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Title: Mass spectrometric studies on alkali metal cation affinities
Authors: Tam, Kam-fai
Keywords: Hong Kong Polytechnic University -- Dissertations
Mass spectrometry
Alkali metals
Issue Date: 2002
Publisher: The Hong Kong Polytechnic University
Abstract: Alkali metals are the most abundant metals found in living systems. The determination of Li+/ Na+ cation binding sites and binding energies (affinities) of biologically relevant model ligands (molecules) are useful in further understanding the interactions between alkali metal cations and peptides / proteins / carbohydrates, which in term determine the occurrence of many Li+ / Na+ mediated important biological processes. In this project, the lithium and sodium cation bound heterodimers between aliphatic amino acids, and their methyl / ethyl esters, and monosaccharides were successfully prepared by fast-atom bombardment (FAB) and electrospray ionization (ESI). The Li+ or Na+ cation binding energy or affinitiy of aliphatic amino acids and their ester derivatives, as well as that of glucose and its derivatives, mannose and galactose were determined by the kinetic method based on the relative rates of decomposition of the heterodimer to its monomer complexes at different internal or collisional excitation energies inside a tandem mass spectrometer. Our study is the first systematic kinetic method measurement in which the affinity of the unknown amino acid (or its ester derivatives) was measured with a set of heterodimers consisting of reference amides of known and validated Li+ and Na+ affinities. Also, entropic effects on the dissociation of the heterodimer complex was also considered in our measurements. The aliphatic amino acids and their ester derivatives, with their Li+ and Na+ affinities established, were then used as reference compounds to form heterodimers with monosaccharides, from the dissociation of which the corresponding Li+ and Na+ affinities of the monosaccharides were determined. The Li+ and Na+ affinities at 0K of aliphatic amino acids and their -OMe/-OEt esters were determined as follows (Li+/Na+ affinities in kJ mol-1): Leu-OEt (--/179.8) > Val-OEt (--/178.2) > Leu-OMe (/175.7) > Val-OMe (--/174.3) > a-AB-OMe (250.1/172.1) > Ala-OMe (246.3/168.9) > Leu (243.7/166.4) > Val (242.9/164.8) > Gly-OMe (241.7/163.7) > a-AB(240.0/161.8) > Ala (237.2/157.6) > Gly(231.8/152.3) Our Li+ and Na+ affinities for glycine and alanine are in excellent agreement with high level ab intio theoretical G2 affinity values to within +-2 kJ mol-1, lending confidence to the quantitative results obtained in our experimental and theoretical studies. Our results indicate that the Li+ affinity values previous reported by Bojesen et al. [Bojesen et al., 1993] may be systematically too low by about 18 kJ mol-1, while the Na+ affinities may be systematically too high by about 5-6 kJ mol-1. The Li+ and Na+ affinities at 0K of representative monosaccharides were determined as follows (Li+/Na+ affinities in kJ mol-1): b-D-galactose (255.2/---) ~ a-D-mannose (255.2/180.9) ~ a-D-galactose (254.9/180.5) > 3-O-methyl-D-glucose (252.9/178.5) ~ methyl b-D-glucoside (252.6/177.5) ~ methyl a-D-glucoside (250.9/177.0) > 2-deoxy-D-glucose (---/176.3) ~ 3-deoxy-D-glucose (---/176.2) > b-D-glucose (250.9/175.3) > a-D-glucose (249.6/175.1) ~ methyl b-D-xyloside (249.9/172.5) > 6-deoxy-D-glucose (247.1/169.7) For glucose, our theoretical DFT-B3/LYP Li+ and Na+ absolute affinity values are in agreement with experimental values to within ± 20 kJ mol-1. However, the experimental Li+ and Na+ absolute affinity values are 31 -36 kJ mol-1 lower than the theoretical DFT-B3/LYP values. The discrepancy between our theoretical and experimental values for mannose is rationalized in terms of complications arising from the kinetic method measurements, leading to determination of binding affinity of the less stable, bidentately bound, but not the most stable tridentately bound Li+ / Na+- mannose complexes. The Li+ and Na+ binding affinities of glucose, mannose and galactose are found to be greater than that of simple amino acids with aliphatic side chains, such as glycine, alanine and valine. Hence, glucose, mannose, galactose and other monosaccharides could compete effectively with amino acid residues in peptides in their binding to Li+ and Na+ ions in biological systems where solvent effects are minimal. Furthermore, our findings in the gas phase are contrary to reported behaviour in the solution phase, in which bivalent but similar sized cations were found to favour tridentate binding to neighbouring hydroxy groups in axial-equatorial-axial configurations. Our findings will promote the understanding and have implications for biological processes involving the interaction of Li+ or Na+ with glucose, such as the absorption of glucose by Na+-glucose co-transporter system in plants
Description: xviii, 202 leaves : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577M ABCT 2002 Tam
Rights: All rights reserved.
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