Enrichment, preferential

Renick SE, Kleven DT, Chan J, Stenius K, Milner TA, Pickel VM, Fremeau RT, Jr. (1999) The mammalian brain high-affinity L-proline transporter is enriched preferentially in synaptic vesicles in a subpopulation of excitatory nerve terminals in rat forebrain. J Neurosci 19 21-33. 

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

Effusion separator (or effusion enricher). An interface in which carrier gas is preferentially removed from the gas entering the mass spectrometer by effusive flow (e.g., through a porous tube or through a slit). This flow is usually molecular flow, such that the mean free path is much greater than the largest dimension of a traverse section of the channel. The flow characteristics are determined by collisions of the gas molecules with surfaces flow effects from molecular collisions are insignificant. 

Jet separator. An interface in which carrier gas is preferentially removed by diffusion out of a gas jet flowing from a nozzle. Jet separator, jet-orifice separator, jet enricher, and jet orifice are synonymous terms. 

Chemical analysis of the metal can serve various purposes. For the determination of the metal-alloy composition, a variety of techniques has been used. In the past, wet-chemical analysis was often employed, but the significant size of the sample needed was a primary drawback. Nondestmctive, energy-dispersive x-ray fluorescence spectrometry is often used when no high precision is needed. However, this technique only allows a surface analysis, and significant surface phenomena such as preferential enrichments and depletions, which often occur in objects having a burial history, can cause serious errors. For more precise quantitative analyses samples have to be removed from below the surface to be analyzed by means of atomic absorption (82), spectrographic techniques (78,83), etc. 

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). 

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

Chiral chemical reagents can react with prochiral centers in achiral substances to give partially or completely enantiomerically pure product. An example of such processes is the preparation of enantiomerically enriched sulfoxides from achiral sulfides with the use of chiral oxidant. The reagent must preferential react with one of the two prochiral faces of the sulfide, that is, the enantiotopic electron pairs. 

When dealing with polymer blends or blockcopolymers, surface enrichment or microstructures may be observed as already discussed in Sect. 3.1. Quite similar effects may be expected for buried interfaces e.g. between polymer and substrate where one component may be preferentially enriched. In a system of PS, PVP and diblock copolymer PS-6-PVP it has been shown by FRS that the copolymer enrichment is strongly concentration dependent [158]. In a mixed film of PS(D) and end-functionalized PS on a silicon wafer the end-functionalized chains will be attached to the silicon interface and can be detected by NR [159],... 

Figure 10.4. Effect on apatite-collagen isotopic fractionation due to inhibition of amino acid production and preferred use of exogenous amino acids. Carnivore and herbivore, both based on C3 plants, have similar bulk isotopic composition of total edible tissues (T), leading to similar 5 C for apatite carbonate (AP). Collagen (CO) of carnivore is more enriched in Cthan that of herbivore, because of preferential utilization of amino acids derived from protein (P) of herbivore flesh in construction of carnivore s proteins. C ss = assimilated carbon.

Two types of sulfoximinocarboxylates (analogous to sulfinylcarboxylates 16), namely 5 -aryl-5 -methoxycarbonylmethyl-A(-methyl sulfoximine 36 and -methyl-5 -phenyl-A(-ethoxycarbonyl sulfoximine 37, were subjected to hydrolysis in the presence of PLE in a phosphate buffer. As a result of a kinetic resolution, both the enantiomerically enriched recovered substrates and the products of hydrolysis and subsequent decarboxylation 38 and 39, respectively, were obtained with moderate to good ees (Equations 20 and 21). Interestingly, in each case the enantiomers of the substrates, having opposite spatial arrangement of the analogous substituents, were preferentially hydrolysed. This was explained in terms of the Jones PLE active site model. ... 

The enantioselectivity was greatly improved by the copolymerization with 7- or 13-membered non-substituted lactone using lipase CA catalyst (Scheme 8) the ee value reached ca. 70% in the copolymerization of (3-BL with DDL. ft is to be noted that in the case of lipase CA catalyst, the (5 )-isomer was preferentially reacted to give the (5 )-enriched optically active copolymer. The lipase CA-catalyzed copolymerization of 8-caprolactone (6-membered) with DDL enan-tioselectively proceeded, yielding the (/ )-enriched optically active polyester with ee of 76%. 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

The difference in the kinds of metals enriched in Kuroko, base metal vein-type and precious metal vein-type deposits could be explained in terms of the HSAB (hard, soft, acids and bases) principle (Pearson, 1963). According to this principle, relatively hard cations (base metal (Cu, Pb, Zn, Fe, Mn, Ag) ions) tend to combine preferentially with chloride ion in hydrothermal solution, while soft cations (Au, Ag, Tl, Hg ions etc.) combine with H2S and HS . The differences in salinity of ore fluids in base-metal-rich deposits (base metal vein-type deposits and Kuroko deposits) and base-metal-poor deposits (precious metal vein-type deposits) is also in accordance with the HSAB principle. 

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