Acceptors In Reactions Research Articles

Synthetic and theoretical studies on group-transfer imidoylation of organotellurium compounds. remarkable reactivity of isonitriles in comparison with carbon monoxide in radical-mediated reactions.

Imidoylation of organotellurium compounds with isonitriles has been investigated in conjunction with the radical-mediated C1 homologation reaction by using CO and isonitriles. Carbon-centered radicals generated photochemically or thermally from organotellurium compounds react with isonitriles in a group-transfer manner to give the corresponding imidoylated products. Organotellurium compounds have been found to serve as effective precursors of a wide variety of stabilized radicals, namely benzyl, alpha-alkoxy, alpha-amino, and acyl radicals, which take part in the imidoylation with high efficiency. The reactions are compatible with various functional groups, and can be carried out in various solvents including environmentally benign water. The reactivity of isonitriles has been compared with that of CO through competition experiments, and the results indicate that isonitriles are superior to CO as radical acceptors in reactions with stabilized radicals. The origin of the differences has been addressed in theoretical studies with density functional theory calculations using the B3LYP hybrid functional. The calculations suggest that both carbonylation and imidoylation proceed with low activation energies, and that there are virtually no differences in the kinetic sense. Instead, it indicates that thermodynamic effects, namely differences in the stability of the acyl and the imidoyl radicals, control the overall course of the reactions.

SPV Monitoring of Near Surface Doping – Role of Boron-Hydrogen Interaction; Boron Passivation and Reactivation

ABSTRACTHydrogen is known to cause the passivation of boron acceptors after such processing steps as wet etching, reactive ion etching, sputter deposition of metal contacts, and Ar ion beam etching. Previous studies of this effect employed CV profiling, spreading resistance profiling, and SIMS measurements on samples diffused with deuterium. These methods are either destructive to the Si surface or require deposition of metal contact. In the present study we used a non-contact small signal ac-surface photovoltage technique, currentlyavailable in commercial diagnostic tools. Simultaneous measurements of the semiconductor surface barrier, Vsb, and the capacitance of the surface depletion layer, CD, give the concentration of boron acceptors in a submicron distance from the Si surface or Si/SiO2 interface. The technique has proven very successful in monitoring low dose implants and also near surface doping in oxidized wafers. Inbare silicon wafers the method occasionally indicated surface boron concentration noticeably below the bulk value. We found such behavior in wafers after the chemical cleaning, used to prepare a hydrogen terminated surface. Thermal annealing at temperatures from 150°C to 200°C reactivates the boron dopant. We will discuss the effect of various cleaning and annealing conditions on passivation and reactivation of boron acceptors in the near surface region. The results obtained with the non-contact SPV technique show excellent agreement with previous studies. They also provide a basis for reliable measurement of the boron concentration free of interference from hydrogen passivation.

Passivation and thermal reactivation of Mg acceptors inp-type GaAs

This paper reports on the efficient passivation of Mg-doped GaAs. The extent of the passivation was demonstrated to be highly temperature dependent, with the passivation being even more efficient than that of Zn-doped GaAs at temperatures below $140\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ By performing reverse bias annealing measurements between 60 and $130\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ we observed the reactivation of the Mg acceptors within the high-field region of the Schottky diode. This reactivation was found to obey first-order kinetics, with a dissociation energy ${E}_{\mathrm{MgH}}=(0.90\ifmmode\pm\else\textpm\fi{}0.03) \mathrm{eV}$ and an attempt frequency ${\ensuremath{\nu}}_{0}{=(3}_{\ensuremath{-}2}^{+5})\ifmmode\times\else\texttimes\fi{}{10}^{8} {\mathrm{s}}^{\ensuremath{-}1}.$ These results suggest that the Mg-H configuration is not bond centered, as also suggested by previous infrared absorption studies.

One-pot oligosaccharide synthesis exploiting solvent reactivity effects.

[reaction: see text] One-pot syntheses of trisaccharides have been accomplished simply by changing the solvent system between the two subsequent glycosylation reactions and utilizing the difference in glycosylation rate between different solvents. By tuning the reactivity of acceptors and donors and performing the first glycosylation in Et(2)O (low glycosylation rate) and the second in CH(2)Cl(2)/Et(2)O (higher glycosylation rate), trisaccharides were synthesized in high yields (76-84%).

Dermatan Is a Better Substrate for 4-O-Sulfation Than Chondroitin: Implications in the Generation of 4-O-Sulfated, [formula omitted]-Iduronate-Rich Galactosaminoglycans

The biosynthesis of dermatan sulfate is a complex process that involves, inter alia, formation of l-iduronic acid residues by C5-epimerization of d-glucuronic acid residues already incorporated into the growing polymer. It has been shown previously that this reaction is promoted by the presence of the sulfate donor 3′-phosphoadenosine-5′-phosphosulfate. In the present investigation, the role of sulfation in the biosynthesis of l-iduronic acid-rich galactosaminoglycans was examined more closely by a study of the substrate specificities and kinetic properties of the sulfotransferases involved in dermatan sulfate biosynthesis. Comparison of the acceptor reactivities of oligosaccharides from chondroitin and dermatan, in an in vitro system containing microsomes from cultured human skin fibroblasts and 3′-phosphoadenosine-5′-phosphosulfate, showed that Km values for the dermatan fragments were substantially lower than those for their chondroitin counterparts. Calculation of Vmax values likewise showed that dermatan was the better substrate. Whereas dermatan incorporated [35S]sulfate exclusively at the C4 position of N-acetylgalactosamine residues, approximately equal amounts of radioactivity were found at the C4 and C6 positions in the labelled chondroitin. Under standard assay conditions, the 4-O-sulfation of dermatan proceeded about six times faster than the 4-O-sulfation of chondroitin. On the basis of these results, we propose that l-iduronic acids, formed in the course of the biosynthesis of dermatan sulfates, enhance sulfation of their adjacent N-acetylgalactosamine residues, and will thereby be locked in the l-ido configuration.

ChemInform Abstract: Synthesis and Novel Reactivity of Halomethyldimethylsulfonium Salts.

Iodomethyl-, chloromethyl-, and fluoromethyldimethylsulfonium salts, 4b−d, have been synthesized and are observed to be highly reactive molecules that exhibit extraordinary diversity with respect to the nature of their reactivity, undergoing facile direct substitution (SN2) reactions, but also being highly susceptible to electron-transfer reactions. Cyclic voltametry experiments indicated that the iodomethyldimethylsulfonium compound, 4b, is a potent electron acceptor, even surpassing the reactivity of perfluoro-n-alkyl iodides in that capacity. The iodo- and chloromethyldimethylsulfonium salts, 4b,c, as well as the analogous iodomethyltrimethylammonium salt, 3a, are shown to be reactive SET acceptors.

Reactivation of Hydrogen-Passivated Aluminum Acceptors in p-type SiC

Reactivation of Hydrogen-Passivated Aluminum Acceptors in p-type SiC

Asymmetric synthesis of chiral amines with ?-transaminase

The asymmetric synthesis of chiral amines using prochiral ketones was carried out with (S)-specific omega-transaminase (omega-TA) from Vibrio fluvialis JS17. This reaction is inhibited severely by both products, (S)-amine and deaminated ketone. In addition, thermodynamic equilibrium strongly favored the reverse reaction. L-Alanine proved to be the best amino donor based on easy removal of the products. Optimal pH of the reactions with both whole cells and cell-free extract was 7. Amino acceptor reactivities of ketone substrates and reaction profiles of the asymmetric synthesis showed that the initial rate as well as the reaction yield were lower when the resulting (S)-amine from a prochiral ketone substrate was a more reactive amino donor. The yield could be increased dramatically by removing pyruvate, which is a more inhibitory product than (S)-alpha-methylbenzylamine [(S)-alpha-MBA] when acetophenone and L-alanine are used as an amino acceptor and donor, respectively. The removal of pyruvate was carried out by incorporating lactate dehydrogenase (LDH) in cell-free extract or by using whole cells. The whole cell reaction yielded a much better result. When 25 mM benzylacetone and 30 mM acetophenone were used as an amino acceptor with 300 mM L-alanine, 90.2% and 92.1% of the reaction yields after 1 day were obtained with whole cells, respectively. Enantiomeric excesses of both (S)-alpha-MBA and (S)-1-methyl-3-phenylpropylamine [(S)-MPPA] were all above 99%.

ChemInform Abstract: Chemoenzymic Synthesis of β‐D‐Gal(6‐SO4)‐(1→4)‐D‐GlcNAc, β‐D‐Gal‐(1→4)‐D‐GlcNAc(6‐SO4) and β‐D‐GlcNAc‐(1→4)‐D‐GlcNAc(6‐SO4) and Their Roles as Fucosyl Acceptors in Reactions Catalyzed by Human α‐3‐Fucosyltransferases.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

BF3·OEt2 Enhanced Yb(OTf)3-Promoted Glycosidation of 1-O-Acyl-D-Glucopyranose1

Much attention has been paid to the synthesis of glycosides and oligosaccharides for the preparation of natural products and their analogues to investigate biological functions.2 Most known glycosidation methods are based on the activation of a leaving group at the anorneric center of a glycosyl donor.3 In some cases alcohol derivatives such as ROSnBu3 and ROSiMe3 are used to increase the reactivity of glycosyl acceptors.3 1. Dedicated to the memory of Professor Akira Hasegawa.

Chemoenzymic synthesis of β-D-Gal(6-SO4)-(1→4)-D-GlcNAc, β-D-Gal-(1→4)-D-GlcNAc(6-SO4) and β-D-GlcNAc-(1→4)-D-GlcNAc(6-SO4) and their roles as fucosyl acceptors in reactions catalysed by human α-3-fucosyltransferases

N-Acetyllactosamine 5 is obtained by transglycoslyation between p-nitrophenyl β-D-galactopyranoside 3 and N-acetyl-D-glucosamine 4 by using the β-galactosidase from Bacillus circulans as the biocatalyst. Sodium salts of β-Gal(6-SO4)-(1→4)-GlcNAc 1 and β-Gal-(1→4)-GlcNAc(6-SO4) 2 have been prepared from N-acetyllactosamine 5 in five and three steps respectively. β-D-GlcNAc-(1→4)-D-GlcNAc(6-SO4) (di-N-acetylchitobiose 6-sulfate) 10 is synthesised from chitin in three steps. The products have been tested as fucosyl acceptors in reactions catalysed by recombinant human fucosyltransferases III to VII.

8-O-Sialylation of neuraminic acid acceptor reactivity and anomeric stereocontrol

2-O-Unprotected and 2-deoxy-2,3-dehydro-neuraminic acid derivatives 2, 4, and 10 exhibit enhanced acceptor properties at their 8-hydroxy group in sialylation reactions with phosphite 5 as donor; yet, mainly (10) or exclusively (2, 4) β-glycoside bond formation was observed. The 3-phenylthionocarbonate group as stereodirecting auxiliary group in the sialyl donor 11 led with 10 as acceptor to exclusive formation of α(2–8)-linked disaccharide.

Minority-carrier-enhanced reactivation of hydrogen-passivated Mg in GaN

The influence of minority-carrier injection on the reactivation of hydrogen passivated Mg in GaN at 175 °C has been investigated in p-n junction diodes. The dissociation of the neutral MgH complexes is greatly enhanced in the presence of minority carrier and the reactivation process follows second-order kinetics. Conventional annealing under zero-bias conditions does not produce Mg-H dissociation until temperatures ≥450 °C. These results provide an explanation for the e-beam-induced reactivation of Mg acceptors in hydrogenated GaN.

Stereoselective alpha-Sialylation with Sialyl Xanthate and Phenylsulfenyl Triflate as a Promotor.

Reaction alpha- and beta-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH(3)CN/CH(2)Cl(2) at low temperature affords alpha-sialosides in good yield and stereoselectivity. PST is prepared in situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher alpha/beta ratio than more reactive allylic alcohol 7 and primary alcohol 8; alpha-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate alpha- and beta-nitrilium cations 16 and 17 is discussed.

Reactivation of Si Donors and Zn Acceptors in Plasma-Irradiated GaAs by Reverse Bias Annealing

Reactivation of Si Donors and Zn Acceptors in Plasma-Irradiated GaAs by Reverse Bias Annealing

Bradhyrhizobium japonicum hydrogen-ubiquinone oxidoreductase activity: quinone specificity, inhibition by quinone analogs, and evidence for separate sites of electron acceptor reactivity

The purified H 2-uptake hydrogenase of Bradyrhizobium japonicum, containing no cytochrome b, catalyzed efficient H 2-ubiquinone oxidoreductase activity. Hydrogen-oxidizing membranes also catalyzed H 2-ubiquinone oxidoreductase activity, and the site of ubiquinone reduction was localized to the H 2-quinone oxidoreductase complex based on comparative antimycin A and HQNO titrations of both H 2-ubiquinone-1 oxidoreductase and ubiquinol-1 oxidase activities. A variety of quinones could function as electron acceptors of both pure or membrane-bound hydrogenase, including ubiquinone-0 Q 0), ubiquinone-1 (Q 1), duroquinone and menadione, indicating relatively loose substrate specificity with regard to the quinone head group. Both the redox potential and the quinone structure determined the efficiency of hydrogenase turnover. Among short-chain ubiquinones, the isoprenoid chain length had a profound affect on K m, with each additional isoprenoid unit resulting in the K m of the membrane-bound enzyme to decrease more than an order of magnitude. For pure enzyme, the K m values for Q 0, Q 1 and Q 2 were 1.97 mM, 68.8 μM and 3.1 μM, respectively. V max was also influenced by the substrate isoprenoid chain length for the pure enzyme. The inhibition patterns of H 2-dependent Q 1 versus MB reduction by the quinone analogs (2- n-heptyl-4-hydroxyquinoline N-oxide and Antimycin A) were significantly different, and clear differences in pH optima for the two activities were observed. In addition, the two hydrogen-dependent electron acceptor activities (Q 1 and MB) exhibited different time-dependent inactivation patterns by the chemical modification reagent diazobenzene sulfonate. Ubiquinone and MB therefore react by different mechanisms (perhaps at different sites) within the hydrogenase complex in situ. The inhibition pattern of hydrogen-ubiquinone oxidoreductase activity by antimycin A was clearly different than antimycin A inhibition of ubiquinol oxidation at the bc 1 complex. This is, to our knowledge, the first report of antimycin A inhibition of a hydrogenase complex, and also of a quinone reducing site of a primary dehydrogenase. When pure hydrogenase is assayed in the absence of dithionite, a delay (lag phase) is observed prior to attainment of full activity. The length of this lag period (in minutes) was inversely dependent on ubiquinone concentration, and was greatly reduced (but not eliminated) at saturating ubiquinone levels. These effects were obtained with both Q 1 and MB as electron acceptor, and the lag phases with Q 1 were significantly longer than with MB. Electron acceptor binding to hydrogenase is thus required for reductive activation of hydrogenase during turnover.

Lithium passivation and electric-field-assisted reactivation of acceptors in GaAs.

We performed reverse-bias annealing of Schottky diodes on GaAs samples in which Zn and Cu acceptors had been partially passivated by Li diffusion. Using capacitance-voltage and deep level transient spectroscopy measurements we observed thermal dissociation of Zn-Li and Cu-Li complexes in the electric field of reverse-biased junctions and a consequent increase of electrically active acceptors in the depletion region of the diodes as well as enhanced passivation of acceptors in the low-field region of the depletion layer and in the neutral bulk. The direction of the electric field is consistent with lithium drifting as a positively charged ion in p-type GaAs. We also investigated the interaction between lithium and the reactivated acceptors under zero-bias conditions. The effect of zero-bias annealing between room temperature and 150 \ifmmode^\circ\else\textdegree\fi{}C on both the shallow Zn and the deep Cu acceptors was found to be consistent with a mobile ionized ${\mathrm{Li}}_{\mathit{i}}$ donor passivating negatively charged acceptors through Coulomb interaction.

Drastic improvement of Zn doping characteristics in p-AlAs grown by metalorganic chemical vapor deposition for low-resistance vertical-cavity surface-emitting lasers

To reduce the resistance of p-type GaAs/AlAs distributed Bragg reflectors of vertical-cavity surface-emitting lasers, we point out theoretically that heavy doping into wide band-gap layers is very effective. We have demonstrated experimentally a high Zn doping in AlAs by metalorganic chemical vapor deposition. A re-activation of Zn acceptors is realized by N 2 annealing and a maximum hole concentration of 7 X 10 18 cm -3 for AlAs is obtained.

Light-induced reactivation of shallow acceptors in hydrogenated n-type AlxGa1−xAs

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Bi-antennary oligo-(N-acetyllactosamino)glycans of I-type are galactosylated preferentially at the GlcNAc?1-6Gal linked arms by ?1,3-galactosyltransferase of bovine thymus

alpha 1,3-Galactosylation of radiolabelled bi-antennary acceptors Gal beta 1-4GlcNAc beta 1-3(Gal beta 1-4GlcNAc beta 1-6)Gal-R (R = 1-OH, beta 1-4GlcNAc or beta 1-4Glc) with bovine thymus alpha 1,3-galactosyltransferase was studied. At all stages of the reactions the three acceptors reacted faster at the 1-->6 linked arm than at the 1-->3 linked branch. Hence, in addition to the doubly alpha 1,3-galactosylated products, practically pure Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal-R could be obtained from the three acceptors in reactions that had proceeded to near completion. The isomeric mono-alpha 1,3-galactosylated products were identified by using exoglycosidases to remove the branches unprotected by alpha 1,3-galactoses and by subsequently identifying the resulting linear glycans chromatographically.