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In quantum field theory (QFT), the fabric of "empty" space is visualized as consisting of fields, with the field at every point in space and time being a quantum harmonic oscillator, with neighboring oscillators interacting with each other. According to QFT the universe is made up of matter fields whose quanta are fermions (e.g. electrons and quarks), force fields whose quanta are bosons (i.e. photons and gluons) and a Higgs field whose quantum is the Higgs boson. The matter and force fields have zero-point energy. A related term is zero-point field (ZPF), which is the lowest energy state of a particular field. The vacuum can be viewed not as empty space, but as the combination of all zero-point fields.
In QFT the zero-point energy of the vacuum state is called the vacuum energy and the average expectation value of the Hamiltonian is called the vacuum expectation value (also called condensate or simply VEV). The QED vacuum is a part of the vacuum state which specifically deals with quantum electrodynamics (e.g. electromagnetic interactions between photons, electrons and the vacuum) and the QCD vacuum deals with quantum chromodynamics (e.g. color charge interactions between quarks, gluons and the vacuum). Recent experiments advocate the idea that particles themselves can be thought of as excited states of the underlying quantum vacuum, and that all properties of matter are merely vacuum fluctuations arising from interactions with the zero-point field.
Each point in space makes a contribution of , resulting in a calculation of infinite zero-point energy in any finite volume; this is one reason renormalization is needed to make sense of quantum field theories. In cosmology, the vacuum energy is one possible explanation for the cosmological constant and the source of dark energy.
Scientists are not in agreement about how much energy is contained in the vacuum. Quantum mechanics requires the energy to be large as Paul Dirac claimed it is, like a sea of energy. Other scientists specializing in General Relativity require the energy to be small enough for curvature of space to agree with observed astronomy. The Heisenberg uncertainty principle allows the energy to be as large as needed to promote quantum actions for a brief moment of time, even if the average energy is small enough to satisfy relativity and flat space. To cope with disagreements, the vacuum energy is described as a virtual energy potential of positive and negative energy.
In quantum perturbation theory, it is sometimes said that the contribution of one-loop and multi-loop Feynman diagrams to elementary particle propagators are the contribution of vacuum fluctuations, or the zero-point energy to the particle masses. | 7 | Physical Chemistry |
Using matrix matched calibration standards can compensate for ion suppression. Using this technique, calibration standards are prepared in identical sample matrix to that used for analysis (e.g. plasma) by spiking a normal sample with known concentrations of analyte. This is not always possible for biological samples, since the analyte of interest is often endogenously present in a clinically significant, albeit normal, quantity. For matrix matched calibration standards to be effective in compensating for ion suppression, the sample matrix must be free of the analyte of interest. Additionally, it is important that there is little variation in test sample composition since both the test sample and the prepared calibration sample must be affected in the same way by ion suppression. Again, in complex biological samples from different individuals, or even the same individual at a different time, there may be large fluctuations in the concentrations of ion suppressing species. | 3 | Analytical Chemistry |
Ancestral sequence reconstruction (ASR) – also known as ancestral gene/sequence reconstruction/resurrection – is a technique used in the study of molecular evolution. The method uses related sequences to reconstruct an "ancestral" gene from a multiple sequence alignment.
The method can be used to resurrect ancestral proteins and was suggested in 1963 by Linus Pauling and Emile Zuckerkandl. In the case of enzymes, this approach has been called paleoenzymology (British: palaeoenzymology). Some early efforts were made in the 1980s and 1990s, led by the laboratory of Steven A. Benner, showing the potential of this technique. Thanks to the improvement of algorithms and of better sequencing and synthesis techniques, the method was developed further in the early 2000s to allow the resurrection of a greater variety of and much more ancient genes. Over the last decade, ancestral protein resurrection has developed as a strategy to reveal the mechanisms and dynamics of protein evolution. | 1 | Biochemistry |
In an open system, there is an exchange of energy and matter between the system and the surroundings. The presence of reactants in an open beaker is an example of an open system. Here the boundary is an imaginary surface enclosing the beaker and reactants. It is named closed, if borders are impenetrable for substance, but allow transit of energy in the form of heat, and isolated, if there is no exchange of heat and substances. The open system cannot exist in the equilibrium state. To describe deviation of the thermodynamic system from equilibrium, in addition to constitutive variables that was described above, a set of internal variables have been introduced. The equilibrium state is considered to be stable and the main property of the internal variables, as measures of non-equilibrium of the system, is their trending to disappear; the local law of disappearing can be written as relaxation equation for each internal variable
where is a relaxation time of a corresponding variable. It is convenient to consider the initial value equal to zero.
The specific contribution to the thermodynamics of open non-equilibrium systems was made by Ilya Prigogine, who investigated a system of chemically reacting substances. In this case the internal variables appear to be measures of incompleteness of chemical reactions, that is measures of how much the considered system with chemical reactions is out of equilibrium. The theory can be generalized, to consider any deviations from the equilibrium state, such as structure of the system, gradients of temperature, difference of concentrations of substances and so on, to say nothing of degrees of completeness of all chemical reactions, to be internal variables.
The increments of Gibbs free energy and entropy at and are determined as
The stationary states of the system exist due to exchange of both thermal energy and a stream of particles. The sum of the last terms in the equations presents the total energy coming into the system with the stream of particles of substances that can be positive or negative; the quantity is chemical potential of substance . The middle terms in equations (2) and (3) depict energy dissipation (entropy production) due to the relaxation of internal variables , while are thermodynamic forces.
This approach to the open system allows describing the growth and development of living objects in thermodynamic terms. | 7 | Physical Chemistry |
Roy Jensen in 1976 theorised that primordial enzymes had to be highly promiscuous in order for metabolic networks to assemble in a patchwork fashion (hence its name, the patchwork model). This primordial catalytic versatility was later lost in favour of highly catalytic specialised orthologous enzymes. As a consequence, many central-metabolic enzymes have structural homologues that diverged before the last universal common ancestor. | 1 | Biochemistry |
The specificity constant (also known as the catalytic efficiency) is a measure of how efficiently an enzyme converts a substrate into product. Although it is the ratio of and it is a parameter in its own right, more fundamental than . Diffusion limited enzymes, such as fumarase, work at the theoretical upper limit of , limited by diffusion of substrate into the active site.
If we symbolize the specificity constant for a particular substrate A as the Michaelis–Menten equation can be written in terms of and as follows:
At small values of the substrate concentration this approximates to a first-order dependence of the rate on the substrate concentration:
Conversely it approaches a zero-order dependence on when the substrate concentration is high:
The capacity of an enzyme to distinguish between two competing substrates that both follow Michaelis–Menten kinetics depends only on the specificity constant, and not on either or alone. Putting for substrate and for a competing substrate , then the two rates when both are present simultaneously are as follows:
Although both denominators contain the Michaelis constants they are the same, and thus cancel when one equation is divided by the other:
and so the ratio of rates depends only on the concentrations of the two substrates and their specificity constants. | 7 | Physical Chemistry |
Imidoyl chlorides are organic compounds that contain the functional group RC(NR)Cl. A double bond exist between the RN and the carbon centre. These compounds are analogues of acyl chloride. Imidoyl chlorides tend to be highly reactive and are more commonly found as intermediates in a wide variety of synthetic procedures. Such procedures include Gattermann aldehyde synthesis, Houben-Hoesch ketone synthesis, and the Beckmann rearrangement. Their chemistry is related to that of enamines and their tautomers when the α hydrogen is next to the C=N bond. Many chlorinated N-heterocycles are formally imidoyl chlorides, e.g. 2-chloropyridine, 2, 4, and 6-chloropyrimidines. | 0 | Organic Chemistry |
Hybridization is a basic property of nucleotide sequences and is taken advantage of in numerous molecular biology techniques. Overall, genetic relatedness of two species can be determined by hybridizing segments of their DNA (DNA-DNA hybridization). Due to sequence similarity between closely related organisms, higher temperatures are required to melt such DNA hybrids when compared to more distantly related organisms. A variety of different methods use hybridization to pinpoint the origin of a DNA sample, including the polymerase chain reaction (PCR). In another technique, short DNA sequences are hybridized to cellular mRNAs to identify expressed genes. Pharmaceutical drug companies are exploring the use of antisense RNA to bind to undesired mRNA, preventing the ribosome from translating the mRNA into protein. | 1 | Biochemistry |
Mechanical alloying (MA) is a solid-state and powder processing technique involving repeated cold welding, fracturing, and re-welding of blended powder particles in a high-energy ball mill to produce a homogeneous material. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or pre-alloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. | 8 | Metallurgy |
In chemistry, a ladder polymer is a type of double stranded polymer with the connectivity of a ladder. In a typical one-dimensional polymer, e.g. polyethylene and polysiloxanes, the monomers form two bonds, giving a chain. In a ladder polymer the monomers are interconnected by four bonds. Inorganic ladder polymers are found in synthetic and natural settings. Ladder polymers are a special case of cross-linked polymers because the crosslinks exist only with pairs of chains.
According to one definition, a ladder polymer, adjacent rings have two or more atoms in common. | 7 | Physical Chemistry |
The surface-area-to-volume ratio or surface-to-volume ratio (denoted as SA:V, SA/V, or sa/vol) is the ratio between surface area and volume of an object or collection of objects.
SA:V is an important concept in science and engineering. It is used to explain the relation between structure and function in processes occurring through the surface the volume. Good examples for such processes are processes governed by the heat equation, that is, diffusion and heat transfer by thermal conduction. SA:V is used to explain the diffusion of small molecules, like oxygen and carbon dioxide between air, blood and cells, water loss by animals, bacterial morphogenesis, organism's thermoregulation, design of artificial bone tissue, artificial lungs and many more biological and biotechnological structures. For more examples see Glazier.
The relation between SA:V and diffusion or heat conduction rate is explained from flux and surface perspective, focusing on the surface of a body as the place where diffusion, or heat conduction, takes place, i.e., the larger the SA:V there is more surface area per unit volume through which material can diffuse, therefore, the diffusion or heat conduction, will be faster. Similar explanation appears in the literature: "Small size implies a large ratio of surface area to volume, thereby helping to maximize the uptake of nutrients across the plasma membrane", and elsewhere.
For a given volume, the object with the smallest surface area (and therefore with the smallest SA:V) is a ball, a consequence of the isoperimetric inequality in 3 dimensions. By contrast, objects with acute-angled spikes will have very large surface area for a given volume. | 7 | Physical Chemistry |
Under high pressure, solubility of increases. On opening the bottle to atmospheric pressure, solubility decreases and the gas bubbles are released from the liquid. | 7 | Physical Chemistry |
The stationary phase or adsorbent in column chromatography is a solid. The most common stationary phase for column chromatography is silica gel, the next most common being alumina. Cellulose powder has often been used in the past. A wide range of stationary phases are available in order to perform ion exchange chromatography, reversed-phase chromatography (RP), affinity chromatography or expanded bed adsorption (EBA). The stationary phases are usually finely ground powders or gels and/or are microporous for an increased surface, though in EBA a fluidized bed is used. There is an important ratio between the stationary phase weight and the dry weight of the analyte mixture that can be applied onto the column. For silica column chromatography, this ratio lies within 20:1 to 100:1, depending on how close to each other the analyte components are being eluted. | 3 | Analytical Chemistry |
Like some fatty acids, carotenoids are lipophilic due to the presence of long unsaturated aliphatic chains. As a consequence, carotenoids are typically present in plasma lipoproteins and cellular lipid structures. | 5 | Photochemistry |
Tetrafluoromethane, like other fluorocarbons, is very stable due to the strength of its carbon–fluorine bonds. The bonds in tetrafluoromethane have a bonding energy of 515 kJ⋅mol. As a result, it is inert to acids and hydroxides. However, it reacts explosively with alkali metals. Thermal decomposition or combustion of CF produces toxic gases (carbonyl fluoride and carbon monoxide) and in the presence of water will also yield hydrogen fluoride.
It is very slightly soluble in water (about 20 mg⋅L), but miscible with organic solvents. | 2 | Environmental Chemistry |
Common inspection methods for steel castings are magnetic particle testing and liquid penetrant testing. Common inspection methods for aluminum castings are radiography, ultrasonic testing, and liquid penetrant testing. | 8 | Metallurgy |
Although bleaching has long been practiced, the first photochemical reaction was described by Trommsdorff in 1834. He observed that crystals of the compound α-santonin when exposed to sunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal.
The first step is a rearrangement reaction to a cyclopentadienone intermediate (2), the second one a dimerization in a Diels–Alder reaction (3), and the third one an intramolecular [2+2]cycloaddition (4). The bursting effect is attributed to a large change in crystal volume on dimerization. | 5 | Photochemistry |
Pigulevsky was married to Nina Viktorovna Pigulevskaya (1894-1970), a soviet historian, corresponding member of the Academy of Sciences of the USSR, specialist in the history of Byzantium, the Near and Middle East in the early Middle Ages. | 0 | Organic Chemistry |
Bacterial ADP-ribosylating exotoxins (bAREs) covalently transfer an ADP-ribose moiety of NAD to target proteins of infected eukaryotes, to yield nicotinamide and a free hydrogen ion. bAREs are produced as enzyme precursors, consisting of a "A" and "B" domains: the "A" domain is responsible for ADP-ribosylation activity; and, the "B" domain for translocation of the enzyme across the membrane of the cell. These domains can exist in concert in three forms: first, as single polypeptide chains with A and B domains covalently linked; second, in multi-protein complexes with A and B domains bound by non-covalent interactions; and, third, in multi-protein complexes with A and B domains not directly interacting, prior to processing.
Upon activation, bAREs ADP-ribosylate any number of eukaryotic proteins; such mechanism is crucial to the instigation of the diseased states associated with ADP-ribosylation. GTP-binding proteins, in particular, are well-established in bAREs pathophysiology. For examples, cholera and heat-labile enterotoxin target the α-subunit of Gs of heterotrimeric GTP-binding proteins. As the α-subunit is ADP-ribosylated, it is permanently in an "active", GTP-bound state; subsequent activation of intracellular cyclic AMP stimulates the release of fluid and ions from intestinal epithelial cells. Furthermore, C. Botulinum C3 ADP-ribosylates GTP-binding proteins Rho and Ras, and Pertussis toxin ADP-ribosylates Gi, Go, and Gt. Diphtheria toxin ADP-ribosylates ribosomal elongation factor EF-2, which attenuates protein synthesis.
There are a variety of bacteria which employ bAREs in infection: CARDS toxin of Mycoplasma pneumoniae, cholera toxin of Vibrio cholerae; heat-labile enterotoxin of E. coli; exotoxin A of Pseudomonas aeruginosa; pertussis toxin of B. pertussis; C3 toxin of C. botulinum; and diphtheria toxin of Corynebacterium diphtheriae. | 1 | Biochemistry |
The formyl group readily oxidizes to the corresponding carboxyl group (). The preferred oxidant in industry is oxygen or air. In the laboratory, popular oxidizing agents include potassium permanganate, nitric acid, chromium(VI) oxide, and chromic acid. The combination of manganese dioxide, cyanide, acetic acid and methanol will convert the aldehyde to a methyl ester.
Another oxidation reaction is the basis of the silver-mirror test. In this test, an aldehyde is treated with Tollens reagent, which is prepared by adding a drop of sodium hydroxide solution into silver nitrate solution to give a precipitate of silver(I) oxide, and then adding just enough dilute ammonia solution to redissolve the precipitate in aqueous ammonia to produce complex. This reagent converts aldehydes to carboxylic acids without attacking carbon–carbon double bonds. The name silver-mirror test' arises because this reaction produces a precipitate of silver, whose presence can be used to test for the presence of an aldehyde.
A further oxidation reaction involves Fehling's reagent as a test. The complex ions are reduced to a red-brick-coloured precipitate.
If the aldehyde cannot form an enolate (e.g., benzaldehyde), addition of strong base induces the Cannizzaro reaction. This reaction results in disproportionation, producing a mixture of alcohol and carboxylic acid. | 0 | Organic Chemistry |
The x-axis is usually some function of compressor entry mass flow, usually corrected flow or non-dimensional flow, as opposed to real flow. This axis can be considered a rough measure of the axial Mach number of the flow through the device. | 7 | Physical Chemistry |
Because of their excellent characteristics and manufacturing methods, liquid metals are often used in wearable devices, medical devices, interconnected devices and so on.
Typical uses of liquid metals include thermostats, switches, barometers, heat transfer systems, and thermal cooling and heating designs. Uniquely, they can be used to conduct heat and/or electricity between non-metallic and metallic surfaces.
Liquid metal is sometimes used as a thermal interface material between coolers and processors because of its high thermal conductivity. The PlayStation 5 video game console uses liquid metal to help cool high temperatures inside the console. Liquid metal cooled reactors also use them.
Liquid metal can be used for wearable devices and for spare parts.
Liquid metal can sometimes be used for biological applications, i.e., making interconnects that flex without fatigue. As Galinstan is not particularly toxic, wires made from silicone with a core of liquid metal would be ideal for intracardiac pacemakers and neural implants where delicate brain tissue cannot tolerate a conventional solid implant. In fact, a wire constructed of this material can be stretched to 3 or even 5 times its length and still conduct electricity, returning to its original size and shape with no loss.
Due to their unique combination of high surface tension and fluidic deformability, liquid metals have been found to be a remarkable material for creating soft actuators. The force-generating mechanisms in liquid metal actuators are typically achieved by modulation of their surface tension. For instance, a liquid metal droplet can be designed to bridge two moving parts (e.g., in robotic systems) in such a way to generate contraction when the surface tension increases. The principles of muscle-like contraction in liquid metal actuators have been studied for their potential as a next-generation artificial muscle that offers several liquid-specific advantages over other solid materials. | 8 | Metallurgy |
Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or unintentional holes in ceilings, like ceiling fans and recessed lights. The rising warm air reduces the pressure in the base of the building, drawing cold air in through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.
In a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, while interior partitions, floors, and fire separations can mitigate it. Especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke and fire, and to maintain tenable conditions for occupants and firefighters. While natural ventilation methods may be effective, such as air outlets being installed closer to the ground, mechanical ventilation is often preferred for taller structures or in buildings with limited space. Smoke extraction is a key consideration in new constructions and must be evaluated in design stages.
The Grenfell Tower fire, as a result of which 72 people died, was in part exacerbated by the stack effect. A cavity between the outer aluminium cladding and the inner insulation formed a chimney and drew the fire upwards. | 7 | Physical Chemistry |
In one large-scale application, para-xylene is converted to terephthalic acid. The major application of ortho-xylene is as a precursor to phthalate esters, used as plasticizer. Meta-xylene is converted to isophthalic acid derivatives, which are components of alkyd resins. | 2 | Environmental Chemistry |
*The lac operon consists of 3 structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: lacZ, lacY, and lacA.
**lacZ encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose.
** lacY encodes Beta-galactoside permease (LacY), a transmembrane symporter that pumps β-galactosides including lactose into the cell using a proton gradient in the same direction. Permease increases the permeability of the cell to β-galactosides.
** lacA encodes β-galactoside transacetylase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to thiogalactoside.
Only lacZ and lacY appear to be necessary for lactose catabolic pathway. | 1 | Biochemistry |
Morphological control on the nanoscale is still limited as COFs lack synthetic control in higher dimensions due to the lack of dynamic chemistry during synthesis. To date, researchers have attempted to establish better control through different synthetic methods such as solvothermal synthesis, interface-assisted synthesis, solid templation as well as seeded growth. First one of the precursors is deposited onto the solid support followed by the introduction of the second precursor in vapor form. This results in the deposition of the COF as a thin film on the solid support. | 6 | Supramolecular Chemistry |
Ruppeiner geometry is thermodynamic geometry (a type of information geometry) using the language of Riemannian geometry to study thermodynamics. George Ruppeiner proposed it in 1979. He claimed that thermodynamic systems can be represented by Riemannian geometry, and that statistical properties can be derived from the model.
This geometrical model is based on the inclusion of the theory of fluctuations into the axioms of equilibrium thermodynamics, namely, there exist equilibrium states which can be represented by points on two-dimensional surface (manifold) and the distance between these equilibrium states is related to the fluctuation between them. This concept is associated to probabilities, i.e. the less probable a fluctuation between states, the further apart they are. This can be recognized if one considers the metric tensor g in the distance formula (line element) between the two equilibrium states
where the matrix of coefficients g is the symmetric metric tensor which is called a Ruppeiner metric, defined as a negative Hessian of the entropy function
where U is the internal energy (mass) of the system and N refers to the extensive parameters of the system. Mathematically, the Ruppeiner geometry is one particular type of information geometry and it is similar to the Fisher-Rao metric used in mathematical statistics.
The Ruppeiner metric can be understood as the thermodynamic limit (large systems limit) of the more general Fisher information metric. For small systems (systems where fluctuations are large), the Ruppeiner metric may not exist, as second derivatives of the entropy are not guaranteed to be non-negative.
The Ruppeiner metric is conformally related to the Weinhold metric via
where T is the temperature of the system under consideration. Proof of the conformal relation can be easily done when one writes down the first law of thermodynamics (dU=TdS+...) in differential form with a few manipulations. The Weinhold geometry is also considered as a thermodynamic geometry. It is defined as a Hessian of the internal energy with respect to entropy and other extensive parameters.
It has long been observed that the Ruppeiner metric is flat for systems with noninteracting underlying statistical mechanics such as the ideal gas. Curvature singularities signal critical behaviors. In addition, it has been applied to a number of statistical systems including Van der Waals gas. Recently the anyon gas has been studied using this approach. | 7 | Physical Chemistry |
The F portion of ATP synthase is hydrophilic and responsible for hydrolyzing ATP. The F unit protrudes into the mitochondrial matrix space. Subunits α and β make a hexamer with 6 binding sites. Three of them are catalytically inactive and they bind ADP.
Three other subunits catalyze the ATP synthesis. The other F subunits γ, δ, and ε are a part of a rotational motor mechanism (rotor/axle). The γ subunit allows β to go through conformational changes (i.e., closed, half open, and open states) that allow for ATP to be bound and released once synthesized. The F particle is large and can be seen in the transmission electron microscope by negative staining. These are particles of 9 nm diameter that pepper the inner mitochondrial membrane. | 5 | Photochemistry |
Multiplex ligation-dependent probe amplification (MLPA) is a variation of the multiplex polymerase chain reaction that permits amplification of multiple targets with only a single primer pair. It detects copy number changes at the molecular level, and software programs are used for analysis. Identification of deletions or duplications can indicate pathogenic mutations, thus MLPA is an important diagnostic tool used in clinical pathology laboratories worldwide. | 1 | Biochemistry |
-Photo-leucine acquires its function after being exposed to UV light. This causes diazirine ring of -photo-leucine to lose its nitrogen atoms in form of nitrogen gas, leaving its carbon atom as a reactive free radical. The bonds established between this carbon, belonging to one protein (A), and atoms belonging to another protein (B) are responsible for the cross-linking properties of -photo-leucine, which allow it to attach these two peptide chains into a single complex (AB).
The appropriate wavelength to activate the -photo-leucine molecule ranges from 320 to 370 nanometers. Lamps with higher power are more effective in accomplishing this objective and do so in less time. The ideal wavelength for the activation of the photo-leucine amino acid is of 345 nm.
To increase efficiency, a shallow and uncovered plate must be used. Also, rotation of the samples located under the UV right may be necessary to make sure they receive even UV irradiation, and thus to, yet again, improve the cross-linking efficiency. If the cross-linking is done in vivo, within living cells, these must be exposed to the UV radiation during a period of 15 minutes or less. | 5 | Photochemistry |
Marine carbon is further separated into particulate and dissolved phases. These pools are operationally defined by physical separation – dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.
There are two main types of inorganic carbon that are found in the oceans:
* Dissolved inorganic carbon (DIC) is made up of bicarbonate (), carbonate () and carbon dioxide (including both dissolved CO and carbonic acid HCO). DIC can be converted to particulate inorganic carbon (PIC) through precipitation of CaCO (biologically or abiotically). DIC can also be converted to particulate organic carbon (POC) through photosynthesis and chemoautotrophy (primary production). DIC increases with depth as organic carbon particles sink and are respired. Free oxygen decreases as DIC increases because oxygen is consumed during aerobic respiration.
* Particulate inorganic carbon (PIC) is the other form of inorganic carbon found in the ocean. Most PIC is the CaCO that makes up shells of various marine organisms, but can also form in whiting events. Marine fish also excrete calcium carbonate during osmoregulation.
Some of the inorganic carbon species in the ocean, such as bicarbonate and carbonate, are major contributors to alkalinity, a natural ocean buffer that prevents drastic changes in acidity (or pH). The marine carbon cycle also affects the reaction and dissolution rates of some chemical compounds, regulates the amount of carbon dioxide in the atmosphere and Earth's temperature. | 9 | Geochemistry |
According to the description of an atom interacting with the electromagnetic field, the absorption of light by the atom depends on the frequency of the incident photons. More precisely, the absorption is characterized by a Lorentzian of width Γ/2 (for reference, for common rubidium D-line transitions). If we have a cell of atomic vapour at room temperature, then the distribution of velocity will follow a Maxwell–Boltzmann distribution
where is the number of atoms, is the Boltzmann constant, and is the mass of the atom. According to the Doppler effect formula in the case of non-relativistic speeds,
where is the frequency of the atomic transition when the atom is at rest (the one which is being probed). The value of as a function of and can be inserted in the distribution of velocities. The distribution of absorption as a function of the pulsation will therefore be proportional to a Gaussian with full width at half maximum
For a rubidium atom at room temperature,
Therefore, without any special trick in the experimental setup probing the maximum of absorption of an atomic vapour, the uncertainty of the measurement will be limited by the Doppler broadening and not by the fundamental width of the resonance. | 7 | Physical Chemistry |
Greenhouse Gases Observing Satellite (GOSAT), also known as , is an Earth observation satellite and the worlds first satellite dedicated to greenhouse gas monitoring. It measures the densities of carbon dioxide and methane from 56,000 locations on the Earths atmosphere. The GOSAT was developed by the Japan Aerospace Exploration Agency (JAXA) and launched on 23 January 2009, from the Tanegashima Space Center. Japan's Ministry of the Environment, and the National Institute for Environmental Studies (NIES) use the data to track gases causing the greenhouse effect, and share the data with NASA and other international scientific organizations. | 2 | Environmental Chemistry |
* 999.99—five nines fine: The purest silver ever produced. This was achieved by the Royal Silver Company of Bolivia.
* 999.9—four nines fine: ultra-fine silver used by the Royal Canadian Mint for their Silver Maple Leaf and other silver coins
* 999—fine silver or three nines fine: used in Good Delivery bullion bars and most current silver bullion coins. Used in U.S. silver commemorative coins and silver proof coins starting in 2019.
* 980: common standard used in Mexico ca. 1930–1945
* 958: () Britannia silver
* 950: French 1st Standard
* 947.9: 91 zolotnik Russian silver
* 935: Swiss standard for watchcases after 1887, to meet the British Merchandise Marks Act and to be of equal grade to 925 sterling. Sometimes claimed to have arisen as a Swiss misunderstanding of the standard required for British sterling. Usually marked with three Swiss bears.
* 935: used in the Art Deco period in Austria and Germany. Scandinavian silver jewellers used 935 silver after the 2nd World War
* 925: () Sterling silver The UK has used this alloy from the early 12th century. Equivalent to plata de primera ley in Spain (first law silver)
* 917: a standard used for the minting of Indian silver (rupees), during the British raj and for some coins during the first Brazilian Republic.
* 916: 88 zolotnik Russian silver
* 900: one nine fine, coin-silver, or 90% silver: e.g. Flowing Hair and 1837–1964 U.S. silver coins. Also used in U.S. silver commemorative coins and silver proof coins 1982–2018.
* 892.4: US coinage fine "standard silver" as defined by the Coinage Act of 1792: e.g. Draped Bust and Capped Bust U.S. silver coins (1795–1836)
* 875: 84 zolotnik is the most common fineness for Russian silver. Swiss standard, commonly used for export watchcases (also 800 and later 935).
* 868: 83 zolotnik. Imperial Russian coinage between 1797 and 1885.
* 835: A standard predominantly used in Germany after 1884, and for some Dutch silver; and for the minting of coins in countries of the Latin Monetary Union
* 833: () a common standard for continental silver especially among the Dutch, Swedish, and Germans
* 830: A common standard used in older Scandinavian silver
* 800: The minimum standard for silver in Germany after 1884; the French 2nd standard for silver; "plata de segunda ley" in Spain (second law silver); Egyptian silver; Canadian silver circulating coinage from 1920-1966/7
* 750: An uncommon silver standard found in older German, Swiss and Austro-Hungarian silver
* 720: Decoplata: many Mexican and Dutch silver coins use this standard, as well as some coins from Portugal's former colonies, Japan, Uruguay, Ecuador, Egypt, and Morocco.
* 600: Used in some examples of postwar Japanese coins, such as the 1957-1966 100 yen coin
* 500: Standard used for making British coinage 1920–1946 as well as Canadian coins from 1967-1968, and some coins from Colombia and Brazil.
* 400: Standard used for US half dollars between 1965 and 1970, and commemorative issue Eisenhower dollars between 1971 and 1978. Also used in some Swedish Krona coins.
* 350: Standard used for US Jefferson "war nickels" minted between 1942 and 1945. | 8 | Metallurgy |
By 1982 the technology was sufficiently advanced for the technique to be called "high-speed" countercurrent chromatography (HSCCC). Peter Carmeci initially commercialized the PC Inc. Ito Multilayer Coil Separator/Extractor which utilized a single bobbin (onto which the coil is wound) and a counterbalance, plus a set of "flying leads" which are tubing that connect the bobbins. Dr. Walter Conway & others later evolved the bobbin design such that multiple coils, even coils of different tubing sizes, could be placed on the single bobbin. Edward Chou later evolved and commercialized a triple bobbin design as the Pharmatech CCC which had a de-twist mechanism for leads between the three bobbins. The Quattro CCC released in 1993 further evolved the commercially available instruments by utilizing a novel mirror image, twin bobbin design that did not need the de-twist mechanism of the Pharmatech between the multiple bobbins, so could still accommodate multiple bobbins on the same instrument. Hydrodynamic CCC are now available with up to 4 coils per instrument. These coils can be in PTFE, PEEK, PVDF, or stainless steel tubing. The 2, 3 or 4 coils can all be of the same bore to facilitate "2D" CCC (see below). The coils may be connected in series to lengthen the coil and increase the capacity, or the coils may be linked in parallel so that 2, 3, or 4 separations may be done simultaneously. The coils can also be of different sizes, on one instrument, ranging from 1 to 6 mm on one instrument, thus allowing a single instrument to optimize from mg to kilos per day. More recently instrument derivatives have been offered with rotating seals for various hydrodynamic CCC designs, instead of flying leads, either as custom or standard options. | 3 | Analytical Chemistry |
β-Catenin also acts as a morphogen in later stages of embryonic development. Together with TGF-β, an important role of β-catenin is to induce a morphogenic change in epithelial cells. It induces them to abandon their tight adhesion and assume a more mobile and loosely associated mesenchymal phenotype. During this process, epithelial cells lose expression of proteins like E-cadherin, Zonula occludens 1 (ZO1), and cytokeratin. At the same time they turn on the expression of vimentin, alpha smooth muscle actin (ACTA2), and fibroblast-specific protein 1 (FSP1). They also produce extracellular matrix components, such as type I collagen and fibronectin. Aberrant activation of the Wnt pathway has been implicated in pathological processes such as fibrosis and cancer. In cardiac muscle development, β-catenin performs a biphasic role. Initially, the activation of Wnt/β-catenin is essential for committing mesenchymal cells to a cardiac lineage; however, in later stages of development, the downregulation of β-catenin is required. | 1 | Biochemistry |
Britannia metal (also called britannium or Britannia ware) is a specific type of pewter alloy, favoured for its silvery appearance and smooth surface. The composition by weight is typically about 92% tin, 6% antimony, and 2% copper.
Britannia metal is usually spun rather than cast, and melts at 255 degrees Celsius. | 8 | Metallurgy |
Increased intracellular levels of lactate can act as a signalling hormone, inducing changes in gene expression that will upregulate genes involved in lactate removal. These genes include MCT1, cytochrome c oxidase (COX), and other enzymes involved in the lactate oxidation complex. Additionally, lactate will increase levels of peroxisome proliferator activated receptor gamma coactivator 1-alpha (PGC1-α), suggesting that lactate stimulates mitochondrial biogenesis. | 1 | Biochemistry |
However, most plants do not have CAM and must therefore open and close their stomata during the daytime, in response to changing conditions, such as light intensity, humidity, and carbon dioxide concentration. When conditions are conducive to stomatal opening (e.g., high light intensity and high humidity), a proton pump drives protons (H) from the guard cells. This means that the cells electrical potential becomes increasingly negative. The negative potential opens potassium voltage-gated channels and so an uptake of potassium ions (K) occurs. To maintain this internal negative voltage so that entry of potassium ions does not stop, negative ions balance the influx of potassium. In some cases, chloride ions enter, while in other plants the organic ion malate is produced in guard cells. This increase in solute concentration lowers the water potential inside the cell, which results in the diffusion of water into the cell through osmosis. This increases the cells volume and turgor pressure. Then, because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, and thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held firmly in place by surrounding epidermal cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can diffuse.
When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the cytosol of the cells and cause the concentration of free Ca to increase in the cytosol due to influx from outside the cell and release of Ca from internal stores such as the endoplasmic reticulum and vacuoles. This causes the chloride (Cl) and organic ions to exit the cells. Second, this stops the uptake of any further K into the cells and, subsequently, the loss of K. The loss of these solutes causes an increase in water potential, which results in the diffusion of water back out of the cell by osmosis. This makes the cell plasmolysed, which results in the closing of the stomatal pores.
Guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial. | 5 | Photochemistry |
Soil contamination from heavy elements can be found in the urban environments, which can be attributed to the transportation and industries along with the background levels (minerals-leaching heavy elements from weathering). Also, some of the most soil contaminated areas are around the mines such as the ones in Slovenia, Bosnia and Herzegovina, and in United States (Sulphur Bank Superfund Site, in California). In a study area, GIS is used for the analysis of spatial relationship of the contaminants within the soil. | 2 | Environmental Chemistry |
Camphorsultam is synthesized by reduction of camphorsulfonylimine. This reaction was originally performed as a catalytic hydrogenation using Raney Nickel, but the modern preparation instead uses lithium aluminium hydride for the reduction. These reductive methods are stereoselective: although both the endo and exo diastereomeric forms are theoretically possible, only the exo isomer is actually produced due to steric effects of one of the methyl groups. Camphorsultam is often referred to as Oppolzers sultam' in reference to Wolfgang Oppolzer and colleagues, who developed the lithium aluminium hydride approach to this compound and pioneered its use in asymmetric synthesis. | 0 | Organic Chemistry |
The radical pair mechanism is currently accepted as the most common cause of CIDNP. This theory was proposed by Closs, and, independently, by Kaptein and Oosterhoff. There are, however, exceptions, and the DNP mechanism was found to be operational, for example, in many fluorine-containing radicals.
The chemical bond is a pair of electrons with opposite spins. Photochemical reactions or heat can cause an electron in the bond to change its spin. The electrons are now unpaired, in what is known as a triplet state, and the bond is broken. The orientation of some of the nuclear spins will favour some unpaired electrons changing their spins and so revert to the normal pairs as chemical bonds. This quantum interaction is known as spin–orbit coupling. Other nuclear spins will exert a different influence on the triplet pairs, giving the radical pairs more time to separate and react with other molecules. Consequently, the products of recombination will have different distributions of nuclear spins from the products produced by separated radicals. | 7 | Physical Chemistry |
To find the shape of the minimal surface bounded by some arbitrary shaped frame using strictly mathematical means can be a daunting task. Yet by fashioning the frame out of wire and dipping it in soap-solution, a locally minimal surface will appear in the resulting soap-film within seconds.
The reason for this is that the pressure difference across a fluid interface is proportional to the mean curvature, as seen in the Young–Laplace equation. For an open soap film, the pressure difference is zero, hence the mean curvature is zero, and minimal surfaces have the property of zero mean curvature. | 6 | Supramolecular Chemistry |
In vitro studies using purified 20S proteasomes showed that salinosporamide A has lower EC50 for trypsin-like (T-L) activity than does bortezomib. In vivo animal model studies show marked inhibition of T-L activity in response to salinosporamide A, whereas bortezomib enhances T-L proteasome activity.
Initial results from early-stage clinical trials of salinosporamide A in relapsed/refractory multiple myeloma patients were presented at the 2011 American Society of Hematology annual meeting. Further early-stage trials of the drug in a number of different cancers are ongoing. | 0 | Organic Chemistry |
The van der Waals surface of a molecule is an abstract representation or model of that molecule, illustrating where, in very rough terms, a surface might reside for the molecule based on the hard cutoffs of van der Waals radii for individual atoms, and it represents a surface through which the molecule might be conceived as interacting with other molecules.
Also referred to as a van der Waals envelope, the van der Waals surface is named for Johannes Diderik van der Waals, a Dutch theoretical physicist and thermodynamicist who developed theory to provide a liquid-gas equation of state that accounted for the non-zero volume of atoms and molecules, and on their exhibiting an attractive force when they interacted (theoretical constructions that also bear his name).
van der Waals surfaces are therefore a tool used in the abstract representations of molecules, whether accessed, as they were originally, via hand calculation, or via physical wood/plastic models, or now digitally, via computational chemistry software.
Practically speaking, CPK models, developed by and named for Robert Corey, Linus Pauling, and Walter Koltun, were the first widely used physical molecular models based on van der Waals radii, and allowed broad pedagogical and research use of a model showing the van der Waals surfaces of molecules. | 6 | Supramolecular Chemistry |
The inscription is undated, and contains a eulogy of a king named Candra, whose dynasty it does not mention. The identity of this king, and thus the date of the pillar, has been the subject of much debate. The various viewpoints about the identity of the issuer were assembled and analyzed in a volume edited by M. C. Joshi and published in 1989.
The king is now generally identified with the Gupta King Chandragupta II. This identification is based on several points:
* The script and the poetic style of the inscription, which point to a date in the late fourth or early fifth century CE: the Gupta period.
* The inscription describes the king as a devotee of the God Vishnu, and records the erection of a dhvaja ("standard", or pillar) of Vishnu, on a hill called Viṣṇupada ("hill of the footprint of Viṣṇu"). Other Gupta inscriptions also describe Chandragupta II as a Bhagavata (devotee of Vishnu). The names of the places mentioned in the inscription are also characteristic of the Gupta Era. For example, (the Indian Ocean) and (the Bengal region).
* The short name Candra is inscribed on the archer-type gold coins of Chandragupta II, while his full name and titles appear in a separate, circular legend on the coin.
* A royal seal of Chandragupta's wife Dhruvadevi contains the phrase ("Nārāyaṇa, the lord of the illustrious Viṣṇupada").
As the inscription is a eulogy and states that the king has abandoned the earth, there has been some discussion as to whether it is posthumous, i.e. whether King Chandra was dead when the record was created. Dasharatha Sharma (1938) argued that it was non-posthumous. According to B. Chhabra and G. S. Gai, the inscription states that the king's mind is "fixed upon Vishnu with devotion", and therefore, indicates that the king was alive at the time. They theorize that it may have been recorded when Chandragupta II abdicated his throne, and settled down as a vanaprastha (retiree) in Viṣṇupada. | 8 | Metallurgy |
It is common to model such a network with a set of coupled ordinary differential equations (ODEs) or SDEs, describing the reaction kinetics of the constituent parts. Suppose that our regulatory network has nodes, and let represent the concentrations of the corresponding substances at time . Then the temporal evolution of the system can be described approximately by
where the functions express the dependence of on the concentrations of other substances present in the cell. The functions are ultimately derived from basic principles of chemical kinetics or simple expressions derived from these e.g. Michaelis–Menten enzymatic kinetics. Hence, the functional forms of the are usually chosen as low-order polynomials or Hill functions that serve as an ansatz for the real molecular dynamics. Such models are then studied using the mathematics of nonlinear dynamics. System-specific information, like reaction rate constants and sensitivities, are encoded as constant parameters.
By solving for the fixed point of the system:
for all , one obtains (possibly several) concentration profiles of proteins and mRNAs that are theoretically sustainable (though not necessarily stable). Steady states of kinetic equations thus correspond to potential cell types, and oscillatory solutions to the above equation to naturally cyclic cell types. Mathematical stability of these attractors can usually be characterized by the sign of higher derivatives at critical points, and then correspond to biochemical stability of the concentration profile. Critical points and bifurcations in the equations correspond to critical cell states in which small state or parameter perturbations could switch the system between one of several stable differentiation fates. Trajectories correspond to the unfolding of biological pathways and transients of the equations to short-term biological events. For a more mathematical discussion, see the articles on nonlinearity, dynamical systems, bifurcation theory, and chaos theory. | 1 | Biochemistry |
The foundations of Feigls work on spot analysis were the works of Hugo Schiff (the earliest publication about "spot test" was Shiffs detection of uric acid in 1859) and of Christian Friedrich Schonberg and Friedrich Goppelsröder on capillary analysis.
On the occasion of Feigl's 70th birthday the Chemical Society of Midland sponsored a symposium in 1952, attended by 500 scientists from 24 countries, in which all plenary sessions were related to spot tests.
The test uses the qualitative characteristics of colored compounds to account for performed chemical reactions. This technique has been used to develop new quantification methods using modern technology. | 3 | Analytical Chemistry |
Bronze statuettes were also made in every period of antiquity for votive use, and at least in Hellenistic and Roman times for domestic ornaments and furniture of household shrines. But the art of bronze statuary hardly existed before the introduction of hollow casting, about the middle of the 6th century BC. The most primitive votive statuettes are oxen and other animals, which evidently represent victims offered to the gods. They have been found abundantly on many temple sites. But classical art preferred the human subject, votaries holding gifts or in their ordinary guise, or gods themselves in human form. Such figures are frequently inscribed with formulas of dedication. Gods and goddesses posed conformably with their traditional characters and bearing their distinctive attributes are the most numerously represented class of later statuettes. They are a religious genre, appearing first in 4th-century sculpture and particularly favoured by Hellenistic sentiment and Roman pedantry. Many of them were doubtless votive figures, others were images in domestic shrines, and some were certainly ornaments. Among the cult-idols are the dancing Lares, who carry cornucopias and libation-bowls. The little Heracles that Lysippus made for Alexander was a table-ornament (epitrapezios): he was reclining on the lion's skin, his club in one hand, a wine-cup in the other. | 8 | Metallurgy |
There are also symbols that appear to be chemical element symbols, but represent certain very common substituents or indicate an unspecified member of a group of elements. These are called pseudoelement symbols or organic elements and are treated like univalent "elements" in skeletal formulae. A list of common pseudoelement symbols: | 0 | Organic Chemistry |
The term "redox" stands for reduction-oxidation. It refers to electrochemical processes involving electron transfer to or from a molecule or ion, changing its oxidation state. This reaction can occur through the application of an external voltage or through the release of chemical energy. Oxidation and reduction describe the change of oxidation state that takes place in the atoms, ions or molecules involved in an electrochemical reaction. Formally, oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. An atom or ion that gives up an electron to another atom or ion has its oxidation state increase, and the recipient of the negatively charged electron has its oxidation state decrease.
For example, when atomic sodium reacts with atomic chlorine, sodium donates one electron and attains an oxidation state of +1. Chlorine accepts the electron and its oxidation state is reduced to −1. The sign of the oxidation state (positive/negative) actually corresponds to the value of each ion's electronic charge. The attraction of the differently charged sodium and chlorine ions is the reason they then form an ionic bond.
The loss of electrons from an atom or molecule is called oxidation, and the gain of electrons is reduction. This can be easily remembered through the use of mnemonic devices. Two of the most popular are "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) and "LEO" the lion says "GER" (Lose Electrons: Oxidation, Gain Electrons: Reduction). Oxidation and reduction always occur in a paired fashion such that one species is oxidized when another is reduced. For cases where electrons are shared (covalent bonds) between atoms with large differences in electronegativity, the electron is assigned to the atom with the largest electronegativity in determining the oxidation state.
The atom or molecule which loses electrons is known as the reducing agent, or reductant, and the substance which accepts the electrons is called the oxidizing agent, or oxidant. Thus, the oxidizing agent is always being reduced in a reaction; the reducing agent is always being oxidized. Oxygen is a common oxidizing agent, but not the only one. Despite the name, an oxidation reaction does not necessarily need to involve oxygen. In fact, a fire can be fed by an oxidant other than oxygen; fluorine fires are often unquenchable, as fluorine is an even stronger oxidant (it has a weaker bond and higher electronegativity, and thus accepts electrons even better) than oxygen.
For reactions involving oxygen, the gain of oxygen implies the oxidation of the atom or molecule to which the oxygen is added (and the oxygen is reduced). In organic compounds, such as butane or ethanol, the loss of hydrogen implies oxidation of the molecule from which it is lost (and the hydrogen is reduced). This follows because the hydrogen donates its electron in covalent bonds with non-metals but it takes the electron along when it is lost. Conversely, loss of oxygen or gain of hydrogen implies reduction. | 7 | Physical Chemistry |
Baculovirus, a rod-shaped virus which infects insect cells, is used as the expression vector in this system. Insect cell lines derived from Lepidopterans (moths and butterflies), such as Spodoptera frugiperda, are used as host. A cell line derived from the cabbage looper is of particular interest, as it has been developed to grow fast and without the expensive serum normally needed to boost cell growth. The shuttle vector is called bacmid, and gene expression is under the control of a strong promoter pPolh. Baculovirus has also been used with mammalian cell lines in the BacMam system.
Baculovirus is normally used for production of glycoproteins, although the glycosylations may be different from those found in vertebrates. In general, it is safer to use than mammalian virus as it has a limited host range and does not infect vertebrates without modifications. | 1 | Biochemistry |
The International Generic Sample Number or IGSN is a persistent identifier for sample. As an active persistent identifier it can be resolved through the Handle System. The system is used in production by the System for Earth Sample Registration (SESAR), Geoscience Australia, Commonwealth Scientific and Industrial Research Organisation Mineral Resources, Australian Research Data Commons (ARDC), University of Bremen MARUM, German Research Centre for Geosciences (GFZ), IFREMER Institut Français de Recherche pour l'Exploitation de la Mer, Korea Institute of Geoscience & Mineral Resources (KIGAM), and University of Kiel. Other organisations are preparing the introduction of the IGSN.
The IGSN was developed as the International Geo Sample Number to provide a persistent, globally unique, web resolvable identifier for physical samples. IGSN is both a governance and technical system for assigning globally unique persistent identifiers to physical samples. Even though initially developed for samples in the geosciences, the application of IGSN can be and has already been expanded to other domains that rely on physical samples and collections. To take into account the expanded scope of the application of IGSN beyond the earth and environmental sciences, the IGSN Implementation Organization (IGSN e.V.) voted to change the name of the identifier to International Generic Sample Number (IGSN) and rename the organisation accordingly.
The IGSN preserves the identity of a sample even as it is moved from lab to lab and as data appear in different publications, thus eliminating ambiguity that stems from similar names for samples from the earth. The IGSN unique identifier allows researchers to track the analytical history of a sample and build on previously collected data as new techniques are developed. Additionally, the IGSN provides a link between disparate data generated by different investigators and published in different scientific articles.
In September 2021, the members of IGSN e.V. and DataCite agreed to enter a partnership. Under the partnership, DataCite will provide the IGSN ID registration services and supporting technology to enable the ongoing sustainability of the IGSN PID infrastructure. The IGSN e.V. will facilitate a Community of Communities to promote and support new research and innovation for standard methods of identifying, citing, and locating physical samples. | 9 | Geochemistry |
George Hermann Büchi (August 1, 1921 – August 28, 1998) was a Swiss organic chemist and professor at the Massachusetts Institute of Technology. "Paternòs reaction", known since the early twentieth century, was renamed to the "Paternò–Büchi reaction" based on enhancements made to it by Büchis research group.
Büchi died at the age of 77 of heart failure while hiking with his wife in Switzerland. | 0 | Organic Chemistry |
For mutually soluble compounds, Raoult's law states that the partial pressure of a compound is equal to its vapor pressure times its mole fraction. This means that mutually soluble contaminants will volatilize slower than if there was only one compound present. | 2 | Environmental Chemistry |
One strand of the DNA, the template strand (or noncoding strand), is used as a template for RNA synthesis. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complementarity with the DNA template to create an RNA copy (which elongates during the traversal). Although RNA polymerase traverses the template strand from 3 → 5, the coding (non-template) strand and newly formed RNA can also be used as reference points, so transcription can be described as occurring 5 → 3. This produces an RNA molecule from 5 → 3, an exact copy of the coding strand (except that thymines are replaced with uracils, and the nucleotides are composed of a ribose (5-carbon) sugar where DNA has deoxyribose (one fewer oxygen atom) in its sugar-phosphate backbone).
mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene. The characteristic elongation rates in prokaryotes and eukaryotes are about 10–100 nts/sec. In eukaryotes, however, nucleosomes act as major barriers to transcribing polymerases during transcription elongation. In these organisms, the pausing induced by nucleosomes can be regulated by transcription elongation factors such as TFIIS.
Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure.
Double-strand breaks in actively transcribed regions of DNA are repaired by homologous recombination during the S and G2 phases of the cell cycle. Since transcription enhances the accessibility of DNA to exogenous chemicals and internal metabolites that can cause recombinogenic lesions, homologous recombination of a particular DNA sequence may be strongly stimulated by transcription. | 1 | Biochemistry |
Latimer diagrams can be used in the construction of Frost diagrams, as a concise summary of the standard electrode potentials relative to the element. Since ΔG to HO, then from HO to HO) is the same as the Gibbs energy change for the overall reduction (i.e. from O to HO), in accordance with Hess's law. This can be used to find the electrode potential for non-adjacent steps, which gives all the information necessary for the Frost diagram.
A simple examination of a Latimer diagram can also indicate if a species will disproportionate in solution under the conditions for which the electrode potentials are given: if the potential to the right of the species is higher than the potential on the left, it will disproportionate. Therefore, hydrogen peroxide is unstable and will disproportionate (see diagram above). | 7 | Physical Chemistry |
An isothermal titration calorimeter is composed of two identical cells made of a highly efficient thermally conducting and chemically inert material such as Hastelloy alloy or gold, surrounded by an adiabatic jacket. Sensitive thermopile/thermocouple circuits are used to detect temperature differences between the reference cell (filled with buffer or water) and the sample cell containing the macromolecule. Prior to addition of ligand, a constant power (<1 mW) is applied to the reference cell. This directs a feedback circuit, activating a heater located on the sample cell. During the experiment, ligand is titrated into the sample cell in precisely known aliquots, causing heat to be either taken up or evolved (depending on the nature of the reaction). Measurements consist of the time-dependent input of power required to maintain equal temperatures between the sample and reference cells.
In an exothermic reaction, the temperature in the sample cell increases upon addition of ligand. This causes the feedback power to the sample cell to be decreased (remember: a reference power is applied to the reference cell) in order to maintain an equal temperature between the two cells. In an endothermic reaction, the opposite occurs; the feedback circuit increases the power in order to maintain a constant temperature (isothermal operation).
Observations are plotted as the power needed to maintain the reference and the sample cell at an identical temperature against time. As a result, the experimental raw data consists of a series of spikes of heat flow (power), with every spike corresponding to one ligand injection. These heat flow spikes/pulses are integrated with respect to time, giving the total heat exchanged per injection. The pattern of these heat effects as a function of the molar ratio [ligand]/[macromolecule] can then be analyzed to give the thermodynamic parameters of the interaction under study.
To obtain an optimum result, each injection should be given enough time for a reaction equilibrium to reach. Degassing samples is often necessary in order to obtain good measurements as the presence of gas bubbles within the sample cell will lead to abnormal data plots in the recorded results. The entire experiment takes place under computer control.
Direct titration is performed most commonly with ITC to obtain the thermodynamic data, by binding two components of the reaction directly to each other. However, many of the chemical reactions and binding interactions may have higher binding affinity above what is desirable with the c-window. To troubleshoot the limitation of c-window and conditions for certain binding interactions, various different methods of titration can be performed. In some cases, simply doing a reverse titration of changing the samples between the injection syringe and sample cell can solve the issue, depending on the binding mechanism. For most of the high or low affinity bindings require chelation or competitive titration. This method is done by loading pre-bound complex solution in the sample cell, and chelating one of the components out with a reagent of higher observed binding affinity within the desirable c-window. | 7 | Physical Chemistry |
An electrolithoautotroph is an organism which feeds on electricity. These organisms use electricity to convert carbon dioxide into organic matter by using electrons directly taken from solid-inorganic electron donors. Electrolithoautotrophs are microorganisms which are found in the deep crevices of the ocean. The warm, mineral-rich environment provides a rich source of nutrients. The electron source for carbon assimilation from diffusible Fe ions to an electrode under the condition that electrical current is the only source of energy and electrons. Electrolithoautotrophs form a third metabolic pathway compared to photosynthesis (plants converting light into sugar) and chemosynthesis (bacteria converting chemical energy into food). | 1 | Biochemistry |
A biomonitoring assessment requires a baseline dataset which, ideally, defines the environment in its natural or default state. This is then used for comparison against any subsequent measurements, in order to assess potential alterations or trends.
In some cases, these datasets are used to create standardised tools for assessing water quality via biomonitoring data, such as the Specific Pollution Index (SPI) and South African Diatom Index (SADI). | 2 | Environmental Chemistry |
The satellite was originally launched from Vandenberg Air Force Base in California on a dedicated Taurus XL rocket. However, the payload fairing—a clam shell-shaped covering that protects the satellite during launch—apparently failed to separate from the spacecraft. "We have not had a successful launch tonight and will not be able to have a successful OCO mission", NASA commentator George Diller said.
* Date: 24 February 2009, 09:55:31 UTC
* Launch Vehicle: Orbital Sciences, Taurus-XL
* Launch Site: Vandenberg Air Force Base, Launch Complex 576-E
A payload fairing is a clamshell-shaped cover that encloses and protects a payload on the pad and during early flight. Fairings are a standard component of expendable launch vehicles, and are always jettisoned as soon as possible after a rocket has climbed high enough for heating from air friction to no longer risk damaging the payload. The Taurus XL's fairing was intended to separate several seconds after stage 2 ignition. Its extra mass was not a significant factor during the flight of the larger lower stages, but kept the relatively small stage 3 from adding enough velocity to reach orbit. 17 minutes after liftoff the payload fell into the ocean near Antarctica. NASA investigators later determined the cause for the launch failure to be faulty materials provided by aluminum manufacturer Sapa Profiles. | 2 | Environmental Chemistry |
A regulatory sequence is a segment of a nucleic acid molecule which is capable of increasing or decreasing the expression of specific genes within an organism. Regulation of gene expression is an essential feature of all living organisms and viruses. | 1 | Biochemistry |
The carbon cycle was first described by Antoine Lavoisier and Joseph Priestley, and popularised by Humphry Davy. The global carbon cycle is now usually divided into the following major reservoirs of carbon (also called carbon pools) interconnected by pathways of exchange:
* Atmosphere
* Terrestrial biosphere
* Ocean, including dissolved inorganic carbon and living and non-living marine biota
* Sediments, including fossil fuels, freshwater systems, and non-living organic material.
* Earth's interior (mantle and crust). These carbon stores interact with the other components through geological processes.
The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.
The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments are fairly balanced; so carbon levels would be roughly stable without human influence. | 5 | Photochemistry |
The periodic presence of euxinic conditions in the Lower Cambrian has been supported by evidence found on the Yangtze platform in South China. Sulfur isotopes during the transition from Proterozoic to Phanerozoic give evidence for widespread euxinia, perhaps lasting throughout the Cambrian period. Towards the end of the Lower Cambrian, the euxinic chemocline grew deeper until euxinia was present only in the sediments, and once sulfate became limiting, conditions became anoxic instead of euxinic. Some areas eventually became oxic, while others eventually returned to euxinic for some time.
Geological records from the paleozoic in the Selwyn Basin in Northern Canada have also shown evidence for episodic stratification and mixing, where, using δS, it was determined that hydrogen sulfide was more prevalent than sulfate. Although this was not originally attributed to euxinia, further studies found that seawater in that time likely had low concentrations of sulfate, meaning that the sulfur in the water was primarily in the form of sulfide. This combined with organic-rich black shale provide strong evidence for euxinia.
There is similar evidence in the black shales in the mid-continent North America from the Devonian and early Mississippian periods. Isorenieratene, a pigment known as a proxy for an anoxic photic zone, has been found in the geological record in Illinois and Michigan. Although present, these events were probably ephemeral and did not last for longer periods of time. Similar periodic evidence of euxinia can also be found in the Sunbury shales of Kentucky.
Evidence for euxinia has also been tied to the Kellwasser events of the Late Devonian Extinction event. Euxinia in basinal waters in what is now central Europe (Germany, Poland, and France) persisted for part of the late Devonian, and may have spread up into shallow waters, contributing to the extinction event.
There was perhaps a period of oxygenation of bottom waters during the Carboniferous, most likely between the Late Devonian Extinction and the Permian-Triassic Extinction, at which point euxinia would be very rare in the paleo oceans.
The Permian–Triassic extinction event may also have some ties to euxinia, with hypercapnia and hydrogen sulfide toxicity killing off many species. Presence of a biomarker for anaerobic photosynthesis by green sulfur bacteria has been found spanning from the Permian to early Triassic in sedimentary rock in both Australia and China, meaning that euxinic conditions extended up quite shallow in the water column, contributing to the extinctions and perhaps even slowed the recovery. It is uncertain, however, just how widespread photic zone euxinia was during this period. Modelers have hypothesized that due to environmental conditions anoxia and sulfide may have been brought up from a deep, vast euxinic reservoir in upwelling areas, but stable, gyre-like areas remained oxic. | 9 | Geochemistry |
Z-FA-FMK, abbreviating for benzyloxycarbonyl-phenylalanyl-alanyl-fluoromethyl ketone, is a very potent irreversible inhibitor of cysteine proteases, including cathepsins B, L, and S, cruzain, and papain. It also selectively inhibits effector caspases 2, 3, 6, and 7 but not caspases 8 and 10. This compound has been shown to block the production of IL1-α, IL1-β, and TNF-α induced by LPS in macrophages by inhibiting NF-κB pathways. | 1 | Biochemistry |
Serum iron is a medical laboratory test that measures the amount of circulating iron that is bound to transferrin and freely circulate in the blood. Clinicians order this laboratory test when they are concerned about iron deficiency, which can cause anemia and other problems. 65% of the iron in the body is bound up in hemoglobin molecules in red blood cells. About 4% is bound up in myoglobin molecules. Around 30% of the iron in the body is stored as ferritin or hemosiderin in the spleen, the bone marrow and the liver. Small amounts of iron can be found in other molecules in cells throughout the body. None of this iron is directly accessible by testing the serum.}
However, some iron is circulating in the serum. Transferrin is a molecule produced by the liver that binds one or two iron(III) ions, i.e. ferric iron, Fe; transferrin is essential if stored iron is to be moved and used. Most of the time, about 30% of the available sites on the transferrin molecule are filled. The test for serum iron uses blood drawn from veins to measure the iron ions that are bound to transferrin and circulating in the blood. This test should be done after 12 hours of fasting. The extent to which sites on transferrin molecules are filled by iron ions can be another helpful clinical indicator, known as percent transferrin saturation. Another lab test saturates the sample to measure the total amount of transferrin; this test is called total iron-binding capacity (TIBC). These three tests are generally done at the same time, and taken together are an important part of the diagnostic process for conditions such as anemia, iron deficiency anemia, anemia of chronic disease and haemochromatosis. | 1 | Biochemistry |
For a redox reaction
R O + e, without mass-transfer limitation, the relationship between the current density and the electrode overpotential is given by the Butler–Volmer equation:
with
is the exchange current density and and are the symmetry factors.
The curve vs. is not a straight line (Fig. 1), therefore a redox reaction is not a linear system. | 7 | Physical Chemistry |
The first reported synthesis and characterisation of phosphaethynolate came from Becker et al. in 1992. They were able to isolate the anion as a lithium salt (in 87% yield) by reacting lithium bis(trimethylsilyl)phosphide with dimethyl carbonate . The x-ray crystallographic analysis of the anion determined the bond length to be (indicative of a phosphorus-carbon triple bond) and the bond length to be . Similar studies were performed on derivatives of this structure and the results indicated that dimerisation to form a four-membered Li ring is favoured by this molecule.
Ten years later, in 2002, Westerhausen et al. published the use of Becker's method to make a family of alkaline earth metal salts of PCO ; this work involved the synthesis of the magnesium, calcium, strontium and barium bis-phosphaethynolates. Like the salts previously reported by Becker, the alkali-earth metal analogues were unstable to moisture and air and thus were required to be stored at low temperatures (around ) in dimethoxyethane solutions.
It was not until 2011 that the first stable salt of the phosphaethynolate anion was reported by Grutzmacher and co-workers . They managed to isolate the compound as a brown solid in 28% yield. The structure of the stable sodium salt, formed by carbonylation of sodium phosphide, contains bridging PCO units in contrast to the terminal anions found in the previously reported structures. The authors noted that this sodium salt could be handled in air as well as water without major decomposition; this emphasises the significance of the accompanying counter cation in stabilisation of PCO.
Direct carbonylation was a method also employed by Goicoechea in 2013 in order to synthesis a phosphaethynolate anion stabilised by a potassium cation sequestered in 18-crown-6 . This method required the carbonylation of solutions of at and produced by-products that were readily separated during aqueous work ups. The use of aqueous work ups reflects the high stability of the salt in water. This method afforded the PCO anion in reasonable yields around 43%. Characterisation of the compound involved infra-red spectroscopy; the band indicative of the triple bond stretch was observed at . | 7 | Physical Chemistry |
Bioanalysis is a sub-discipline of analytical chemistry covering the quantitative measurement of xenobiotics (drugs and their metabolites, and biological molecules in unnatural locations or concentrations) and biotics (macromolecules, proteins, DNA, large molecule drugs, metabolites) in biological systems. | 3 | Analytical Chemistry |
Oxidative deamination is the first step to breaking down the amino acids so that they can be converted to sugars. The process begins by removing the amino group of the amino acids. The amino group becomes ammonium as it is lost and later undergoes the urea cycle to become urea, in the liver. It is then released into the blood stream, where it is transferred to the kidneys, which will secrete the urea as urine. The remaining portion of the amino acid becomes oxidized, resulting in an α-keto acid. The alpha-keto acid will then proceed into the TCA cycle, in order to produce energy. The acid can also enter glycolysis, where it will be eventually converted into pyruvate. The pyruvate is then converted into acetyl-CoA so that it can enter the TCA cycle and convert the original pyruvate molecules into ATP, or usable energy for the organism.
Transamination leads to the same result as deamination: the remaining acid will undergo either glycolysis or the TCA cycle to produce energy that the organism's body will use for various purposes. This process transfers the amino group instead of losing the amino group to be converted into ammonium. The amino group is transferred to α-ketoglutarate, so that it can be converted to glutamate. Then glutamate transfers the amino group to oxaloacetate. This transfer is so that the oxaloacetate can be converted to aspartate or other amino acids. Eventually, this product will also proceed into oxidative deamination to once again produce alpha-ketoglutarate, an alpha-keto acid that will undergo the TCA cycle, and ammonium, which will eventually undergo the urea cycle.
Transaminases are enzymes that help catalyze the reactions that take place in transamination. They help catalyze the reaction at the point when the amino group is transferred from the original amino acid, like glutamate to α-ketoglutarate, and hold onto it to transfer it to another α-ketoacid. | 1 | Biochemistry |
This gene encodes an E26 transformation-specific related transcription factor. The encoded protein is primarily expressed in lymphoid cells and can act as both an enhancer and a repressor to regulate transcription of various genes. Alternative splicing results in multiple transcript variants. | 1 | Biochemistry |
In order to understand how life arose, knowledge is required of the chemical pathways that permit formation of the key building blocks of life under plausible prebiotic conditions. The RNA world hypothesis holds that in the primordial soup there existed free-floating ribonucleotides, the fundamental molecules that combine in series to form RNA. Complex molecules such as RNA must have emerged from relatively small molecules whose reactivity was governed by physico-chemical processes. RNA is composed of pyrimidine and purine nucleotides, both of which are necessary for reliable information transfer, and thus natural selection and Darwinian evolution. Becker et al. showed how pyrimidine nucleosides can be synthesized from small molecules and ribose, driven solely by wet-dry cycles. Purine nucleosides can be synthesized by a similar pathway. 5’-mono-and diphosphates also form selectively from phosphate-containing minerals, allowing concurrent formation of polyribonucleotides with both the pyrimidine and purine bases. Thus a reaction network towards the pyrimidine and purine RNA building blocks can be established starting from simple atmospheric or volcanic molecules. | 1 | Biochemistry |
Larive worked in the field of bioanalytical chemistry, applying analytical tools such as nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry to the products of chemical separations. Much of her research focused on reducing the amount of sample needed for analysis, such as constructing microcoil NMR probes that can measure as little as 25 nL of sample and are usable as part of an capillary isotachophoresis apparatus. This has been applied to structure determination of heparin and heparan sulfate. She developed NMR pulse sequences to study protein ligand interactions in complexes with multiple ligands. She also researches analytical methods for metabolomics and chemogenomics for the reaction of plants to pesticides and hypoxia using NMR and mass spectroscopy.
NMR is often thought of as a low-sensitivity method, but Larive's laboratory has developed ways of increasing the sensitivity of their measurements to obtain precise chemical and structural information. The techniques she has developed are relevant to understanding carbohydrate structure and biosynthesis, designing new drugs and measuring the purity of pharmaceuticals. Her work in developing chemical profiles for substances also has relevance for the authentication of foodstuffs such as wine, olive oil, and pomegranate juice. | 3 | Analytical Chemistry |
A dipole-induced dipole interaction (Debye force) is due to the approach of a molecule with a permanent dipole to another non-polar molecule with no permanent dipole. This approach causes the electrons of the non-polar molecule to be polarized toward or away from the dipole (or "induce" a dipole) of the approaching molecule. Specifically, the dipole can cause electrostatic attraction or repulsion of the electrons from the non-polar molecule, depending on orientation of the incoming dipole. Atoms with larger atomic radii are considered more "polarizable" and therefore experience greater attractions as a result of the Debye force. | 6 | Supramolecular Chemistry |
An American IPM system is designed around six basic components:
* Acceptable pest levels—The emphasis is on control, not eradication. IPM holds that wiping out an entire pest population is often impossible, and the attempt can be expensive and unsafe. IPM programmes first work to establish acceptable pest levels, called action thresholds, and apply controls if those thresholds are crossed. These thresholds are pest and site specific, meaning that it may be acceptable at one site to have a weed such as white clover, but not at another site. Allowing a pest population to survive at a reasonable threshold reduces selection pressure. This lowers the rate at which a pest develops resistance to a control, because if almost all pests are killed then those that have resistance will provide the genetic basis of the future population. Retaining a significant number of unresistant specimens dilutes the prevalence of any resistant genes that appear. Similarly, the repeated use of a single class of controls will create pest populations that are more resistant to that class, whereas alternating among classes helps prevent this.
* Preventive cultural practices—Selecting varieties best for local growing conditions and maintaining healthy crops is the first line of defense. Plant quarantine and cultural techniques such as crop sanitation are next, e.g., removal of diseased plants, and cleaning pruning shears to prevent spread of infections. Beneficial fungi and bacteria are added to the potting media of horticultural crops vulnerable to root diseases, greatly reducing the need for fungicides.
* Monitoring—Regular observation is critically important. Observation is broken into inspection and identification. Visual inspection, insect and spore traps, and other methods are used to monitor pest levels. Record-keeping is essential, as is a thorough knowledge of target pest behavior and reproductive cycles. Since insects are cold-blooded, their physical development is dependent on area temperatures. Many insects have had their development cycles modeled in terms of degree-days. The degree days of an environment determines the optimal time for a specific insect outbreak. Plant pathogens follow similar patterns of response to weather and season. Recently, automated systems based on AI have been developed to identify and monitor flies using e-trapping devices.
* Mechanical controls—Should a pest reach an unacceptable level, mechanical methods are the first options. They include simple hand-picking, barriers, traps, vacuuming and tillage to disrupt breeding.
* Biological controls—Natural biological processes and materials can provide control, with acceptable environmental impact, and often at lower cost. The main approach is to promote beneficial insects that eat or parasitize target pests. Biological insecticides, derived from naturally occurring microorganisms (e.g.—Bt, entomopathogenic fungi and entomopathogenic nematodes), also fall in this category. Further biology-based or ecological techniques are under evaluation.
* Responsible use—Synthetic pesticides are used as required and often only at specific times in a pests life cycle. Many newer pesticides are derived from plants or naturally occurring substances (e.g.'—nicotine, pyrethrum and insect juvenile hormone analogues), but the toxophore or active component may be altered to provide increased biological activity or stability. Applications of pesticides must reach their intended targets. Matching the application technique to the crop, the pest, and the pesticide is critical. The use of low-volume spray equipment reduces overall pesticide use and labor cost.
An IPM regime can be simple or sophisticated. Historically, the main focus of IPM programmes was on agricultural insect pests. Although originally developed for agricultural pest management, IPM programmes are now developed to encompass diseases, weeds and other pests that interfere with management objectives for sites such as residential and commercial structures, lawn and turf areas, and home and community gardens. Predictive models have proved to be suitable tools supporting the implementation of IPM programmes. | 9 | Geochemistry |
The SSA can be measured by adsorption using the BET isotherm. This has the advantage of measuring the surface of fine structures and deep texture on the particles. However, the results can differ markedly depending on the substance adsorbed. The BET theory has inherent limitations but has the advantage to be simple and to yield adequate relative answers when the solids are chemically similar. In relatively rare cases, more complicated models based on thermodynamic approaches, or even quantum chemistry, may be applied to improve the consistency of the results, but at the cost of much more complex calculations requiring advanced knowledge and a good understanding from the operator. | 7 | Physical Chemistry |
The output of CAGE is a set of short nucleotide sequences (often called tags in analogy to expressed sequence tags) with their observed counts. Copy numbers of CAGE tags provide a digital quantification of the RNA transcript abundances in biological samples. Using a reference genome, a researcher can usually determine, with some confidence, the original mRNA (and therefore which gene) the tag was extracted from.
Unlike a similar technique serial analysis of gene expression (SAGE) in which tags come from other parts of transcripts, CAGE is primarily used to locate exact transcription start sites in the genome. This knowledge in turn allows a researcher to investigate promoter structure necessary for gene expression.
CAGE tags tend to start with an extra guanine (G) that is not encoded in the genome, which is attributed to the template-free 5′-extension during the first-strand cDNA synthesis or reverse-transcription of the cap itself. When not corrected, this can induce erroneous mapping of CAGE tags, for instance to nontranscribed pseudogenes. On the other hand, this addition of Gs was also utilised as a signal to filter more reliable TSS peaks. | 1 | Biochemistry |
Endangered animal species and breeds are preserved using similar techniques. Animal species can be preserved in genebanks, which consist of cryogenic facilities used to store living sperm, eggs, or embryos. For example, the Zoological Society of San Diego has established a "frozen zoo" to store such samples using cryopreservation techniques from more than 355 species, including mammals, reptiles, and birds.
A potential technique for aiding in reproduction of endangered species is interspecific pregnancy, implanting embryos of an endangered species into the womb of a female of a related species, carrying it to term. It has been carried out for the Spanish ibex. | 1 | Biochemistry |
The lower flammability limit or lower explosive limit (LFL/LEL) represents the lowest air to fuel vapor concentration required for combustion to take place when ignited by an external source, for any particular chemical. Any concentration lower than this could not produce a flame or result in combustion. The upper flammability limit or upper explosive limit (UFL/UEL) represents the highest air to fuel vapor concentration at which combustion can take place when ignited by an external source. Any fuel-air mixture higher than this would be too concentrated to result in combustion. The values existing between these two limits represent the flammable or explosive range. Within this threshold, give an external ignition source, combustion of the particular fuel would likely happen. | 7 | Physical Chemistry |
Centromeres are the sites where spindle fibers attach to newly replicated chromosomes in order to segregate them into daughter cells when the cell divides. Each eukaryotic chromosome has a single functional centromere that is seen as a constricted region in a condensed metaphase chromosome. Centromeric DNA consists of a number of repetitive DNA sequences that often take up a significant fraction of the genome because each centromere can be millions of base pairs in length. In humans, for example, the sequences of all 24 centromeres have been determined and they account for about 6% of the genome. However, it is unlikely that all of this noncoding DNA is essential since there is considerable variation in the total amount of centromeric DNA in different individuals. Centromeres are another example of functional noncoding DNA sequences that have been known for almost half a century and it is likely that they are more abundant than coding DNA. | 1 | Biochemistry |
Every nitrate ion reduced to ammonia produces one OH ion. To maintain a pH balance, the plant must either excrete it into the surrounding medium or neutralize it with organic acids. This results in the medium around the plants roots becoming alkaline when they take up nitrate.
To maintain ionic balance, every NO taken into the root must be accompanied by either the uptake of a cation or the excretion of an anion. Plants like tomatoes take up metal ions like K, Na, Ca and Mg to exactly match every nitrate taken up and store these as the salts of organic acids like malate and oxalate. Other plants like the soybean balance most of their NO intake with the excretion of OH or HCO.
Plants that reduce nitrates in the shoots and excrete alkali from their roots need to transport the alkali in an inert form from the shoots to the roots. To achieve this they synthesize malic acid in the leaves from neutral precursors like carbohydrates. The potassium ions brought to the leaves along with the nitrate in the xylem are then sent along with the malate to the roots via the phloem. In the roots, the malate is consumed. When malate is converted back to malic acid prior to use, an OH is released and excreted. (RCOO + HO -> RCOOH +OH) The potassium ions are then recirculated up the xylem with fresh nitrate. Thus the plants avoid having to absorb and store excess salts and also transport the OH.
Plants like castor reduce a lot of nitrate in the root itself, and excrete the resulting base. Some of the base produced in the shoots is transported to the roots as salts of organic acids while a small amount of the carboxylates are just stored in the shoot itself. | 1 | Biochemistry |
A manual or automated process injects the sample onto a catalyst in a combustion tube operated from 680 up to 950 degrees C in an oxygen rich atmosphere. The concentration of carbon dioxide generated is measured with a non-dispersive infrared (NDIR) detector.
Oxidation of the sample is complete after injection into the furnace, turning oxidizable material in the sample into gaseous form. A carbon-free carrier gas transports the CO, through a moisture trap and halide scrubbers to remove water vapor and halides from the gas stream before it reaches the detector. These substances can interfere with the detection of the CO gas. The HTCO method may be useful in those applications where difficult to oxidize compounds, or high molecular weight organics, are present as it provides almost complete oxidation of organics including solids and particulates small enough to be injected into the furnace. The major drawback of HTCO analysis is its unstable baseline resulting from the gradual accumulation of non-volatile residues within the combustion tube. These residues continuously change TOC background levels requiring continuous background correction. Because aqueous samples are injected directly into a very hot, usually quartz, furnace only small aliquots (less than 2 milliliters and usually less than 50 - 100 microliter, with a maximum of approximately 300 - 400 micro-liters under special conditions; such as repetitive injections) of sample can be handled making the methods less sensitive than chemical oxidation methods capable of digesting as much as 10 times more sample. Also, the salt content of the samples do not combust, and so therefore, gradually build a residue inside the combustion tube eventually clogging the catalyst resulting in poor peak shapes, and degraded accuracy or precision, unless appropriate maintenance procedures are followed. The catalyst should be regenerated or replaced as needed. To avoid this problem the manufacturing industry has developed several concepts, such as matrix separation, ceramic reactors, better process control or methods without catalysts. | 3 | Analytical Chemistry |
In 1993 Philex Mining Corporation, a Philippines mining company, replaced the mechanical cleaner circuit with Cells at its Benguet copper concentrator. Following their successful operation, Philex replaced the mechanical cells in its cleaner-scavenger circuit in 1994 and began the phased introduction of Cell rougher and scavenger lines that was completed in early 1996. This was the first operation in which the external recycle mechanism system was applied. By the time the last Jameson Cell was installed, the entire flotation circuit was composed of Jameson Cells.
The motivation for installing Jameson Cells was, in part, to take advantage of their space-saving capabilities and to improve copper recovery at a minimum cost. The Cell circuit occupied 60% less floor area and achieved equivalent results to the mechanical banks with 40% of their residence time. They provided a power saving of 18%.
In addition to these benefits, the use of the Jameson Cells in the rougher and rougher–scavenger section of the plant resulted in a 3.3% increase in copper recovery and a 4.5% increase in gold recovery. When combined with the other Cells in the cleaner, recleaner and cleaner–scavenger section, there was a 2.6% increase in final copper concentrate grade and a 3.5% increase in plant copper recovery, with a 2.6% increase in plant gold recovery. | 8 | Metallurgy |
There is an increased risk of central nervous system depression when eszopiclone is taken together with other CNS depressant agents, including antipsychotics, sedative hypnotics (like barbiturates or benzodiazepines), antihistamines, opioids, phenothiazines, and some antidepressants. There is also increased risk of central nervous system depression with other medications that inhibit the metabolic activities of the CYP3A4 enzyme system of the liver. Medications that inhibit this enzyme system include nelfinavir, ritonavir, ketoconazole, itraconazole and clarithromycin. Alcohol also has an additive effect when used concurrently with eszopiclone. Eszopiclone is most effective if it is not taken after a heavy meal with high fat content. | 4 | Stereochemistry |
If one could observe a gas under a powerful microscope, one would see a collection of particles without any definite shape or volume that are in more or less random motion. These gas particles only change direction when they collide with another particle or with the sides of the container. This microscopic view of gas is well-described by statistical mechanics, but it can be described by many different theories. The kinetic theory of gases, which makes the assumption that these collisions are perfectly elastic, does not account for intermolecular forces of attraction and repulsion. | 7 | Physical Chemistry |
The stability of a colloidal system is defined by particles remaining suspended in solution and depends on the interaction forces between the particles. These include electrostatic interactions and van der Waals forces, because they both contribute to the overall free energy of the system.
A colloid is stable if the interaction energy due to attractive forces between the colloidal particles is less than kT, where k is the Boltzmann constant and T is the absolute temperature. If this is the case, then the colloidal particles will repel or only weakly attract each other, and the substance will remain a suspension.
If the interaction energy is greater than kT, the attractive forces will prevail, and the colloidal particles will begin to clump together. This process is referred to generally as aggregation, but is also referred to as flocculation, coagulation or precipitation. While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible, permanent aggregation where the forces holding the particles together are stronger than any external forces caused by stirring or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called a floc. The term precipitation is normally reserved for describing a phase change from a colloid dispersion to a solid (precipitate) when it is subjected to a perturbation. Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if either of these processes occur the colloid will no longer be a suspension.
Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.
* Electrostatic stabilization is based on the mutual repulsion of like electrical charges. The charge of colloidal particles is structured in an electrical double layer, where the particles are charged on the surface, but then attract counterions (ions of opposite charge) which surround the particle. The electrostatic repulsion between suspended colloidal particles is most readily quantified in terms of the zeta potential. The combined effect of van der Waals attraction and electrostatic repulsion on aggregation is described quantitatively by the DLVO theory. A common method of stabilising a colloid (converting it from a precipitate) is peptization, a process where it is shaken with an electrolyte.
* Steric stabilization consists absorbing a layer of a polymer or surfactant on the particles to prevent them from getting close in the range of attractive forces. The polymer consists of chains that are attached to the particle surface, and the part of the chain that extends out is soluble in the suspension medium. This technique is used to stabilize colloidal particles in all types of solvents, including organic solvents.
A combination of the two mechanisms is also possible (electrosteric stabilization).
A method called gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped, and the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. Examples of such substances are xanthan and guar gum. | 7 | Physical Chemistry |
Electrofusion welding requires electrical current to be passed through the coils implanted in the fittings. Since the electrical energy input is an excellent indicator of the joint strength that develops during fusion, it is necessary to have consistent electrical power input. Energy input during the joining process is typically measured by controlling the time it takes for the current to pass through the fitting. However, energy input can also be monitored by controlling overall temperature, molten polymer temperature, or molten polymer pressure.
A control box takes electrical power from a generator and converts it into an appropriate voltage and current for electrofusion joining. This provides consistent energy input for each application. The most common input voltage for electrofusion welding fittings is 39.5V, as it provides the best results without risking operator safety. The current is input as an alternating current (AC) waveform. | 7 | Physical Chemistry |
The existence of backsplicing was first suggested in 2012. This backsplicing explains the genesis of circular RNAs resulting from the exact junction between the 3 boundary of an exon with the 5 boundary of an exon located upstream. In these exonic circular RNAs, the junction is a classic 3-5link.
The exclusion of intronic sequences during splicing can also leave traces, in the form of circular RNAs. In some cases, the intronic lariat is not destroyed and the circular part remains as a lariat-derived circRNA.In these lariat-derived circular RNAs, the junction is a 2-5link. | 1 | Biochemistry |
Nickel allergy is the most common contact allergy in industrialized countries, affecting around 8% to 19% of adults and 8% to 10% of children. Women are affected 4–10 times as frequently as men. | 1 | Biochemistry |
Due to the relatively small sample size achievable with dealloying, the mechanical properties of these materials are often probed using the following techniques:
* Nanoindentation
* Micropillar compression
* Deflection testing of bridges
* Thin-film wrinkling | 8 | Metallurgy |
The Polanyi potential adsorption theory is based on the assumption that the molecules near a surface move according to a potential, similar to that of gravity or electric fields. This model is applicable in the case of gases at a surface at constant temperature. Gas molecules move closer to that surface when the pressure is higher than the equilibrium vapor pressure. The change in potential relative to the distance from the surface can be calculated using the formula for difference of the chemical potential,
where is the chemical potential, is the molar entropy, is the molar volume, and is the molar internal energy.
At equilibrium, the chemical potential of a gas at a distance from a surface, , is equal to the chemical potential of the gas at an infinitely large distance from the surface, . As a result, the integration from an infinitely far distance to r distance from the surface leads to
where is the partial pressure at distance r and is the partial pressure at infinite distance from the surface.
Since the temperature remains constant, the difference in chemical potential formula can be integrated over pressures and
By setting the , the equation can be simplified to
Using the ideal gas law, , the following formula is obtained
Since gas condenses into a liquid on a surface when the pressure of the gas exceeds the equilibrium vapor pressure, , we can assume a liquid film forms over the surface of thickness, . The energy at is
Considering that the partial pressure of the gases relates to the concentration, the adsorption potential, can be calculated as
where is the saturated concentration of adsorbate and is the equilibrium concentration of the adsorbate. | 7 | Physical Chemistry |
Scientists observed the thermogenic activity in brown adipose tissue, which eventually led to the discovery of UCP1, initially known as "Uncoupling Protein". The brown tissue revealed elevated levels of mitochondria respiration and another respiration not coupled to ATP synthesis, which symbolized strong thermogenic activity. UCP1 was the protein discovered responsible for activating a proton pathway that was not coupled to ADP phosphorylation (ordinarily done through ATP Synthase). | 1 | Biochemistry |
Corrosion coupons are made in different shapes and sizes. These coupons are often made from same material of the pipe or tank/vessel which should be monitored for corrosion. The most common used coupons are as follows:
* strip coupon
* ladder strip coupon
* flush disc coupon
* multi-disc coupon
* scale coupon | 8 | Metallurgy |
Patricia Ana Matrai is a marine scientist known for her work on the cycling of sulfur. She is a senior research scientist at Bigelow Laboratory for Ocean Sciences. | 9 | Geochemistry |
Moexipril was an angiotensin converting enzyme inhibitor (ACE inhibitor) used for the treatment of hypertension and congestive heart failure. Moexipril can be administered alone or with other antihypertensives or diuretics.
It works by inhibiting the conversion of angiotensin I to angiotensin II.
It was patented in 1980 and approved for medical use in 1995. Moexipril is available from Schwarz Pharma under the trade name Univasc. | 4 | Stereochemistry |
Morpholino oligos were conceived by Summerton (Gene Tools) at AntiVirals Inc. (now Sarepta Therapeutics) and originally developed in collaboration with Weller. | 1 | Biochemistry |
In chemistry, a chemical trap is a chemical compound that is used to detect unstable compounds. The method relies on efficiency of bimolecular reactions with reagents to produce a more easily characterize trapped product. In some cases, the trapping agent is used in large excess. | 3 | Analytical Chemistry |
There have been disputes on metamerism being included with other isomerisms such as position as well as chain isomerism, some authors still keep using it in their textbooks, mostly citing the examples of ethers and secondary amines. | 4 | Stereochemistry |
* R: (:ja:理科年表): An (mostly) annual reference book published in Japan since 1925CE. Note that the actual published year is typically one year earlier than the nominal (book title) year.
</references>
* I: : Revised roughly by each decade. First edition 1935CE.
</references>
*D: Kyoritsu Great Dictionary of Chemistry
</references>
*X: Other usage examples
</references> | 9 | Geochemistry |
Inadequate magnesium intake frequently causes muscle spasms, and has been associated with cardiovascular disease, diabetes, high blood pressure, anxiety disorders, migraines, osteoporosis, and cerebral infarction. Acute deficiency (see hypomagnesemia) is rare, and is more common as a drug side-effect (such as chronic alcohol or diuretic use) than from low food intake per se, but it can occur in people fed intravenously for extended periods of time.
The most common symptom of excess oral magnesium intake is diarrhea. Supplements based on amino acid chelates (such as glycinate, lysinate etc.) are much better-tolerated by the digestive system and do not have the side-effects of the older compounds used, while sustained-release dietary supplements prevent the occurrence of diarrhea. Since the kidneys of adult humans excrete excess magnesium efficiently, oral magnesium poisoning in adults with normal renal function is very rare. Infants, which have less ability to excrete excess magnesium even when healthy, should not be given magnesium supplements, except under a physician's care.
Pharmaceutical preparations with magnesium are used to treat conditions including magnesium deficiency and hypomagnesemia, as well as eclampsia. Such preparations are usually in the form of magnesium sulfate or chloride when given parenterally. Magnesium is absorbed with reasonable efficiency (30% to 40%) by the body from any soluble magnesium salt, such as the chloride or citrate. Magnesium is similarly absorbed from Epsom salts, although the sulfate in these salts adds to their laxative effect at higher doses. Magnesium absorption from the insoluble oxide and hydroxide salts (milk of magnesia) is erratic and of poorer efficiency, since it depends on the neutralization and solution of the salt by the acid of the stomach, which may not be (and usually is not) complete.
Magnesium orotate may be used as adjuvant therapy in patients on optimal treatment for severe congestive heart failure, increasing survival rate and improving clinical symptoms and patient's quality of life.
In 2021, magnesium salts were the 211th most commonly prescribed medication in the United States, with more than 2million prescriptions. | 1 | Biochemistry |
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