to 1860s. | 7 | Physical Chemistry |
Due to its similar structure and properties, pentavalent arsenic metabolites are capable of replacing the phosphate group of many metabolic pathways. The replacement of phosphate by arsenate is initiated when arsenate reacts with glucose and gluconate in vitro. This reaction generates glucose-6-arsenate and 6-arsenogluconate, which act as analogs for glucose-6-phosphate and 6-phosphogluconate. At the substrate level, during glycolysis, glucose-6-arsenate binds as a substrate to glucose-6-phosphate dehydrogenase, and also inhibits hexokinase through negative feedback. Unlike the importance of phosphate in glycolysis, the presence of arsenate restricts the generation of ATP by forming an unstable anhydride product, through the reaction with D-glyceraldehyde-3-phosphate. The anhydride 1-arsenato-3-phospho-D-glycerate generated readily hydrolyzes due to the longer bond length of As-O compared to P-O. At the mitochondrial level, arsenate uncouples the synthesis of ATP by binding to ADP in the presence of succinate, thus forming an unstable compound that ultimately results in a decrease of ATP net gain. Arsenite (III) metabolites, on the other hand, have limited effect on ATP production in red blood cells. | 1 | Biochemistry |
Clonal derivation exists in nature in some animal species and is referred to as parthenogenesis (reproduction of an organism by itself without a mate). This is an asexual form of reproduction that is only found in females of some insects, crustaceans, nematodes, fish (for example the hammerhead shark), Cape honeybees, and lizards including the Komodo dragon and several whiptails. The growth and development occurs without fertilization by a male. In plants, parthenogenesis means the development of an embryo from an unfertilized egg cell, and is a component process of apomixis. In species that use the XY sex-determination system, the offspring will always be female. An example is the little fire ant (Wasmannia auropunctata), which is native to Central and South America but has spread throughout many tropical environments. | 1 | Biochemistry |
Anhydrous reacts vigorously with bases, so suitable precautions are required.
It can cause irritation to the eyes, skin, and the respiratory system if inhaled or on contact. | 0 | Organic Chemistry |
Pinoresinol biosynthesis involved a protein called a dirigent protein. The first dirigent protein was discovered in Forsythia intermedia. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol from coniferyl alcohol monomers. Recently, a second, enantiocomplementary dirigent protein was identified in Arabidopsis thaliana, which directs enantioselective synthesis of (−)-pinoresinol. | 4 | Stereochemistry |
Gibbs paradox arises when entropy is calculated using an dimensional phase space, where is also the number of particles in the gas. These particles are spatially confined to the one-dimensional interval . The volume of the surface of fixed energy is
The subscripts on are used to define the state variables and will be discussed later, when it is argued that the number of particles, lacks full status as a state variable in this calculation. The integral over configuration space is . As indicated by the underbrace, the integral over velocity space is restricted to the "surface area" of the dimensional hypersphere of radius , and is therefore equal to the "area" of that hypersurface. Thus
After approximating the factorial and dropping the small terms, we obtain
In the second expression, the term was subtracted and added, using the fact that . This was done to highlight exactly how the "entropy" defined here fails to be an extensive property of matter. The first two terms are extensive: if the volume of the system doubles, but gets filled with the same density of particles with the same energy, then each of these terms doubles. But the third term is neither extensive nor intensive and is therefore wrong.
The arbitrary constant has been added because entropy can usually be viewed as being defined up to an arbitrary additive constant. This is especially necessary when entropy is defined as the logarithm of a phase space volume measured in units of momentum-position. Any change in how these units are defined will add or subtract a constant from the value of the entropy. | 7 | Physical Chemistry |
In order to fully understand complex ocean systems, a wide variety of sensor arrays, capable of surviving for long periods of time in harsh conditions, are necessary. A suite of sensors (over 100) were selected and strategically placed throughout RSN. They are located at Axial Seamount, Hydrate Ridge, and also on the water-column moorings.
Instruments connected to the RSN include:
* Conductivity Temperature Depth (located on profilers)
* Dissolved Oxygen,
* 3-D Single Point Current Meter
* Temperature
* Fluorometers
* CDOM,
* Chlorophyll-a,
* Optical Backscatter
The instruments are the final spot of each regional network branch. | 9 | Geochemistry |
Malonyl-CoA signals glucose utilization and it controls the entry and oxidation of long-chain fatty acids (LCFA) in the mitochondria. Circulating glucose in the liver stimulates its uptake. Glucose oxidation produces citrate which can be converted to malonyl-CoA by acetyl-CoA carboxylase. Malonyl-CoA inhibits the carnitine palmitoyltransferase (CPT) that controls the entry and oxidation of LCFA. The glucose-derived malonyl-CoA prevents the oxidation of fatty acids and favors fatty acid esterification. | 1 | Biochemistry |
NEM is a Michael acceptor in the Michael reaction, which means that it adds nucleophiles such as thiols. The resulting thioether features a strong C-S bond and the reaction is virtually irreversible. Reaction with thiols occur in the pH range 6.5–7.5, NEM may react with amines or undergo hydrolysis at a more alkaline pH. NEM has been widely used to probe the functional role of thiol groups in enzymology. NEM is an irreversible inhibitor of all cysteine peptidases, with alkylation occurring at the active site thiol group (see schematic). | 1 | Biochemistry |
A variety of methods for the generation of diimide exist. The most synthetically useful methods are:
* Oxidation of hydrazine with oxygen, in the presence of a copper(II) catalyst and/or a carboxylic acid
* Decarboxylation of dipotassium azodicarboxylate in the presence of an acid
* Thermal decomposition of sulfonylhydrazides
Procedures (particularly those employing air as an oxidant) are typically straightforward and do not require special handling techniques. | 0 | Organic Chemistry |
For a box of radiation, ignoring quantum mechanics, the energy of a classical field in thermal equilibrium is infinite, since equipartition demands that each field mode has an equal energy on average, and there are infinitely many modes. This is physically ridiculous, since it means that all energy leaks into high-frequency electromagnetic waves over time.
Still, without quantum mechanics, there are some things that can be said about the equilibrium distribution from thermodynamics alone, because there is still a notion of adiabatic invariance that relates boxes of different size.
When a box is slowly expanded, the frequency of the light recoiling from the wall can be computed from the Doppler shift. If the wall is not moving, the light recoils at the same frequency. If the wall is moving slowly, the recoil frequency is only equal in the frame where the wall is stationary. In the frame where the wall is moving away from the light, the light coming in is bluer than the light coming out by twice the Doppler shift factor v/c:
On the other hand, the energy in the light is also decreased when the wall is moving away, because the light is doing work on the wall by radiation pressure. Because the light is reflected, the pressure is equal to twice the momentum carried by light, which is E/c. The rate at which the pressure does work on the wall is found by multiplying by the velocity:
This means that the change in frequency of the light is equal to the work done on the wall by the radiation pressure. The light that is reflected is changed both in frequency and in energy by the same amount:
Since moving the wall slowly should keep a thermal distribution fixed, the probability that the light has energy E at frequency f must only be a function of E/f.
This function cannot be determined from thermodynamic reasoning alone, and Wien guessed at the form that was valid at high frequency. He supposed that the average energy in high-frequency modes was suppressed by a Boltzmann-like factor:
This is not the expected classical energy in the mode, which is by equipartition, but a new and unjustified assumption that fit the high-frequency data.
When the expectation value is added over all modes in a cavity, this is Wiens distribution, and it describes the thermodynamic distribution of energy in a classical gas of photons. Wiens law implicitly assumes that light is statistically composed of packets that change energy and frequency in the same way. The entropy of a Wien gas scales as the volume to the power N, where N is the number of packets. This led Einstein to suggest that light is composed of localizable particles with energy proportional to the frequency. Then the entropy of the Wien gas can be given a statistical interpretation as the number of possible positions that the photons can be in. | 7 | Physical Chemistry |
Pulsed sources allow for the utilization of Fourier-transform spectroscopy principles in scanning near-field optical microscopy techniques. Particularly in nano-FTIR, where the scattering from a sharp probe-tip is used to perform spectroscopy of samples with nanoscale spatial resolution, a high-power illumination from pulsed infrared lasers makes up for a relatively small scattering efficiency (often < 1%) of the probe. | 7 | Physical Chemistry |
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