Paul L. Weber

Conformational analysis—XV : Force field calculations and NMR determination of conformational equilibria of organosilicon compounds

Abstract Force field calculations of the conformational energies of fifteen silanes are described. The calculated structures of silaethane, 2-silapropane and 2-methyl-2-silapropane are in acceptable agreement with available microwave data. The calculated torsional barriers of silaethane 2-silapropane; 2-methyl- 2-silapropane and 1-silapropane are consistent with reported barriers. In 2-silabutane and related compounds, the gauche conformation is more stable than the anti conformation as a result of attractive van der Waals energy terms. The strain energies of the two eclipsed conformations of 2-silabutane are identical and substantially lower than the strain energies of the two eclipsed conformations of butane which are of unequal energy. The shape of the torsional curve for 2-silabutane differs dramatically from that of butane. 1-Silabutane is stable in the anti conformation and the gauche-anti energy difference is similar to butane. The two eclipsed conformations of 1-silabutane stand in the same order as those for butane but are of higher energy. A comparison of the torsional curve for 1-silabutane with butane illustrates the steeper barriers for the former compound. Conformational equilibrium constants for 2-silabutane, 1-silabutane, and several compounds containing the 2-silabutane structure are obtained by NMR analysis of vicinal coupling constants and are in agreement with the calculated force field values. The conformational preferences of SiH3, SiH2, SiH(CH3)2, and Si(CH3)3 on cyclohexane are calculated. Unique features of silacyclohexane and the conformational preferences of hydrogen, methyl, and t-butyl on this ring are discussed.

Separation and quantitation of the amino acid neurotransmitters in rat brain by capillary electrophoresis

Capillary electrophoresis (CE) has been used to separate the CBI derivatives of the neurotransmitters gamma-aminobutyric acid, glycine, glutamate, aspartate, norepinephrine and dopamine from 18 other amino acids present in the rat brain. The procedure, which requires an injection volume of < 10 nl, gave detection limits of 24-40 fmol using UV detection at 420 nm. Efficiencies for the derivatized amino acids varied from 344,000 to 444,000 theoretical plates. The method was applied successfully to the quantitation of the amino acid neurotransmitters and several other amino acids in a rat brain homogenate.

Structural and immunochemical characterization of the acidic arabinomannan of mycobacterium smegmatis

A serologically active, acidic arabinomannan has been isolated from Mycobacterium smegmatis. The polysaccharide contains approximately 56 arabinosyl and 11 mannosyl residues, and 2 phosphate, 6 monoesterified succinate, and 4 ether-linked lactate groups. After saponification to remove succinyl groups, the polysaccharide can be separated into phosphorylated (55%) and nonphosphorylated (45%) forms, the former containing a little more arabinose and a little less mannose than the latter. The structures of these polysaccharides were investigated by 1H- and 13C-n.m.r. spectroscopy and methylation analysis, before and after selective cleavage of furanosyl linkages. The phosphorylated and nonphosphorylated forms of the polysaccharide were found to have similar, if not identical, structures. The main structural feature of the polysaccharides is the presence of chains of contiguous arabinofuranosyl residues linked alpha-(1 leads to 5). These chains are attached at 0-4 of arabinopyranosyl residues that are present in a core region of the polysaccharide that also contains mannopyranosyl residues. Immunochemical studies demonstrated that the polysaccharide is an effective, precipitating antigen with antisera from rabbits immunized with cell walls or heat-killed cells of M. smegmatis. The polysaccharide is, however, more effective as a precipitating antigen after removal of the succinate groups, and completely ineffective after removal of arabinofuranosyl residues. The polysaccharide therefore contains an important antigen in common with the arabinogalactan lipopolysaccharide of the cell wall of the bacterium, i.e., chains of contiguous alpha-(1 leads to 5)-linked arabinofuranosyl residues.

Capillary zone electrophoresis separation of tryptophan and its metabolites, including quinolinic acid

Capillary zone electrophoresis with UV absorbance detection was used to separate tryptophan and ten of its metabolites. Run buffers of pH 4.0-10.0 were evaluated for their effect on resolution; a pH 9.6 buffer was found to give optimum separation of all components. Ethylenediaminetetraacetic acid (EDTA), which prevents complexation of some analytes with polyvalent cations, was included in the run buffer to insure good peak shape and reproducible mobilities. The resulting method was used to detect the presence of quinolinic acid in a urine sample.

A simultaneous assay system for n-acetylgalactosamine and N-acetylgalactosaminitol using gas chromatography/mass spectrometry.

Abstract A procedure is described, using gas chromatography/mass spectrometry to identify and quantitate various sugar alditol acetates, which will simultaneously quantitate N-acetylgalactosamine and N-acetylgalactosaminitol. N-acetylgalactosamine is distinguished from N-acetylgalactosaminitol by reduction with sodium borodeuteride and determining relative intensities of the ion pairs m e 84, 85; 102, 103; and 144, 145. The method is adequate for determining amounts of these sugars in the range of 40–1000 nmol.

Biomedical applications and biological systems

The potential of capillary electrophoresis (CE) in the analysis of the many compounds of biological interest can been deduced from a quick scan of the variety of topics covered in other chapters of this book. These chapters have dealt with analysis of drugs and endogenous compounds present as either standard aqueous solutions or in biological fluids routinely used for clinical analysis, such as serum, plasma, urine or cerebrospinal fluid. In this chapter the review will focus on those reports that involve tissue analysis, particularly of the brain. Also reviewed will be the use of CE in the analysis of single cells in which the human erythrocyte seems to be the most popular cell studied. Table 28.1 summarizes the conditions used, compounds analyzed and gives a sample matrix for cited applications. Also reviewed are studies on the use of microdialysis and related methods for obtaining samples for CE analysis. The literature covered includes only those papers published in 1992 or later.

Chapter 8 Capillary electrophoresis in the study of amino acids

Publisher Summary Peptide or protein products contain primarily only the 20 common amino acids or their derivatives and thus represent a narrower range of amino acid structures than are encountered in endogenous samples. The methodologies developed for peptide and protein product analyses are well established with respect to efficient separations and reproducible and accurate quantitation. Because of the success of liquid chromatography (LC) in the routine analysis of amino acid products of peptide or protein origin, there is little advantage in using capillary electrophoresis (CE) for these applications. Because the basis of LC separation rests primarily on differences in polarity rather than size and charge, resolution of such amino acid mixtures is inherently difficult. The simplest form of CE—capillary zone electrophoresis (CZE)—separates analytes on the basis of their effective size and charge and is thus ideally suited to such a task. Compared to LC, CE exhibits higher efficiencies and provides much easier means to explore different separation conditions for optimization.

Synthetic adventure (Williams, Fred D.)

A software program that is an adventure game which utilizes synthetic organic chemistry to overcome a series of problems or obstacles encountered in the game.