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Aluminium chloride is manufactured on a large scale by the exothermic reaction of aluminium metal with chlorine or hydrogen chloride at temperatures between .
Aluminium chloride may be formed via a single displacement reaction between copper(II) chloride and aluminium.
In the US in 1993, approximately 21,000 tons were produced, not counting the amounts consumed in the production of aluminium.
Hydrated aluminium trichloride is prepared by dissolving aluminium oxides in hydrochloric acid. Metallic aluminium also readily dissolves in hydrochloric acid ─ releasing hydrogen gas and generating considerable heat. Heating this solid does not produce anhydrous aluminium trichloride, the hexahydrate decomposes to aluminium hydroxide when heated:
Aluminium also forms a lower chloride, aluminium(I) chloride (AlCl), but this is very unstable and only known in the vapour phase. | 0 | Organic Chemistry |
Mauveine is a mixture of four related aromatic compounds differing in number and placement of methyl groups. Its organic synthesis involves dissolving aniline, p-toluidine, and o-toluidine in sulfuric acid and water in a roughly 1:1:2 ratio, then adding potassium dichromate.
Mauveine A () incorporates 2 molecules of aniline, one of p-toluidine, and one of o-toluidine. Mauveine B () incorporates one molecule each of aniline, p-toluidine, and two of o-toluidine. In 1879, Perkin showed mauveine B related to safranines by oxidative/reductive loss of the p-tolyl group. In fact, safranine is a 2,8-dimethyl phenazinium salt, whereas the parasafranine produced by Perkin is presumed to be the 1,8- (or 2,9-) dimethyl isomer.
The molecular structure of mauveine proved difficult to determine, finally being identified in 1994. In 2007, two more were isolated and identified: mauveine B2, an isomer of mauveine B with methyl on different aryl group, and mauveine C, which has one more p-methyl group than mauveine A.
In 2008, additional mauveines and pseudomauveines were discovered, bringing the total number of these compounds up to 12. In 2015 a crystal structure was reported for the first time. | 7 | Physical Chemistry |
*Addition reaction
**Aldol addition
**Electrophilic addition
**Michael addition
**Mukaiyama aldol addition
**Nucleophilic addition
*Cyclization
**Bergman cyclization
**Nazarov cyclization reaction
*Elimination reaction
**Beta elimination
**Cope elimination
**E1cB elimination reaction
**Hofmann elimination
*Organic redox reaction
**Cannizzaro reaction
**Oxidation
***Baeyer-Villiger oxidation
***Corey-Kim oxidation
***Dess-Martin oxidation
***Fleming-Tamao oxidation
***Jones oxidation
***Nucleophilic epoxidation
***Oppenauer oxidation
***Prilezhaev reaction
***Rubottom oxidation
***Schmidt reaction
***Swern oxidation
***Wacker-Tsuji oxidation
**Reduction
***Birch reduction
***Bouveault-Blanc reduction
***CBS reduction
***Clemmensen reduction
***Corey-Bakshi-Shibata reduction
***Corey–Itsuno reduction
***Fukuyama reduction
***Luche reduction
***Meerwein-Ponndorf-Verley reduction
***Rosenmund reduction
***Staudinger reduction
***Wolff-Kishner reduction
*Pericyclic reaction
**Cheletropic reaction
**Cycloaddition
***1,3-Dipolar cycloaddition
****Azide-alkyne Huisgen cycloaddition
***Diels–Alder reaction
***Nitrone-olefin (3+2) cycloaddition
***Staudinger ketene-imine cycloaddition
**Dyotropic reaction
**Electrocyclic reaction
**Group transfer reaction
**Sigmatropic reaction
*Polymerization
**Ring-opening metathesis polymerisation
*Rearrangement reaction
**Baker–Venkataraman rearrangement
**Beckmann rearrangement
**Benzilic acid rearrangement
**Brook rearrangement
**Claisen rearrangement
**Cope rearrangement
**Curtius rearrangement
**Fries rearrangement
**Ireland–Claisen rearrangement
**Newman–Kwart rearrangement
**Overman rearrangement
**Oxy-Cope rearrangement
**Pinacol rearrangement
**1,2-Wittig rearrangement
**2,3-Wittig rearrangement
*Substitution reaction
**Electrophilic aromatic substitution
**Nucleophilic aromatic substitution
**Electrophilic substitution
**Nucleophilic substitution
***S1 reaction
***S2 reaction
**Vicarious nucleophilic substitution | 0 | Organic Chemistry |
A supramolecular host could bind to a guest molecule in such a way that the guests labile group is positioned close to the reactive group of another reactive species. The proximity of the two groups enhances the probability that the reaction could occur and thus the reaction rate is increased. This concept is similar to the principle of preorganization which states that complexation could be improved if the binding motifs are preorganized in a well-defined position so that the host does not require any major conformational change for complexation. In this case, the catalyst is preorganized such that no major conformational changes is required for the reaction to occur. A notable example of catalysts that employ this mechanism is Jean-Marie Lehns crown ether. In addition, catalysts based on functionalized cyclodextrins often employ this mode of catalysis. | 6 | Supramolecular Chemistry |
Silicon carbide can host point defects in the crystal lattice which are known as color centers. These defects can produce single photons on demand and thus serve as a platform for single-photon source. Such a device is a fundamental resource for many emerging applications of quantum information science. If one pumps a color center via an external optical source or electric current, the color center will be brought to the excited state and then relax with the emission of one photon.
One well known point defect in silicon carbide is the divacancy which has a similar electronic structure as the nitrogen-vacancy center in diamond. In 4H-SiC, the divacancy has four different configurations which correspond to four zero-phonon lines (ZPL). These ZPL values are written using the notation V-V and the unit eV: hh(1.095), kk(1.096), kh(1.119), and hk(1.150). | 8 | Metallurgy |
His academic teaching experience spans in both undergraduate level at the University of Massachusetts and postgraduate level at the University of Connecticut and the University of Patras.
His published work as co-author includes three books, six chapters in other authors' books, 92 scientific papers and 146 research presentations. Vasilis Gregoriou is also co-inventor of 15 patents. His research interests include flexible photovoltaic cells based on organic semiconductors, optically active materials based on conjugated oligomers, and nanostructured polymer materials. He has served as President of Society for Applied Spectroscopy (SAS) in 2001 and now he participates as National Representative of Greece in the Committee of the European Research Council (ERC) for the Horizon 2020 program, the Mari Sklodowska-Curie actions and the Future and Emerging Technologies (FET). Vasilis Gregoriou has been the Director of the National Hellenic Research Foundation since 2013.
As a technology entrepreneur, Vasilis Gregoriou is the co-founder and the CEO of Advent Technologies which is based in Cambridge, Massachusetts. Advent Technologies develops advanced technology and devices in the field of energy and defense and it has also developed research collaborations with Northeastern University in Boston, US, Patras University in Greece, and the Institute of Chemical Engineering Sciences (ICE-HT/FORTH). | 7 | Physical Chemistry |
Electron-counting rules are used to predict the preferred electron count for molecules. The octet rule, the 18-electron rule, and Hückels 4n + 2 pi-electron rule are proven to be useful in predicting the molecular stability. Wades rules were formulated to explain the electronic requirement of monopolyhedral borane clusters. The Jemmis mno rules are an extension of Wade's rules, generalized to include condensed polyhedral boranes as well.
The first condensed polyhedral borane, , is formed by sharing four vertices between two icosahedra. According to Wades n + 1 rule for n-vertex closo structures, should have a charge of +2 (n + 1 = 20 + 1 = 21 pairs required; 16 BH units provide 16 pairs; four shared boron atoms provide 6 pairs; thus 22 pairs are available). To account for the existence of as a neutral species, and to understand the electronic requirement of condensed polyhedral clusters, a new variable, m, was introduced and corresponds to the number of polyhedra (sub-clusters). In Wades n + 1 rule, the 1 corresponds to the core bonding molecular orbital (BMO) and the n corresponds to the number of vertices, which in turn is equal to the number of tangential surface BMOs. If m polyhedra condense to form a macropolyhedron, m core BMOs will be formed. Thus the skeletal electron pair (SEP) requirement of closo-condensed polyhedral clusters is m + n.
Single-vertex sharing is a special case where each subcluster needs to satisfy Wades rule separately. Let a and b be the number of vertices in the subclusters including the shared atom. The first cage requires a + 1 and the second cage requires b + 1 SEPs. Therefore, a total of a + b + 2 or a + b + m SEPs are required; but a + b = n + 1, as the shared atom is counted twice. The rule can be modified to m + n + 1, or generally m + n + o, where o corresponds to the number of single-vertex sharing condensations. The rule can be made more general by introducing a variable, p, corresponding to the number of missing vertices, and q', the number of caps. As such, the generalized Jemmis rule can be stated as follows:
:The SEP requirement of condensed polyhedral clusters is m + n + o + p − q, where m is the number of subclusters, n is the number of vertices, o is the number of single-vertex shared condensations, p is the number of missing vertices and q is the number of caps. | 7 | Physical Chemistry |
The insert is created by PCR using Taq polymerase. This polymerase lacks 3 to 5 proofreading activity and, with a high probability, adds a single, 3-adenine overhang to each end of the PCR product. It is best if the PCR primers have guanines at the 5 end as this maximizes probability of Taq DNA polymerase adding the terminal adenosine overhang. Thermostable polymerases containing extensive 3´ to 5´ exonuclease activity should not be used as they do not leave the 3´ adenine-overhangs. | 1 | Biochemistry |
Thermometric titrimetry is particularly suited to the determination of a range of analytes where a precipitate is formed by reaction with the titrant. In some cases, an alternative to traditional potentiometric titration practice can be offered. In other cases, reaction chemistries may be employed for which there is no satisfactory equivalent in potentiometric titrimetry. | 3 | Analytical Chemistry |
The following panels show sea surface concentrations of fields prepared by GLODAPv1.1. The "pre-industrial" is the 18th century, while "present-day" is approximately the 1990s.
The following panels show sea surface concentrations of fields prepared by GLODAPv2. The "pre-industrial" is the 18th century, while "present-day" is normalised to 2002. Note that these properties are shown in mass units (per kilogram of seawater) rather than the volume units (per cubic metre of seawater) used in the GLODAPv1.1 panels. | 9 | Geochemistry |
There is a hypothesis that states that TEs might provide a ready source of DNA that could be co-opted by the cell to help regulate gene expression. Research showed that many diverse modes of TEs co-evolution along with some transcription factors targeting TE-associated genomic elements and chromatin are evolving from TE sequences. Most of the time, these particular modes do not follow the simple model of TEs and regulating host gene expression. | 1 | Biochemistry |
In terms of applications, a popular allyl complex is allyl palladium chloride.
The reactivity of allyl ligands depends on the overall complex, although the influence of the metal center can be roughly summarized as
:(more reactive) Fe ≫ Pd > Mo > W (less reactive)
Such complexes are usually electrophilic (i.e., react with nucleophiles), but nickel allyl complexes are usually nucleophilic (resp. with electrophiles). In the former case, the addition may occur at unusual locations, and can be useful in organic synthesis. | 0 | Organic Chemistry |
Research has shown that applying 5 to 15 volts of electricity for 50 ms to the segmental nerve that innervates the light organ leads to a glow 1.5 seconds after that lasts for five to ten seconds. Stimulation of the segmental nerve has been found to lead to several different nerve impulses, and frequency of nervous impulses has been found to be proportional to the intensity of the stimulus applied. A high frequency of nervous impulse was found to lead to a constant latency. The light organ is inactive in the absence of nerve impulses. Constant nerve signaling was shown to coincide with constant emission of light from the light organ with a higher frequency coinciding with a higher amplitude of light emitted up to 30 impulses per second. Impulses beyond this frequency were not found to be associated with a more intense glow. The fact that the frequency of nerve impulses was able to exceed beyond the maximum intensity of light emission suggests some limitations in the mechanism either arising from the synapse or the cell's light producing process. Additionally, a series of action potentials have been shown to lead to the sporadic, discontinuous emission to light. It was also found that a higher frequency of action potentials lead to a higher likelihood of any emission of light. Nerve impulses are associated with a depolarization of the photocyte which plays a role in its light emitting mechanism, and greater depolarization events were found to be associated with more intense lightning. The nerve innervating the light organ containing photocytes has only two axons, but they branch repeatedly allowing the numerous photocytes to be innervated with each cell being associated with several nerve terminals with each terminal possibly being associated with several synapses.
It was found that the junction between at the end of the neuron innervating the light organ differs from the kind of junction found between two different neurons or between neurons and muscles in the neuromuscular junction. The depolarization of the photocyte following nervous stimulation was found to be one-hundred times slower than the with the other two kinds of junctions and this slow response cannot be attributed to the rate of diffusion because the synapse between the neuron and photocyte is relatively small. It has been found that the neurons that control the light mechanism terminate at the tracheal cells rather than the photocytes themselves. | 1 | Biochemistry |
Flash photolysis is an alternative to pulse radiolysis that uses high-power light pulses (e.g. from an excimer laser) rather than beams of electrons to initiate chemical reactions. Typically ultraviolet light is used which requires less radiation shielding than required for the X-rays emitted in pulse radiolysis. | 5 | Photochemistry |
In July 2016 Florida declared a state of emergency for four counties as a result of blooms. They were said to be "destroying" a number of businesses and affecting local economies, with many needing to shut down entirely. Some beaches were closed, and hotels and restaurants suffered a drop in business. Tourist sporting activities such as fishing and boating were also affected.
In 2019, the biggest Sargassum bloom ever seen created a crisis in the Tourism industry in North America. This event was likely caused by climate change and nutrient pollution from fertilizers. Several Caribbean countries considered declaring a state of emergency due to the impact on tourism as a result of environmental damage and potentially toxic and harmful health effects. | 3 | Analytical Chemistry |
Size of sea salt aerosols ranges widely from ~0.05 to 10 μm in diameter, with most of masses concentrated in super-micron range (coarse mode), and highest number concentration in sub-micron range. Correspondingly, sea salt aerosols have a wide range of atmospheric lifetimes. As the sea salt aerosols are hygroscopic, their particle sizes may vary with humidity by up to a factor of 2. Sea salt aerosols influence the sulfate aerosol formation in different ways due to the different sizes. Very small sea salt aerosols, which are below the critical diameter for droplet activation at low supersaturations, can serve as nuclei for the growth of sulfate particles, while larger sea salt particles serve as a sink for gaseous hydrogen sulfate (HSO) molecules, reducing the amount of sulfate available for the formation of accumulation mode particles. | 9 | Geochemistry |
The signs and symptoms of acute beryllium pneumonitis usually resolve over several weeks to months, but may be fatal in 10 percent of cases, and about 15–20% of cases may progress to chronic beryllium disease.
Acute beryllium poisoning approximately doubles the risk of lung cancer. The mechanism by which beryllium is carcinogenic is unclear, but may be due to ionic beryllium binding to nucleic acids; it is not mutagenic. | 1 | Biochemistry |
IRsweep was founded in 2014 as a spin-off from the Swiss Federal Institute of Technology (ETH Zurich). The company commercialized its first product after having developed its prototypes for academic research projects. The first derived product is the IRcell, a cylindrical multipass cell combining a long optical path in a small detection volume. | 7 | Physical Chemistry |
Influenza neuraminidase is a mushroom-shaped projection on the surface of the influenza virus. It has a head consisting of four co-planar and roughly spherical subunits, and a hydrophobic region that is embedded within the interior of the virus' membrane. It comprises a single polypeptide chain that is oriented in the opposite direction to the hemagglutinin antigen. The composition of the polypeptide is a single chain of six conserved polar amino acids, followed by hydrophilic, variable amino acids. β-Sheets predominate as the secondary level of protein conformation.
The structure of trans-sialidase includes a catalytic β-propeller domain, a N-terminal lectin-like domain and an irregular beta-stranded domain inserted into the catalytic domain.
Recent emergence of oseltamivir and zanamivir resistant human influenza A(H1N1) H274Y has emphasized the need for suitable expression systems to obtain large quantities of highly pure and stable, recombinant neuraminidase through two separate artificial tetramerization domains that facilitate the formation of catalytically active neuraminidase homotetramers from yeast and Staphylothermus marinus, which allow for secretion of FLAG-tagged proteins and further purification. | 0 | Organic Chemistry |
Schlögl received numerous scientific awards for his work, including the Erwin Schrödinger Prize of the Austrian Academy of Sciences in 1985, the prize for natural sciences of the city of Vienna in 1989, and the Wilhelm Exner Medal of the Austrian Economic Association in 1991. | 0 | Organic Chemistry |
For his work, Woodward received many awards, honors and honorary doctorates, including election to the American Academy of Arts and Sciences in 1948, the National Academy of Sciences in 1953, the American Philosophical Society in 1962, and membership in academies around the world. He was also a consultant to many companies such as Polaroid, Pfizer, and Merck. Other awards include:
* John Scott Medal, from the Franklin Institute and City of Philadelphia, 1945
* Leo Hendrik Baekeland Award, from the North Jersey Section of the American Chemical Society, 1955
* Elected a Foreign Member of the Royal Society (ForMemRS) in 1956
* Davy Medal, from the Royal Society in 1959
* Roger Adams Medal, from the American Chemical Society in 1961
* Pius XI Gold Medal, from the Pontifical Academy of Sciences in 1969
* National Medal of Science from the United States in 1964 ("For an imaginative new approach to the synthesis of complex organic molecules and, especially, for his brilliant syntheses of strychnine, reserphine, lysergic acid, and chlorophyll.")
* Nobel Prize in Chemistry in 1965
* Willard Gibbs Award from the Chicago Section of the American Chemical Society in 1967
* Lavoisier Medal from the Société chimique de France in 1968
* The Order of the Rising Sun, Second Class from the Emperor of Japan in 1970
* Hanbury Memorial Medal from The Pharmaceutical Society of Great Britain in 1970
* Pierre Bruylants Medal from the University of Louvain in 1970
* AMA Scientific Achievement Award in 1971
* Cope Award from the American Chemical Society, shared with Roald Hoffmann in 1973
* Copley Medal from the Royal Society, London in 1978 | 4 | Stereochemistry |
The marine carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon (carbon not associated with a living thing, such as carbon dioxide) and organic carbon (carbon that is, or has been, incorporated into a living thing). Part of the marine carbon cycle transforms carbon between non-living and living matter.
Three main processes (or pumps) that make up the marine carbon cycle bring atmospheric carbon dioxide (CO) into the ocean interior and distribute it through the oceans. These three pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total active pool of carbon at the Earth's surface for durations of less than 10,000 years is roughly 40,000 gigatons C (Gt C, a gigaton is one billion tons, or the weight of approximately 6 million blue whales), and about 95% (~38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon. The speciation of dissolved inorganic carbon in the marine carbon cycle is a primary controller of acid-base chemistry in the oceans. | 9 | Geochemistry |
Weak links play a very important role in modern superconductivity. In most cases weak links are oxide barriers between two superconducting thin films, but it can also be a crystal boundary (in the case of high-Tc superconductors). A schematic representation is given in Fig. 4. Now consider the ring which is thick everywhere except for a small section where the ring is closed via a weak link (Fig. 3b). The velocity is zero except near the weak link. In these regions the velocity contribution to the total phase change in the loop is given by (with Eq. ())
The line integral is over the contact from one side to the other in such a way that the end points of the line are well inside the bulk of the superconductor where . So the value of the line integral is well-defined (e.g. independent of the choice of the end points). With Eqs. (), (), and ()
Without proof we state that the supercurrent through the weak link is given by the so-called DC Josephson relation
The voltage over the contact is given by the AC Josephson relation
The names of these relations (DC and AC relations) are misleading since they both hold in DC and AC situations. In the steady state (constant ) Eq. () shows that V=0 while a nonzero current flows through the junction. In the case of a constant applied voltage (voltage bias) Eq. () can be integrated easily and gives
Substitution in Eq. () gives
This is an AC current. The frequency
is called the Josephson frequency. One μV gives a frequency of about 500 MHz. By using Eq. () the flux quantum is determined with the high precision as given in Eq. ().
The energy difference of a Cooper pair, moving from one side of the contact to the other, is . With this expression Eq. () can be written as which is the relation for the energy of a photon with frequency ν.
:The AC Josephson relation (Eq. ()) can be easily understood in terms of Newtons law, (or from one of the London equations). We start with Newton's law
:Substituting the expression for the Lorentz force and using the general expression for the co-moving time derivative gives
:Eq. () gives so
:Take the line integral of this expression. In the end points the velocities are zero so the ∇v term gives no contribution. Using and Eq. (), with and , gives Eq. (). | 7 | Physical Chemistry |
Temporins are a family of peptides isolated originally from the skin secretion of the European red frog, Rana temporaria. Peptides belonging to the temporin family have been isolated also from closely related North American frogs, such as Rana sphenocephala. | 1 | Biochemistry |
The structure of P680 consists of a heterodimer of two distinct chlorophyll molecules,
referred to as P and P. This “special pair” forms an excitonic dimer that functions as a single unit, excited by light energy as if they were a single molecule. | 5 | Photochemistry |
In medical testing applications and industrial samples with high concentrations or interfering material, there is often a dialyzer module in the instrument in which the analyte permeates through a dialysis membrane into a separate flow path going on to further analysis. The purpose of a dialyzer is to separate the analyte from interfering substances such as protein, whose large molecules do not go through the dialysis membrane but go to a separate waste stream. The reagents, sample and reagent volumes, flow rates, and other aspects of the instrument analysis depend on which analyte is being measured. The autoanalyzer is also a very small machine | 3 | Analytical Chemistry |
The reaction of a strong acid with a strong base is essentially a quantitative reaction. For example,
In this reaction both the sodium and chloride ions are spectators as the neutralization reaction,
does not involve them. With weak bases addition of acid is not quantitative because a solution of a weak base is a buffer solution. A solution of a weak acid is also a buffer solution. When a weak acid reacts with a weak base an equilibrium mixture is produced. For example, adenine, written as AH, can react with a hydrogen phosphate ion, .
The equilibrium constant for this reaction can be derived from the acid dissociation constants of adenine and of the dihydrogen phosphate ion.
The notation [X] signifies "concentration of X". When these two equations are combined by eliminating the hydrogen ion concentration, an expression for the equilibrium constant, is obtained. | 7 | Physical Chemistry |
Liquids with low vapor pressures are preferred for the working fluids to minimize evaporation loss due to flash evaporation.
Liquids have been found that in the range of 300 to 900 K have a vapor pressure so low that the evaporation loss during the normal lifetime of a space system
(possibly as long as 30 years) will be only a small fraction of the total mass of the radiator.
Operating life of the fluid in the LDR environment is affected by thermal stability, oxidative stability, and resistance to radiation.
If a liquid metal is used as the coolant, the pumping of the liquid may use an electromagnetic device. The device induces eddy currents in the metal that generate a Lorentz force with their associated magnetic fields. The effect is the pumping of the liquid metal resulting in a simplified design with no moving parts. This is known as MHD pumping.
For example, a simple mixture of mineral oil and iron filings was found to approximate a suitable ferrofluid for several seconds, before separation of the iron filings and oil was observed in the presence of a magnetic field. At droplet sizes of approximately 200 µm, surface tension will hold the two components at accelerations up to about 1 g.
If an ionic fluid is used as the coolant, the fluid can be used for momentum transfer between spacecraft traveling at different speeds. It may be possible to synthesize the fluid in-situ. For example, BMIM-BF4 ( [CHN]BF) is 42.5% carbon by mass. Lunar regolith typically contains several compounds with carbon and about 5% of asteroids are carbonaceous chondrites which are rich in carbon as well as metals and water.
It may be possible to mine the moon for carbon and combine it with other elements to produce ionic fluid. Another good source of carbon is Mars' largest moon, Phobos, which is a captured asteroid believed to be rich in carbon. | 7 | Physical Chemistry |
The Australian Society for Biochemistry and Molecular Biology (ASBMB) is an academic society founded in 1955. Originally named Australian Biochemical Society, it was renamed to its current title in 1990. Its main activities include hosting scientific conferences, supporting ancillary symposia, workshops and publishing an educational magazine. | 1 | Biochemistry |
Kerr frequency combs (also known as microresonator frequency combs) are optical frequency combs which are generated from a continuous wave pump laser by the Kerr nonlinearity. This coherent conversion of the pump laser to a frequency comb takes place inside an optical resonator which is typically of micrometer to millimeter in size and is therefore termed a microresonator. The coherent generation of the frequency comb from a continuous wave laser with the optical nonlinearity as a gain sets Kerr frequency combs apart from today's most common optical frequency combs. These frequency combs are generated by mode-locked lasers where the dominating gain stems from a conventional laser gain medium, which is pumped incoherently. Because Kerr frequency combs only rely on the nonlinear properties of the medium inside the microresonator and do not require a broadband laser gain medium, broad Kerr frequency combs can in principle be generated around any pump frequency.
While the principle of Kerr frequency combs is applicable to any type of optical resonator, the requirement for Kerr frequency comb generation is a pump laser field intensity above the parametric threshold of the nonlinear process. This requirement is easier to fulfill inside a microresonator because of the possible very low losses inside microresonators (and corresponding high quality factors) and because of the microresonators’ small mode volumes. These two features combined result in a large field enhancement of the pump laser inside the microresonator which allow the generation of broad Kerr frequency combs for reasonable powers of the pump laser.
One important property of Kerr frequency combs, which is a direct consequence of the small dimensions of the microresonators and their resulting large free spectral ranges (FSR), is the large mode spacing of typical Kerr frequency combs. For mode-locked lasers this mode spacing, which defines the distance in between adjacent teeth of the frequency comb, is typically in the range of 10 MHz to 1 GHz. For Kerr frequency combs the typical range is from around 10 GHz to 1 THz.
The coherent generation of an optical frequency comb from a continuous wave pump laser is not a unique property of Kerr frequency combs. Optical frequency combs generated with cascaded optical modulators also possess this property. For certain application this property can be advantageous. For example, to stabilize the offset frequency of the Kerr frequency comb one can directly apply feedback to the pump laser frequency. In principle it is also possible to generate a Kerr frequency comb around a particular continuous wave laser in order to use the bandwidth of the frequency comb to determine the exact frequency of the continuous wave laser.
Since their first demonstration in silica micro-toroid resonators, Kerr frequency combs have been demonstrated in a variety of microresonator platforms which notably also include crystalline microresonators and integrated photonics platforms such as waveguide resonators made from silicon nitride. More recent research has expanded the range of available platforms further which now includes diamond, aluminum nitride,
lithium niobate, and, for mid-infrared pump wavelengths, silicon.
Because both use the nonlinear effects of the propagation medium, the physics of Kerr frequency combs and of supercontinuum generation from pulsed lasers is very similar. In addition to the nonlinearity, the chromatic dispersion of the medium also plays a crucial role for these systems. As a result of the interplay of nonlinearity and dispersion, solitons can form. The most relevant type of solitons for Kerr frequency comb generation are bright dissipative cavity solitons, which are sometimes also called dissipative Kerr solitons (DKS). These bright solitons have helped to significantly advance the field of Kerr frequency combs as they provide a way to generate ultra-short pulses which in turn represent a coherent, broadband optical frequency comb, in a more reliable fashion than what was possible before.
In its simplest form with only the Kerr nonlinearity and second order dispersion the physics of Kerr frequency combs and dissipative solitons can be described well by the Lugiato–Lefever equation.
Other effects such as the Raman effect and higher order dispersion effects require additional terms in the equation. | 7 | Physical Chemistry |
Amylases are used in breadmaking and to break down complex sugars, such as starch (found in flour), into simple sugars. Yeast then feeds on these simple sugars and converts it into the waste products of ethanol and carbon dioxide. This imparts flavour and causes the bread to rise. While amylases are found naturally in yeast cells, it takes time for the yeast to produce enough of these enzymes to break down significant quantities of starch in the bread. This is the reason for long fermented doughs such as sourdough. Modern breadmaking techniques have included amylases (often in the form of malted barley) into bread improver, thereby making the process faster and more practical for commercial use.
α-Amylase is often listed as an ingredient on commercially package-milled flour. Bakers with long exposure to amylase-enriched flour are at risk of developing dermatitis or asthma. | 1 | Biochemistry |
Multiomics currently holds a promise to fill gaps in the understanding of human health and disease, and many researchers are working on ways to generate and analyze disease-related data. The applications range from understanding host-pathogen interactions and infectious diseases, cancer, to understanding better chronic and complex non-communicable diseases and improving personalized medicine. | 1 | Biochemistry |
Free fatty acids cannot penetrate any biological membrane due to their negative charge. Free fatty acids must cross the cell membrane through specific transport proteins, such as the SLC27 family fatty acid transport protein. Once in the cytosol, the following processes bring fatty acids into the mitochondrial matrix so that beta-oxidation can take place.
# Long-chain-fatty-acid—CoA ligase catalyzes the reaction between a fatty acid with ATP to give a fatty acyl adenylate, plus inorganic pyrophosphate, which then reacts with free coenzyme A to give a fatty acyl-CoA ester and AMP.
# If the fatty acyl-CoA has a long chain, then the carnitine shuttle must be utilized (shown in the table below):
#* Acyl-CoA is transferred to the hydroxyl group of carnitine by carnitine palmitoyltransferase I, located on the cytosolic faces of the outer and inner mitochondrial membranes.
#* Acyl-carnitine is shuttled inside by a carnitine-acylcarnitine translocase, as a carnitine is shuttled outside.
#* Acyl-carnitine is converted back to acyl-CoA by carnitine palmitoyltransferase II, located on the interior face of the inner mitochondrial membrane. The liberated carnitine is shuttled back to the cytosol, as an acyl-carnitine is shuttled into the matrix.
# If the fatty acyl-CoA contains a short chain, these short-chain fatty acids can simply diffuse through the inner mitochondrial membrane. | 1 | Biochemistry |
The following is a list of common functional groups. In the formulas, the symbols R and R' usually denote an attached hydrogen, or a hydrocarbon side chain of any length, but may sometimes refer to any group of atoms. | 0 | Organic Chemistry |
Sex pheromones have found applications in pest monitoring and pest control. For monitoring, pheromone traps are used to attract and catch a sample of pest insects to determine whether control measures are needed. For control, much larger quantities of a sex pheromone are released to disrupt the mating of a pest species. This can be either by releasing enough pheromone to prevent males from finding females, effectively drowning out their signals, or by mass trapping, attracting and removing pests directly. For example, research on the control of the spruce bud moth (Zeiraphera canadensis) has focused on the use of the pheromone E-9-tetradecenyl-acetate, a chemical the spruce bud moth releases during mating. | 1 | Biochemistry |
In this method, plant nuclei are isolated by physically grinding tissues and reconstituting the intact nuclei in a unique Nuclear Isolation Buffer (NIB). The plastid DNAs are released from organelles and eliminated with an osmotic buffer by washing and centrifugation. The purified nuclei are then lysed and further cleaned by organic extraction, and the genomic DNA is precipitated with a high concentration of CTAB. The highly pure, high molecular weight gDNA is extracted from the nuclei, dissolved in a high pH buffer, allowing for stable long-term storage. | 1 | Biochemistry |
# During the afterhyperpolarization period after an action potential, the membrane potential is more negative than when the cell is at the resting potential. In the figure to the right, this undershoot occurs at approximately 3 to 4 milliseconds (ms) on the time scale. The afterhyperpolarization is the time when the membrane potential is hyperpolarized relative to the resting potential.
# During the rising phase of an action potential, the membrane potential changes from negative to positive, a depolarization. In the figure, the rising phase is from approximately 1 to 2 ms on the graph. During the rising phase, once the membrane potential becomes positive, the membrane potential continues to depolarize (overshoot) until the peak of the action potential is reached at about +40 millivolts (mV). After the peak of the action potential, a hyperpolarization repolarizes the membrane potential to its resting value, first by making it less positive, until 0 mV is reached, and then by continuing to make it more negative. This repolarization occurs in the figure from approximately 2 to 3 ms on the time scale. | 7 | Physical Chemistry |
Akkermansia is a genus in the phylum Verrucomicrobiota (Bacteria). The genus was first proposed by Derrien et al. (2004), with the type species Akkermansia muciniphila (gen. nov., sp. nov).
Until 2016 the genus contained a single known species, namely A. muciniphila. In 2016, Akkermansia glycaniphila was isolated in the feces of a reticulated python. | 1 | Biochemistry |
Clay minerals can be incorporated in lime-metakaolin mortars to improve mechanical properties. Electrochemical separation helps to obtain modified saponite-containing products with high smectite-group minerals concentrations, lower mineral particles size, more compact structure, and greater surface area. These characteristics open possibilities for the manufacture of high-quality ceramics and heavy-metal sorbents from saponite-containing products.
Furthermore, tail grinding occurs during the preparation of the raw material for ceramics; this waste reprocessing is of high importance for the use of clay pulp as a neutralizing agent, as fine particles are required for the reaction. Experiments on the histosol deacidification with the alkaline clay slurry demonstrated that neutralization with the average pH level of 7.1 is reached at 30% of the pulp added and an experimental site with perennial grasses proved the efficacy of the technique. Moreover, the reclamation of disturbed lands is an integral part of the social and environmental responsibility of the mining company and this scenario addresses the community necessities at both local and regional levels. | 9 | Geochemistry |
Fluoroform is weakly acidic with a pK = 25–28 and quite inert. Attempted deprotonation results in defluorination to generate and difluorocarbene (). Some organocopper and organocadmium compounds have been developed as trifluoromethylation reagents.
Fluoroform is a precursor of the Ruppert-Prakash reagent Trifluoromethyltrimethylsilane|, which is a source of the nucleophilic anion. | 2 | Environmental Chemistry |
A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life on a planet. Measurable attributes of life include its complex physical or chemical structures, its use of free energy, and the production of biomass and wastes.
The field of astrobiology uses biosignatures as evidence in the search for past or present extraterrestrial life. | 2 | Environmental Chemistry |
The lac operon of the model bacterium Escherichia coli was the first operon to be discovered and provides a typical example of operon function. It consists of three adjacent structural genes, a promoter, a terminator, and an operator. The lac operon is regulated by several factors including the availability of glucose and lactose. It can be activated by allolactose. Lactose binds to the repressor protein and prevents it from repressing gene transcription. This is an example of the derepressible (from above: negative inducible) model. So it is a negative inducible operon induced by presence of lactose or allolactose. | 1 | Biochemistry |
According to food chemist Udo Pollmer of the European Institute of Food and Nutrition Sciences in Munich, alcohol can be molecularly encapsulated in cyclodextrines, a sugar derivate. In this way, encapsuled in small capsules, the fluid can be handled as a powder. The cyclodextrines can absorb an estimated 60 percent of their own weight in alcohol. A US patent has been registered for the process as early as 1974. | 6 | Supramolecular Chemistry |
In addition to Cori Cycle, the lactate shuttle hypothesis proposes complementary functions of lactate in multiple tissues. Contrary to the long-held belief that lactate is formed as a result of oxygen-limited metabolism, substantial evidence exists that suggests lactate is formed under both aerobic and anaerobic conditions, as a result of substrate supply and equilibrium dynamics. | 1 | Biochemistry |
Endogenous gaseous mediators have shown anti-inflammatory and cytoprotective properties Combination nonsteroidal anti-inflammatory drugs featuring both a cyclooxygenase inhibitor and gaseous mediator releasing component are being investigated as a safer alternative to current anti-inflammatory drugs due to their potential reduction in risk for gastrointestinal ulcer formation. | 1 | Biochemistry |
The Diels-Alder reaction, also known as cycloaddition, combines a conjugated diene and an alkene to form cycloalkene. This is a concerted process, with bonds forming and breaking simultaneously. | 0 | Organic Chemistry |
Normal sound waves are fluctuations in the displacement and density of molecules in a substance;
second sound waves are fluctuations in the density of quasiparticle thermal excitations (rotons and phonons). Second sound can be observed in any system in which most phonon-phonon collisions conserve momentum, like superfluids and in some dielectric crystals when Umklapp scattering is small.
Contrary to molecules in a gas, quasiparticles are not necessarily conserved. Also gas molecules in a box conserve momentum (except at the boundaries of box), while quasiparticles can sometimes not conserve momentum in the presence of impurities or Umklapp scattering. Umklapp phonon-phonon scattering exchanges momentum with the crystal lattice, so phonon momentum is not conserved, but Umklapp processes can be reduced at low temperatures.
Normal sound in gases is a consequence of the collision rate between molecules being large compared to the frequency of the sound wave . For second sound, the Umklapp rate has to be small compared to the oscillation frequency for energy and momentum conservation. However analogous to gasses, the relaxation time describing the collisions has to be large with respect to the frequency , leaving a window:
for sound-like behaviour or second sound. The second sound thus behaves as oscillations of the local number of quasiparticles (or of the local energy carried by these particles). Contrary to the normal sound where energy is related to pressure and temperature, in a crystal the local energy density is purely a function of the temperature. In this sense, the second sound can also be considered as oscillations of the local temperature. Second sound is a wave-like phenomena which makes it very different from usual heat diffusion. | 7 | Physical Chemistry |
Aromatic acids are a type of aromatic compound. Included in that class are substances containing an aromatic ring and an organic acid functional group.
There are several categories of aromatic acids including:
*Phenolic acids: substances containing an aromatic ring and an organic carboxylic acid function (C6-C1 skeleton).
*Aromatic amino acids | 0 | Organic Chemistry |
The most common side effects of enalapril include increased serum creatinine (20%), dizziness (2–8%), low blood pressure (1–7%), syncope (2%), and dry cough (1–2%). The most serious common adverse event is angioedema (swelling) (0.68%) which often affects the face and lips, endangering the patient's airway. Angioedema can occur at any point during treatment with enalapril, but is most common after the first few doses. Angioedema and fatality therefrom are reportedly higher among black people. Agranulocytosis has been observed with Enalapril.
Some evidence suggests enalapril will cause injury and death to a developing fetus. In pregnancy, enalapril may result in damage to the fetus's kidneys and resulting oligohydramnios (not enough amniotic fluid). Enalapril is secreted in breast milk and is not recommended for use while breastfeeding. | 4 | Stereochemistry |
A grain boundary (GB) is the transition area or interface between adjacent crystallites (or grains) of the same chemical and lattice composition, not to be confused with a phase boundary. The adjacent grains do not have the same orientation of the lattice, thus giving the atoms in GB shifted positions relative to the lattice in the crystals. Due to the shifted positioning of the atoms in the GB they have a higher energy state when compared with the atoms in the crystal lattice of the grains. It is this imperfection that makes it possible to selectively etch the GBs when one wants the microstructure to be visible.
Striving to minimize its energy leads to the coarsening of the microstructure to reach a metastable state within the specimen. This involves minimizing its GB area and changing its topological structure to minimize its energy. This grain growth can either be normal or abnormal, a normal grain growth is characterized by the uniform growth and size of all the grains in the specimen. Abnormal grain growth is when a few grains grow much larger than the remaining majority. | 8 | Metallurgy |
Coward was born on 2 July 1885 in Blackburn, Lancashire. She studied Botany and graduated M.Sc. from University of Manchester. After a few years, she joined University College London to study biochemistry and perform research under J. C. Drummond on Vitamin A, paving the way for her to be nominated to the Fellow of the Chemical Society in 1923. | 3 | Analytical Chemistry |
As ozone in the atmosphere prevents most energetic ultraviolet radiation reaching the surface of the Earth, astronomical data in these wavelengths have to be gathered from satellites orbiting above the atmosphere and ozone layer. Most of the light from young hot stars is in the ultraviolet and so study of these wavelengths is important for studying the origins of galaxies. The Galaxy Evolution Explorer, GALEX, is an orbiting ultraviolet space telescope launched on April 28, 2003, which operated until early 2012. | 5 | Photochemistry |
The cycle comprises three enzyme-catalysed reactions. The first stage is the deamination of the purine nucleotide adenosine monophosphate (AMP) to form inosine monophosphate (IMP), catalysed by the enzyme AMP deaminase:
:AMP + HO + H → IMP + NH
The second stage is the formation of adenylosuccinate from IMP and the amino acid aspartate, which is coupled to the energetically favourable hydrolysis of GTP, and catalysed by the enzyme adenylosuccinate synthetase:
:Aspartate + IMP + GTP → Adenylosuccinate + GDP + P
Finally, adenylosuccinate is cleaved by the enzyme adenylosuccinate lyase to release fumarate and regenerate the starting material of AMP:
:Adenylosuccinate → AMP + Fumarate
A recent study showed that activation of HIF-1α allows cardiomyocytes to sustain mitochondrial membrane potential during anoxic stress by utilizing fumarate produced by adenylosuccinate lyase as an alternate terminal electron acceptor in place of oxygen. This mechanism should help provide protection in the ischemic heart. | 1 | Biochemistry |
Cyclamin can be extracted from cyclamen plants such as the species mirabile and trocopteranthum. Cyclamen are known houseplants; this raises concerns about the awareness of the toxicity of this flower. The compound cyclamin belongs to the family of triterpene saponins, which are derived from the saponin structure. Triterpenoid compounds contain one or more sugar moieties attached to triterpenoid aglycones. The large diversity of structures causes saponins to exhibit a wide range of biological and pharmacological properties. In China, cyclamin has been used as a traditional medicine for years. Cyclamen has been used against menstrual disorders, digestive disorders, and anxiety in women. However, this is only the case for the leaves, the roots of the plants are known to be harmful if ingested. In these roots, cyclamin is found, as well as in the bulbs. Therefore, cyclamin is suspected to be the compound which causes the toxicity of these roots and bulbs in cyclamen plants. | 0 | Organic Chemistry |
The four most common methods of transition metal catalyzed methane activation are the Shilov system, sigma bond metathesis, oxidative addition, and 1,2 addition reactions.
The Shilov system involves platinum based complexes to produce metal alkyls. It was first discovered when a hydrogen-deuterium exchanged was observed in a deuterated solution with the platinum tetrachloride anion. Shilov et al. then was able to catalytically convert methane into methanol or methyl chloride when a Pt(IV) salt was used as a stoichiometric oxidant. The process is simplified down into three main steps: (1) C-H activation, (2) a redox reaction to form an octahedral intermediate, followed by (3) the formation of the carbon-oxygen bond to form methanol ().
Sigma bond metathesis involves the formation of new C-H and metal-carbon bonds, where the metals are typically in the d configuration. Starting with a metal alkyl, a C-H bond coordinates with the metal complex via sigma bonding. A four-member transition state is created, where a new metal-carbon bond is formed, and the former C-H linkage is broken ().
In oxidative addition, the metal center's oxidation state increases by 2 units during the process. First, the metal center coordinates with a sigma C-H bond to form an intermediate called a sigma-methane complex. The C-H linkage is then broken, as the metal becomes covalently bonded each to the carbon and the hydrogen ().
Similar to sigma bond metathesis is the 1,2 addition reaction, where a four-membered transition state is also formed. However, a polarized double or triple metal-ligand bond is required in order to favor the formation of the desired product (). | 0 | Organic Chemistry |
Robert Travis Kennedy is an American chemist specializing in bioanalytical chemistry including liquid chromatography, capillary electrophoresis, and microfluidics. He is currently the Hobart H. Willard Distinguished University Professor of Chemistry and the chair of the department of chemistry at the University of Michigan. He holds joint appointments with the Department of Pharmacology and Department Macromolecular Science and Engineering. Kennedy is an Associate Editor of Analytical Chemistry and ACS Measurement Science AU. | 3 | Analytical Chemistry |
In the solid state, racemic mixtures may have different physical properties from either of the pure enantiomers because of the differential intermolecular interactions (see Biological Significance section). The change from a pure enantiomer to a racemate can change its density, melting point, solubility, heat of fusion, refractive index, and its various spectra. Crystallization of a racemate can result in separate (+) and (−) forms, or a single racemic compound. However, in liquid and gaseous states, racemic mixtures will behave with physical properties that are identical, or near identical, to their pure enantiomers. | 4 | Stereochemistry |
Laurie Ellen Locascio (born November 21, 1961) is an American biomedical engineer, analytical chemist, and former academic administrator serving as the under secretary of commerce for standards and technology and the director of National Institute of Standards and Technology. From 2017 to 2021, Locascio was vice president for research of University of Maryland, College Park and University of Maryland, Baltimore. | 3 | Analytical Chemistry |
Residence time distributions are measured by introducing a non-reactive tracer into the system at the inlet. Its input concentration is changed according to a known function and the output concentration measured. The tracer should not modify the physical characteristics of the fluid (equal density, equal viscosity) or the hydrodynamic conditions and it should be easily detectable.
In general, the change in tracer concentration will either be a pulse or a step. Other functions are possible, but they require more calculations to deconvolute the RTD curve. | 9 | Geochemistry |
The Takehara copper refinery of the Mitsui Mining & Smelting Company Limited of Japan commissioned a BBOC in its precious metals department in 1993.
Prior to the installation of the BBOC, the Takehara refinery refined a mixture of copper and lead anode slimes in a three reverberatory furnaces (two operating and one being rebricked) in a process that had a cycle time of 104 hours for refining 6 t of bullion.
The reverberatory furnaces were replaced with a single BBOC with a charge capacity of 6 t of feed. The cycle time was reduced to 50 hours. The use of the BBOC reduced the energy consumption from 74 GJ/t to 27 GJ/t and also had better bismuth elimination than the reverberatory furnaces. | 8 | Metallurgy |
Carbon-14 goes through radioactive beta decay:
: → + + + 156.5 keV
By emitting an electron and an electron antineutrino, one of the neutrons in the carbon-14 atom decays to a proton and the carbon-14 (half-life of 5,700 ± 30 years) decays into the stable (non-radioactive) isotope nitrogen-14.
As usual with beta decay, almost all the decay energy is carried away by the beta particle and the neutrino. The emitted beta particles have a maximum energy of about 156 keV, while their weighted mean energy is 49 keV. These are relatively low energies; the maximum distance traveled is estimated to be 22 cm in air and 0.27 mm in body tissue. The fraction of the radiation transmitted through the dead skin layer is estimated to be 0.11. Small amounts of carbon-14 are not easily detected by typical Geiger–Müller (G-M) detectors; it is estimated that G-M detectors will not normally detect contamination of less than about 100,000 disintegrations per minute (0.05 µCi). Liquid scintillation counting is the preferred method although more recently, accelerator mass spectrometry has become the method of choice; it counts all the carbon-14 atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly. The G-M counting efficiency is estimated to be 3%. The half-distance layer in water is 0.05 mm. | 9 | Geochemistry |
Exposure assessment is the process of estimating or measuring the magnitude, frequency and duration of exposure to an agent, along
with the number and characteristics of the population exposed. Ideally, it describes the sources, pathways, routes, and the uncertainties in the assessment. It is a necessary part of risk analysis and hence risk assessment.
Exposure analysis is the science that describes how an individual or population comes in contact with a contaminant, including quantification of the amount of contact across space and time. Exposure assessment and exposure analysis are often used as synonyms in many practical contexts. Risk is a function of exposure and hazard. For example, even for an extremely toxic (high hazard) substance, the risk of an adverse outcome is unlikely if exposures are near zero. Conversely, a moderately toxic substance may present substantial risk if an individual or a population is highly exposed. | 2 | Environmental Chemistry |
Hot baths of sodium hydroxide (NaOH), nitrates such as sodium nitrate (), and/or nitrites such as sodium nitrite (NaNO) at are used to convert the surface of the material into magnetite (FeO). Water must be periodically added to the bath, with proper controls to prevent a steam explosion.
Hot blackening involves dipping the part into various tanks. The workpiece is usually dipped by automated part carriers for transportation between tanks. These tanks contain, in order, alkaline detergent, water, sodium hydroxide at (the blackening compound), and finally the sealant, which is usually oil.
The NaOH (caustic soda) and elevated temperature cause FeO (black oxide) to form on the surface of the metal instead of FeO (red oxide; rust). While it is physically denser than red oxide, the fresh black oxide is porous, so oil is then applied as post treatment to the heated part, which seals it by "sinking" into it. The combination prevents corrosion of the workpiece. There are many advantages of blackening, including:
* Blackening can be done in large batches, which is ideal for small parts.
* There is no significant dimensional impact. The blacking process creates a layer about 1μm thick.
* It is far cheaper than similar corrosion protection systems, such as paint and electroplating.
The oldest and most widely used specification for hot black oxide is MIL-DTL-13924, which covers four classes of processes for different substrates. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408.
Iron(III) chloride (FeCl) may also be used for steel blackening by dipping a piece of steel into a hot bath of 50% FeCl solution and then into a hot boiling water. The process is usually repeated several times. | 7 | Physical Chemistry |
The technique has been well utilized in studying carbon nanotubes to determine thermodynamic binding interactions with biological molecules and graphene composite interactions. Another notable use of ITC with carbon nanotubes is optimization of preparation of carbon nanotubes from graphene composite and polyvinyl alcohol (PVA). PVA assembly process can be measured thermodynamically as mixing of the two ingredients is an exothermic reaction, and its binding trend can be easily observed by ITC. | 7 | Physical Chemistry |
The first experimental evidence for the presence of 18 cm absorption lines of the hydroxyl (HO) radical in the radio absorption spectrum of Cassiopeia A was obtained by Weinreb et al. (Nature, Vol. 200, pp. 829, 1963) based on observations made during the period October 15–29, 1963. | 2 | Environmental Chemistry |
This reagent is inexpensively available for laboratory use. It is a by-product from the production of ortho-toluenesulfonyl chloride (a precursor for the synthesis of the common food additive and catalyst saccharin), via the chlorosulfonation of toluene:
: CHCH + SOCl → CHCHSOCl + HCl | 0 | Organic Chemistry |
Thermal treatment of milk by indirect heating (e.g. pasteurization) to reduce microbial load and increase shelf life is generally performed by a plate heat exchanger. Heat exchanger surfaces can become fouled by adsorbed milk protein deposits. Fouling is initiated by formation of a protein monolayer at room temperature, followed by heat induced aggregation and deposition of whey protein and calcium phosphate deposits. Adsorbed proteins decrease efficiency of heat transfer and potentially affect product quality by preventing adequate heating of milk. | 1 | Biochemistry |
Herman van Bekkum (26 September 1932 – 30 November 2020) was a Dutch organic chemist. He was professor of Catalysis in Organic Chemistry between 1971 and 1998 at Delft University of Technology. He served as rector magnificus of the university between 1975 and 1976. He was an expert in the field of carbohydrate chemistry and zeolites. | 0 | Organic Chemistry |
Ion chromatography (or ion-exchange chromatography) is a form of chromatography that separates ions and ionizable polar molecules based on their affinity to the ion exchanger. It works on almost any kind of charged molecule—including small inorganic anions, large proteins, small nucleotides, and amino acids. However, ion chromatography must be done in conditions that are one pH unit away from the isoelectric point of a protein.
The two types of ion chromatography are anion-exchange and cation-exchange. Cation-exchange chromatography is used when the molecule of interest is positively charged. The molecule is positively charged because the pH for chromatography is less than the pI (also known as pH(I)). In this type of chromatography, the stationary phase is negatively charged and positively charged molecules are loaded to be attracted to it. Anion-exchange chromatography is when the stationary phase is positively charged and negatively charged molecules (meaning that pH for chromatography is greater than the pI) are loaded to be attracted to it. It is often used in protein purification, water analysis, and quality control. The water-soluble and charged molecules such as proteins, amino acids, and peptides bind to moieties which are oppositely charged by forming ionic bonds to the insoluble stationary phase. The equilibrated stationary phase consists of an ionizable functional group where the targeted molecules of a mixture to be separated and quantified can bind while passing through the column—a cationic stationary phase is used to separate anions and an anionic stationary phase is used to separate cations. Cation exchange chromatography is used when the desired molecules to separate are cations and anion exchange chromatography is used to separate anions. The bound molecules then can be eluted and collected using an eluant which contains anions and cations by running a higher concentration of ions through the column or by changing the pH of the column.
One of the primary advantages for the use of ion chromatography is that only one interaction is involved the separation, as opposed to other separation techniques; therefore, ion chromatography may have higher matrix tolerance. Another advantage of ion exchange is the predictability of elution patterns (based on the presence of the ionizable group). For example, when cation exchange chromatography is used, certain cations will elute out first and others later. A local charge balance is always maintained. However, there are also disadvantages involved when performing ion-exchange chromatography, such as constant evolution of the technique which leads to the inconsistency from column to column. A major limitation to this purification technique is that it is limited to ionizable group. | 3 | Analytical Chemistry |
The oldest and most widely used expression systems are cell-based and may be defined as the "combination of an expression vector, its cloned DNA, and the host for the vector that provide a context to allow foreign gene function in a host cell, that is, produce proteins at a high level". Overexpression is an abnormally and excessively high level of gene expression which produces a pronounced gene-related phenotype.
There are many ways to introduce foreign DNA to a cell for expression, and many different host cells may be used for expression — each expression system has distinct advantages and liabilities. Expression systems are normally referred to by the host and the DNA source or the delivery mechanism for the genetic material. For example, common hosts are bacteria (such as E. coli, B. subtilis), yeast (such as S. cerevisiae) or eukaryotic cell lines. Common DNA sources and delivery mechanisms are viruses (such as baculovirus, retrovirus, adenovirus), plasmids, artificial chromosomes and bacteriophage (such as lambda). The best expression system depends on the gene involved, for example the Saccharomyces cerevisiae is often preferred for proteins that require significant posttranslational modification. Insect or mammal cell lines are used when human-like splicing of mRNA is required. Nonetheless, bacterial expression has the advantage of easily producing large amounts of protein, which is required for X-ray crystallography or nuclear magnetic resonance experiments for structure determination.
Because bacteria are prokaryotes, they are not equipped with the full enzymatic machinery to accomplish the required post-translational modifications or molecular folding. Hence, multi-domain eukaryotic proteins expressed in bacteria often are non-functional. Also, many proteins become insoluble as inclusion bodies that are difficult to recover without harsh denaturants and subsequent cumbersome protein-refolding.
To address these concerns, expressions systems using multiple eukaryotic cells were developed for applications requiring the proteins be conformed as in, or closer to eukaryotic organisms: cells of plants (i.e. tobacco), of insects or mammalians (i.e. bovines) are transfected with genes and cultured in suspension and even as tissues or whole organisms, to produce fully folded proteins. Mammalian in vivo expression systems have however low yield and other limitations (time-consuming, toxicity to host cells,..). To combine the high yield/productivity and scalable protein features of bacteria and yeast, and advanced epigenetic features of plants, insects and mammalians systems, other protein production systems are developed using unicellular eukaryotes (i.e. non-pathogenic <nowiki/>Leishmania<nowiki/> cells). | 1 | Biochemistry |
Marion Katherine Blight was born in Watford in 1921. Her mother worked in domestic service while her father was a shop assistant. McQuillan attended Wycombe High School before getting a scholarship to Henrietta Barnett’s School. McQuillan went to University in 1939 where she graduated from Girton College, Cambridge with a degree in metallurgy and natural sciences. She got her first job in 1942 in the Royal Aircraft Establishment Farnborough (RAE) in 1942. McQuillan researched jet engine metals and was a member of the first team to research titanium.
In 1946 she travelled through Germany and Austria as member of one of the many teams sent by the British Intelligence Objectives Sub-Committee, collecting technical information from universities, research establishments and factories.
She also worked at the Atomic Energy Research Establishment at Harwell, working on some of the early metallurgical problems of nuclear energy. From 1948-1951 she was at the Australian Royal Aircraft Establishment in Melbourne.
McQuillan returned to the UK where she began to work for ICI Metals (also known as IMI), in the Titanium Alloy Research Department where, within two years later she was head of the section. With her husband McQuillan published the seminal book “Titanium” in 1956. During the 1960s McQuillan registered 8 titanium alloy patents. In 1967 McQuillan was appointed technical director of the New Metals Division and by 1978 she became the first woman managing director of Imperial Metal Industries subsidiary, Enots. | 8 | Metallurgy |
To date, many studies have been conducted on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level, indicating that likely selenium functions as an immunostimulator, i.e. reversing the immunosuppression in tumour microenvironment towards antitumour immunity by activating immune cells (e.g. M1 macrophages and CD8+ T-lymphocytes, the elevated number of neutrophils and activated cytotoxic NK cells) and releasing pro-inflammatory cytokines such as IFNγ and TNFα. | 1 | Biochemistry |
Relative entropy can also be interpreted as the expected discrimination information for over : the mean information per sample for discriminating in favor of a hypothesis against a hypothesis , when hypothesis is true. Another name for this quantity, given to it by I. J. Good, is the expected weight of evidence for over to be expected from each sample.
The expected weight of evidence for over is not the same as the information gain expected per sample about the probability distribution of the hypotheses,
Either of the two quantities can be used as a utility function in Bayesian experimental design, to choose an optimal next question to investigate: but they will in general lead to rather different experimental strategies.
On the entropy scale of information gain there is very little difference between near certainty and absolute certainty—coding according to a near certainty requires hardly any more bits than coding according to an absolute certainty. On the other hand, on the logit scale implied by weight of evidence, the difference between the two is enormous – infinite perhaps; this might reflect the difference between being almost sure (on a probabilistic level) that, say, the Riemann hypothesis is correct, compared to being certain that it is correct because one has a mathematical proof. These two different scales of loss function for uncertainty are both useful, according to how well each reflects the particular circumstances of the problem in question. | 7 | Physical Chemistry |
In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from Latin ligare, which means to bind. In protein-ligand binding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein. The binding typically results in a change of conformational isomerism (conformation) of the target protein. In DNA-ligand binding studies, the ligand can be a small molecule, ion, or protein which binds to the DNA double helix. The relationship between ligand and binding partner is a function of charge, hydrophobicity, and molecular structure.
Binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and Van der Waals forces. The association or docking is actually reversible through dissociation. Measurably irreversible covalent bonding between a ligand and target molecule is atypical in biological systems. In contrast to the definition of ligand in metalorganic and inorganic chemistry, in biochemistry it is ambiguous whether the ligand generally binds at a metal site, as is the case in hemoglobin. In general, the interpretation of ligand is contextual with regards to what sort of binding has been observed.
Ligand binding to a receptor protein alters the conformation by affecting the three-dimensional shape orientation. The conformation of a receptor protein composes the functional state. Ligands include substrates, inhibitors, activators, signaling lipids, and neurotransmitters. The rate of binding is called affinity, and this measurement typifies a tendency or strength of the effect. Binding affinity is actualized not only by host–guest interactions, but also by solvent effects that can play a dominant, steric role which drives non-covalent binding in solution. The solvent provides a chemical environment for the ligand and receptor to adapt, and thus accept or reject each other as partners.
Radioligands are radioisotope labeled compounds used in vivo as tracers in PET studies and for in vitro binding studies. | 1 | Biochemistry |
MTBE gives water an unpleasant taste at very low concentrations. MTBE often is introduced into water-supply aquifers by leaking underground storage tanks (USTs) at gasoline stations or by gasoline containing MTBE being spilled onto the ground. The higher water solubility and persistence of MTBE cause it to travel faster and farther than many other components of gasoline when released into an aquifer.
MTBE is biodegraded by the action of bacteria. In the proper type of bioreactor, such as a fluidized bed bioreactor, MTBE may be removed rapidly and economically from water to undetectable levels. Activated carbon produced from coconut shells and optimized for MTBE adsorption may reduce MTBE to undetectable levels, although this level of reduction is likely to occur only in the most ideal circumstances. There are currently no known published cases of any in-situ treatment method that has been capable of reducing contaminant concentrations to baseline (pre-development) conditions within the aquifer soil matrix.
According to the International Agency for Research on Cancer (IARC), a cancer research agency of the World Health Organization, MTBE is not classified as a human carcinogen. MTBE may be tasted in water at concentrations of 5–15 µg/L (5-15ppb).
As of 2007, researchers have limited data about the health effects of ingestion of MTBE. The United States Environmental Protection Agency (EPA) has concluded that available data are inadequate to quantify health risks of MTBE at low exposure levels in drinking water, but the data support the conclusion that MTBE is a potential human carcinogen at high doses. | 2 | Environmental Chemistry |
In thermodynamics, the volume of a system is an important extensive parameter for describing its thermodynamic state. The specific volume, an intensive property, is the system's volume per unit mass. Volume is a function of state and is interdependent with other thermodynamic properties such as pressure and temperature. For example, volume is related to the pressure and temperature of an ideal gas by the ideal gas law.
The physical region covered by a system may or may not coincide with a control volume used to analyze the system. | 7 | Physical Chemistry |
Amine oxides are used as protecting group for amines and as chemical intermediates. Long-chain alkyl amine oxides are used as amphoteric surfactants and foam stabilizers.
Amine oxides are highly polar molecules and have a polarity close to that of quaternary ammonium salts. Small amine oxides are very hydrophilic and have an excellent water solubility and a very poor solubility in most organic solvents.
Amine oxides are weak bases with a pK of around 4.5 that form , cationic hydroxylamines, upon protonation at a pH below their pK. | 0 | Organic Chemistry |
The dominant industrial method for producing ammonia is the Haber process also known as the Haber-Bosch process. Fertilizer production is now the largest source of human-produced fixed nitrogen in the terrestrial ecosystem. Ammonia is a required precursor to fertilizers, explosives, and other products. The Haber process requires high pressures (around 200 atm) and high temperatures (at least 400 °C), which are routine conditions for industrial catalysis. This process uses natural gas as a hydrogen source and air as a nitrogen source. The ammonia product has resulted in an intensification of nitrogen fertilizer globally and is credited with supporting the expansion of the human population from around 2 billion in the early 20th century to roughly 8 billion people now. | 1 | Biochemistry |
Organolithium reagents are sensitive to moisture and thus should be handled under inert atmosphere in anhydrous conditions. Tetrahydrofuran is the most common solvent employed for lateral lithiation reactions. Measurement of the concentration of commercial or prepared alkyllithium solutions may be accomplished using well-established titration methods.
A useful indicator for the progress of lateral lithiations is the color of the reaction mixture. Benzyllithium compounds range in color from red to deep purple, and in many cases the lack of a color change upon addition of an organolithium reagent to the substrate may indicate the presence of an undesired proton source in solution. | 0 | Organic Chemistry |
In the presence of an appropriate transition metal (typically copper or rhodium), α-diazocarbonyl compounds are converted to transition metal carbenes, which undergo addition reactions in the presence of carbon–carbon double bonds to form cyclopropanes. Insertion into carbon–carbon or carbon–hydrogen bonds is possible in substrates lacking a double bond. The intramolecular version of this reaction forms fused carbocycles, although yields of reactions mediated by copper are typically moderate. For enantioselective cyclopropanations and insertions, both copper- and rhodium-based catalysts are employed, although the latter have been more heavily studied in recent years. | 0 | Organic Chemistry |
White etching cracks (WEC), or white structure flaking or brittle flaking, is a type of rolling contact fatigue (RCF) damage that can occur in bearing steels under certain conditions, such as hydrogen embrittlement, high stress, inadequate lubrication, and high temperature. WEC is characterised by the presence of white areas of microstructural alteration in the material, which can lead to the formation of small cracks that can grow and propagate over time, eventually leading to premature failure of the bearing. WEC has been observed in a variety of applications, including wind turbine gearboxes, automotive engines, and other heavy machinery. The exact mechanism of WEC formation is still a subject of research, but it is believed to be related to a combination of microstructural changes, such as phase transformations and grain boundary degradation, and cyclic loading. | 8 | Metallurgy |
The evolution of interactome complexity is delineated in a study published in Nature. In this study it is first noted that the boundaries between prokaryotes, unicellular eukaryotes and multicellular eukaryotes are accompanied by orders-of-magnitude reductions in effective population size, with concurrent amplifications of the effects of random genetic drift. The resultant decline in the efficiency of selection seems to be sufficient to influence a wide range of attributes at the genomic level in a nonadaptive manner. The Nature study shows that the variation in the power of random genetic drift is also capable of influencing phylogenetic diversity at the subcellular and cellular levels. Thus, population size would have to be considered as a potential determinant of the mechanistic pathways underlying long-term phenotypic evolution. In the study it is further shown that a phylogenetically broad inverse relation exists between the power of drift and the structural integrity of protein subunits. Thus, the accumulation of mildly deleterious mutations in populations of small size induces secondary selection for protein–protein interactions that stabilize key gene functions, mitigating the structural degradation promoted by inefficient selection. By this means, the complex protein architectures and interactions essential to the genesis of phenotypic diversity may initially emerge by non-adaptive mechanisms. | 1 | Biochemistry |
In some cases, such as the decay of , the bremsstrahlung produced by shielding the beta radiation with the normally used dense materials (e.g. lead) is itself dangerous; in such cases, shielding must be accomplished with low density materials, such as Plexiglas (Lucite), plastic, wood, or water; as the atomic number is lower for these materials, the intensity of bremsstrahlung is significantly reduced, but a larger thickness of shielding is required to stop the electrons (beta radiation). | 7 | Physical Chemistry |
Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "felsic" rocks. Those again that contain the least silica and most magnesia and iron, so that quartz is absent while olivine is usually abundant, form the "mafic" group. The "intermediate" rocks include those characterized by the general absence of both quartz and olivine. An important subdivision of these contains a very high percentage of alkalis, especially soda, and consequently has minerals such as nepheline and leucite not common in other rocks. It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of mafic rocks. Lastly, a small sub-group rich in olivine and without feldspar has been called the "ultramafic" rocks. They have very low percentages of silica but much iron and magnesia.
Except these last, practically all rocks contain felspars or feldspathoid minerals. In the acid rocks, the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the mafic rocks labradorite, anorthite, and bytownite prevail, being rich in lime and poor in silica, potash, and soda. Augite is the most common ferromagnesian in mafic rocks, but biotite and hornblende are on the whole more frequent in felsic rocks.
Rocks that contain leucite or nepheline, either partly or wholly replacing felspar, are not included in this table. They are essentially of intermediate or of mafic character. We might in consequence regard them as varieties of syenite, diorite, gabbro, etc., in which feldspathoid minerals occur, and indeed there are many transitions between syenites of ordinary type and nepheline — or leucite — syenite, and between gabbro or dolerite and theralite or essexite. But, as many minerals develop in these "alkali" rocks that are uncommon elsewhere, it is convenient in a purely formal classification like that outlined here to treat the whole assemblage as a distinct series.
This classification is based essentially on the mineralogical constitution of the igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial, but it has grown up with the growth of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, and the peridotites, are far less important than the granites, diorites, and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros. As the different kinds of rock, regarded as aggregates of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. The quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzonites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on. | 9 | Geochemistry |
Minerals produced through hydrothermal alteration and weathering of primary basaltic minerals are also present on Mars. Secondary minerals include hematite, phyllosilicates (clay minerals), goethite, jarosite, iron sulfate minerals, opaline silica, and gypsum. Many of these secondary minerals require liquid water to form (aqueous minerals).
Opaline silica and iron sulphate minerals form in acidic (low pH) solutions. Sulphates have been found in a variety of locations, including near Juventae Chasma, Ius Chasma, Melas Chasma, Candor Chasma, and Ganges Chasma. These sites all contain fluvial landforms indicating that abundant water was once present. Spirit rover discovered sulfates and goethite in the Columbia Hills.
Some of the mineral classes detected may have formed in environments suitable (i.e., enough water and the proper pH) for life. The mineral smectite (a phyllosilicate) forms in near-neutral waters. Phyllosilicates and carbonates are good for preserving organic matter, so they may contain evidence of past life. Sulfate deposits preserve chemical and morphological fossils, and fossils of microorganisms form in iron oxides like hematite. The presence of opaline silica points toward a hydrothermal environment that could support life. Silica is also excellent for preserving evidence of microbes. | 9 | Geochemistry |
Hypersensitive sites are found on every active gene, and many of these genes often have more than one hypersensitive site. Most often, hypersensitive sites are found only in chromatin of cells in which the associated gene is being expressed, and do not occur when the gene is inactive.
In DNA being transcribed, 5'hypersensitive sites appear before transcription begins, and the DNA sequences within the hypersensitive sites are required for gene expression. Note: hypersensitive sites precede active promoters.
Hypersensitive sites are generated as a result of the binding of transcription factors that displace histone octamers.
They can also be located by indirect end labelling. A fragment of DNA is cut once at the hypersensitive site with DNase and at another site with a restriction enzyme. The distance from the known restriction site to the DNase cut is then measured to give the location. | 1 | Biochemistry |
Typical type II restriction enzymes differ from type I restriction enzymes in several ways. They form homodimers, with recognition sites that are usually undivided and palindromic and 4–8 nucleotides in length. They recognize and cleave DNA at the same site, and they do not use ATP or AdoMet for their activity—they usually require only Mg as a cofactor. These enzymes cleave the phosphodiester bond of double helix DNA. It can either cleave at the center of both strands to yield a blunt end, or at a staggered position leaving overhangs called sticky ends. These are the most commonly available and used restriction enzymes. In the 1990s and early 2000s, new enzymes from this family were discovered that did not follow all the classical criteria of this enzyme class, and new subfamily nomenclature was developed to divide this large family into subcategories based on deviations from typical characteristics of type II enzymes. These subgroups are defined using a letter suffix.
Type IIB restriction enzymes (e.g., BcgI and BplI) are multimers, containing more than one subunit. They cleave DNA on both sides of their recognition to cut out the recognition site. They require both AdoMet and Mg cofactors. Type IIE restriction endonucleases (e.g., NaeI) cleave DNA following interaction with two copies of their recognition sequence. One recognition site acts as the target for cleavage, while the other acts as an allosteric effector that speeds up or improves the efficiency of enzyme cleavage. Similar to type IIE enzymes, type IIF restriction endonucleases (e.g. NgoMIV) interact with two copies of their recognition sequence but cleave both sequences at the same time. Type IIG restriction endonucleases (e.g., RM.Eco57I) do have a single subunit, like classical Type II restriction enzymes, but require the cofactor AdoMet to be active. Type IIM restriction endonucleases, such as DpnI, are able to recognize and cut methylated DNA. Type IIS restriction endonucleases (e.g. FokI) cleave DNA at a defined distance from their non-palindromic asymmetric recognition sites; this characteristic is widely used to perform in-vitro cloning techniques such as Golden Gate cloning. These enzymes may function as dimers. Similarly, Type IIT restriction enzymes (e.g., Bpu10I and BslI) are composed of two different subunits. Some recognize palindromic sequences while others have asymmetric recognition sites. | 1 | Biochemistry |
NAD-dependent formate dehydrogenases are important in methylotrophic yeast and bacteria, being vital in the catabolism of C1 compounds such as methanol. The cytochrome-dependent enzymes are more important in anaerobic metabolism in prokaryotes. For example, in E. coli, the formate:ferricytochrome-b1 oxidoreductase is an intrinsic membrane protein with two subunits and is involved in anaerobic nitrate respiration.
NAD-dependent reaction
Formate + NAD CO + NADH + H
Cytochrome-dependent reaction
Formate + 2 ferricytochrome b1 CO + 2 ferrocytochrome b1 + 2 H | 1 | Biochemistry |
Metal-catalyzed cyclopropanations are chemical reactions that result in the formation of a cyclopropane ring from a metal carbenoid species and an alkene. In the Simmons–Smith reaction the metal involved is zinc. Metal carbenoid species can be generated through the reaction of a diazo compound with a transition metal). The intramolecular variant of this reaction was first reported in 1961. Rhodium carboxylate complexes, such as dirhodium tetraacetate, are common catalysts. Enantioselective cyclopropanations have been developed. | 0 | Organic Chemistry |
An example due to photodissociation is triphenylsulfonium triflate. This colourless salt consists of a sulfonium cation and the triflate anion. Many related salts are known including those with other noncoordinating anions and those with diverse substituents on the phenyl rings.
The triphenylsulfonium salts absorb at a wavelength of 233 nm, which induces a dissociation of one of the three phenyl rings. This dissociated phenyl radical then re-combines with remaining diphenylsulfonium to liberate an H ion. The second reaction is irreversible, and therefore the entire process is irreversible, so triphenylsulfonium triflate is a photoacid generator. The ultimate products are thus a neutral organic sulfide and the strong acid triflic acid.
:[(CH)S][CFSO] + hν → [(CH)S][CFSO] + CH
:[(CH)S][CFSO] + CH → (CHCH)(CH)S + [[Triflic acid|[CFSO][H]]]
Applications of these photoacids include photolithography and catalysis of the polymerization of epoxides. | 5 | Photochemistry |
Asymmetric Heck reactions establish quaternary or tertiary stereocenters. If migratory insertion generates a quaternary center adjacent to the palladium-carbon bond (as in reactions of trisubstituted or 1,1-disubstituted alkenes), β-hydride elimination toward that center is not possible and it is retained in the product. Similarly, β-hydride elimination is not possible if a hydrogen syn to the palladium-carbon bond is not available. Thus, tertiary stereocenters can be established in conformationally restricted systems. | 0 | Organic Chemistry |
Amorphous brazing foils are typically used for brazing, a metallurgy process by which two pieces of metal are joined by melting and cooling a third "fill metal" at their joint. The use of preforms increases the capability of ABFs for use on an industrial scale, even being able to be assembled by machine. | 8 | Metallurgy |
Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C complex called La@C was synthesized in 1985. The @ (at sign) in the name reflects the notion of a small molecule trapped inside a shell. Two types of endohedral complexes exist: endohedral metallofullerenes and non-metal doped fullerenes. | 6 | Supramolecular Chemistry |
In 1956 a heterogeneous catalyst made of palladium deposited on silk was shown to effect asymmetric hydrogenation. Later, in 1968, the groups of William Knowles and Leopold Horner independently published the examples of asymmetric hydrogenation using a homogeneous catalysts. While exhibiting only modest enantiomeric excesses, these early reactions demonstrated feasibility. By 1972, enantiomeric excess of 90% was achieved, and the first industrial synthesis of the Parkinson's drug L-DOPA commenced using this technology.
The field of asymmetric hydrogenation continued to experience a number of notable advances. Henri Kagan developed DIOP, an easily prepared C-symmetric diphosphine that gave high ees in certain reactions. Ryōji Noyori introduced the ruthenium-based catalysts for the asymmetric hydrogenated polar substrates, such as ketones and aldehydes. Robert H. Crabtree demonstrated the ability for Iridium compounds to catalyse asymmetric hydrogenation reactions in 1979 with the invention of Crabtrees catalyst. In the early 1990's, the introduction of P,N ligands by several groups independently then further expanded the scope of the C-symmetric ligands, although they are not fundamentally superior to chiral ligands lacking rotational symmetry.
Today, asymmetric hydrogenation is a routine methodology in laboratory and industrial scale organic chemistry. The importance of asymmetric hydrogenation was recognized by the 2001 Nobel Prize in Chemistry awarded to William Standish Knowles and Ryōji Noyori. | 0 | Organic Chemistry |
Biotic material or biological derived material is any material that originates from living organisms. Most such materials contain carbon and are capable of decay.
The earliest life on Earth arose at least 3.5 billion years ago. Earlier physical evidences of life include graphite, a biogenic substance, in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland, as well as, "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. Earth's biodiversity has expanded continually except when interrupted by mass extinctions. Although scholars estimate that over 99 percent of all species of life (over five billion) that ever lived on Earth are extinct, there are still an estimated 10–14 million extant species, of which about 1.2 million have been documented and over 86% have not yet been described.
Examples of biotic materials are wood, straw, humus, manure, bark, crude oil, cotton, spider silk, chitin, fibrin, and bone.
The use of biotic materials, and processed biotic materials (bio-based material) as alternative natural materials, over synthetics is popular with those who are environmentally conscious because such materials are usually biodegradable, renewable, and the processing is commonly understood and has minimal environmental impact. However, not all biotic materials are used in an environmentally friendly way, such as those that require high levels of processing, are harvested unsustainably, or those that are used to produce carbon emissions.
When the source of the recently living material has little importance to the product produced, such as in the production of biofuels, biotic material is simply called biomass. Many fuel sources may have biological sources, and may be divided roughly into fossil fuels, and biofuel.
In soil science, biotic material is often referred to as organic matter. Biotic materials in soil include glomalin, Dopplerite and humic acid. Some biotic material may not be considered to be organic matter if it is low in organic compounds, such as a clam's shell, which is an essential component of the living organism, but contains little organic carbon.
Examples of the use of biotic materials include:
*Alternative natural materials
*Building material, for a stylistic reasons, or to reduce allergic reactions
*Clothing
*Energy production
*Food
*Medicine
*Ink
*Composting and mulch | 1 | Biochemistry |
A column is prepared by packing a solid adsorbent into a cylindrical glass or plastic tube. The size will depend on the amount of compound being isolated. The base of the tube contains a filter, either a cotton or glass wool plug, or glass frit to hold the solid phase in place. A solvent reservoir may be attached at the top of the column.
Two methods are generally used to prepare a column: the dry method and the wet method. For the dry method, the column is first filled with dry stationary phase powder, followed by the addition of mobile phase, which is flushed through the column until it is completely wet, and from this point is never allowed to run dry. For the wet method, a slurry is prepared of the eluent with the stationary phase powder and then carefully poured into the column. The top of the silica should be flat, and the top of the silica can be protected by a layer of sand. Eluent is slowly passed through the column to advance the organic material.
The individual components are retained by the stationary phase differently and separate from each other while they are running at different speeds through the column with the eluent. At the end of the column they elute one at a time. During the entire chromatography process the eluent is collected in a series of fractions. Fractions can be collected automatically by means of fraction collectors. The productivity of chromatography can be increased by running several columns at a time. In this case multi stream collectors are used. The composition of the eluent flow can be monitored and each fraction is analyzed for dissolved compounds, e.g. by analytical chromatography, UV absorption spectra, or fluorescence. Colored compounds (or fluorescent compounds with the aid of a UV lamp) can be seen through the glass wall as moving bands. | 3 | Analytical Chemistry |
To begin HPTLC, a stationary phase has to be determined to separate different compounds within a mixture. Around 90% of all pharmaceutical separations are performed on normal phase silica gel; however, other stationary phases such as alumina can be used for samples with dissociating compounds and cellulose for ionic compounds. The reverse-phase HPTLC method (similar methodology to reverse-phase TLC) is used for compounds with high polarity. After the selection of the stationary phase, plates are generally washed with methanol and dried in an oven to remove excess solvent.
Selection for the mobile phase is one of the most important processes of HPTLC and follows a trial and error pathway. However, the PRISMA system stands as a guideline for finding the optimal mobile phase. The mobile phase is dependent on the absorptivity of the stationary phase and the composition of the compound of interest. The compound is first tested with solutions such as diethyl ether, ethanol, dichloromethane, chloroform for normal phase HPTLC, or solutions such as methanol, acetonitrile, and tetrahydrofuran for reverse phase HPTLC. The retardation factors (Rf) of the compounds with the selected solvent are then analyzed and the solvent that gives the largest Rf is chosen to be the mobile phase for the compound. Then, the mobile solvent strength is tested against hexane (for normal HPTLC) and water (for reverse-phase HPTLC) to determine the need for adjustment.
Notable HPTLC devices such as the Linomat 5 and the Automatic TLC Sampler 4 (ATS 4) by CAMAG function very similarly by having the automated spray-on sample application technique. This automated spray-on technique is useful to overcome the uncertainty in droplet size and position when the sample is applied to the TLC plate by hand. Additionally, automation provides high resolution and narrow bands since the solvent evaporates immediately as the sample makes contact with the plate. One approach to automation has been the use of piezoelectric devices and inkjet printers for applying the sample. Alternatively, the Nanomat 4 and ATS 4 by CAMAG are manually operated where the sample is applied via spot application using a capillary pipette.
Upon chromatographic detection, HPTLC plates are usually developed in saturated twin-trough chambers with filter paper for optimal outcomes. However, flat-bottom chambers and horizontal-development chambers are also used for specific compounds. A general mechanism for the HPTLC device goes as follows. A fitted filter paper is placed in the rear trough of the chamber and the mobile phase is poured through the rear trough to ensure complete solvent absorption of the filter paper. The chamber is then tilted to ~45° so both troughs are equal in solvent volume and left alone to equilibrate for ~20 mins. Finally, the HPTLC plate is placed in the chamber to develop. Between each sample reading, the mobile phase and filter paper are changed to ensure the best outcomes.
The spot capacity (analogous to peak capacity in HPLC) can be increased by developing the plate with two different solvents, using two-dimensional chromatography. The procedure begins with development of a sample loaded plate with first solvent. After removing it, the plate is rotated 90° and developed with a second solvent. | 3 | Analytical Chemistry |
Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called the mesophyll (Greek for "middle leaf"). This assimilation tissue is the primary location of photosynthesis in the plant. The products of photosynthesis are called "assimilates".
In ferns and most flowering plants, the mesophyll is divided into two layers:
* An upper palisade layer of vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis, with intercellular air spaces between them. Its cells contain many more chloroplasts than the spongy layer. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight separation of the cells provides maximum absorption of carbon dioxide. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil are single-layered.
* Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more branched and not so tightly packed, so that there are large intercellular air spaces between them. The pores or stomata of the epidermis open into substomatal chambers, which are connected to the intercellular air spaces between the spongy and palisade mesophyll cell, so that oxygen, carbon dioxide and water vapor can diffuse into and out of the leaf and access the mesophyll cells during respiration, photosynthesis and transpiration.
Leaves are normally green, due to chlorophyll in chloroplasts in the mesophyll cells. Some plants have leaves of different colours due to the presence of accessory pigments such as carotenoids in their mesophyll cells. | 5 | Photochemistry |
Gas chromatography-olfactometry (GC-O) is a technique that integrates the separation of volatile compounds using a gas chromatograph with the detection of odour using an olfactometer (human assessor). It was first invented and applied in 1964 by Fuller and co-workers. While GC separates volatile compounds from an extract, human olfaction detects the odour activity of each eluting compound. In this olfactometric detection, a human assessor may qualitatively determine whether a compound has odour activity or describe the odour perceived, or quantitatively evaluate the intensity of the odour or the duration of the odour activity. The olfactometric detection of compounds allows the assessment of the relationship between a quantified substance and the human perception of its odour, without instrumental detection limits present in other kinds of detectors. Compound identification still requires use of other detectors, such as mass spectrometry, with analytical standards. | 3 | Analytical Chemistry |
FRET in lanthanide probes is a widely used technique to measure the distance between two points separated by approximately 15–100 Angstrom. Measurements can be done under physiological conditions in vitro with genetically encoded dyes, and often in vivo as well. The technique relies on a distant- dependent transfer of energy from a donor fluorophore to an acceptor dye. Lanthanide probes has been used to study DNA-protein interactions (using a terbium chelate complex) to measure distances in DNA complexes bent by the CAP protein. | 1 | Biochemistry |