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31.
In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation. 相似文献
32.
《Tetrahedron》2004,60(22):4781-4787
In addition to the known compounds of the type of nonactic and homononactic acids and their lactones, dilactones and tetralactones, five new compounds, namely homononactyl-nonactoate, a dilactone consisting of nonactic and homononactic acids and three cyclic trimers with nonactic and homononactic acids, were isolated from a strain of Streptomyces globisporus. Their structures, including the absolute configurations of the hydroxyl and methyl groups, were determined by extensive spectroscopic techniques such as UV, IR, MS, 1D and 2D NMR. 相似文献
33.
As an extension of previous reports, the barium carbonate procedure has been optimized in detail for the preparation of apolar columns. The aim was to produce optimum overall column characteristics, and to maintain them unchanged under the prolonged influence of the highest possible temperature. The main parameters under optimization were glass variety, leaching of glass surface with aqueous HCl, and amount of barium carbonate deposited, while deactivation and coating were kept constant. The basic column characteristics were adsorption properties and thermostability of deactivation, acid/base behaviour and separation efficiency. They were determined by a new, quantitative testing procedure. Intense leaching was able to eliminate almost totally the differences between glass varieties and to create a well-defined glass surface. While untreated glass, leached glass, and barium carbonate treated glass showed specific weak points in the respective column quality, the combination of leaching and barium carbonate treatment yielded the highest and most stable quality. Some technical modifications of the preparation procedure are described, including deactivation in the gas phase, and use of pentane as a solvent for static coating. 相似文献
34.
本文报导的方法是在南京大学高鸿教授研究的K_3Fe(CN)_6示波极谱滴定法(在PH8的K_2HPO_4—KH_2PO_4—KBr底液中,用0.04660mol/LK_3Fe(CN)_6标准溶液对纯抗坏血酸测定)的基础上,考虑到L—抗坏血酸(以下简称V_e)在碱性、中性介质中稳定性差,如用于V_e含量很微的果蔬测定,对结果影响大。本文改为H_2C_2O_4—KBr的酸性介质作底液,用 相似文献
35.
《Tetrahedron letters》2004,45(11):2377-2380
The β-substituted captodative olefins methyl 2-aryloxy-3-dimethylaminopropenoates 4a-h were synthesized, via aminomethylenation of the corresponding 2-phenoxyacetic esters 9a-h. Lewis acid promoted intramolecular cyclization of alkenes 4 led to benzofurans 7a-h, in an efficient synthetic approach to the benzofuran frame. 相似文献
36.
Vermeiren K 《Journal of chromatography. A》2005,1085(1):66-73
The two-dimensional ion exclusion chromatography/ion chromatography (ICE-IC) approach is considered to be the method of choice for the determination of trace anions in concentrated hydrofluoric acid (48-50%, w/w). In order to achieve lower detection limits, this method was for the first time used with electrolytically generated and purified hydroxide eluents in combination with a low noise electrochemical suppressor. Compared to carbonate based eluents, the achieved gain in peak height sensitivity for chloride, sulfate, nitrate and phosphate is a factor of 6, 7, 16 and 13, respectively. The instrumental detection limits, based on the background noise, are 2, 0.2, 0.4 and 1.4 microg/kg HF 50% (w/w) for the same anions. Their method detection limits, calculated according to SEMI, are all within the 6-10microg/kg HF 50% (w/w) range and thus at least 10 times lower than the current Tier C grade requirements. The chromatographic run time could be shortened with some 10 min by the use of a relatively fast high-capacity hydroxide selective anion exchange column. 相似文献
37.
Halyna Levytska 《Talanta》2007,71(3):1441-1443
The interaction of Hf(IV) with calconcarboxylic acid (1-(2-hydroxy-4-sulfo-1-naphtylazo)-3-naphtolic acid) was investigated by cyclic voltammetry varying various factors (pH, ionic strength, contents of ethanol and scan rate). Optimal conditions of Hf(IV) determination in the presence of calconcarboxylic acid were found: acetic buffer solution pH 2.6, scan rate 0.5 Vs−1. The detection limit of Hf(IV) concentration was 2.46 × 10−7 mol L−1. The influence of foreign ions on the Hf(IV) determination was studied. It was established that some metals like cadmium, nickel, zinc, copper and titanium could be determined with Hf(IV) simultaneously. The method of voltammetric determination of hafnium was tested on model solutions and used in the determination of Hf(IV) in terbium-base alloy. 相似文献
38.
The complexation equilibria between Ni(II) and Zn(II) metal ions with 3-(1-naphthyl)-2-mercaptopropenoic acid (H2NMP) were studied by glass electrode potentiometry, at 25 °C and 1.0 mol·dm–3 in NaClO4 as constant ionic medium in 50% (v/v) water-ethanol solutions. Formation constants for the complexes Ni(NMP), Ni(NMP)
2
2–
, Zn(NMP) and Zn(NMP)
2
2–
, refined by the MINIGLASS program, are reported. 相似文献
39.
Victor Barba Julián Vázquez Rosa Santillan 《Journal of organometallic chemistry》2005,690(9):2351-2357
The syntheses of eight [4.3.0] heterobicyclic boronates containing a N → B coordinative bond are described. The monomeric compounds were prepared by reaction of arylboronic acids with a tridentate ligand having the ONO donor set of atoms. It was shown that substituents at the para-position of the B-phenyl moiety transmit electronic effects to the CN bond which in turn is polarized by formation of the N → B coordination bond. At the same time, related tridentate ligands were also reacted with 1,4-benzenediboronic acid in order to prepare benzene diboron complexes. The structure of this type of compounds was confirmed by X-ray analysis for one of the derivatives. 相似文献
40.
L. G. Voskressensky T. N. Borisova L. N. Kulikova A. V. Varlamov 《Chemistry of Heterocyclic Compounds》2007,43(7):913-918
The interaction of 7-trifluoroacetyltetrahydropyrrolo[1,2-c]pyrimidine with acetylenedicarboxylic ester (DMAD) and ethyl propiolate
in acetonitrile and alcohols has been studied. It was established that DMAD splits pyrrolopyrimidine at the aminal fragment
in acetonitrile and methanol with the formation of 1-H-and 2-(N-dimethoxycarbonylvinyl-N-methyl)aminoethyl-1-methoxymethyl-3-methyl-5-trifluoroacetylpyrroles.
In acetonitrile ethyl propiolate splits pyrrolopyrimidine both at the aminal fragment and at the C(3)-N(2) bond (Hofmann reaction), but in ethanol only at the C(3)-N(2) bond with the formation of 2-propenylpyrroles.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1082–1087, July, 2007. 相似文献