Title

Synthesis, conformational analysis, and application of bioactive oligosaccharides

Date of Award

1999

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Peng George Wang, Committee Chair

Abstract

Glycosidase-catalyzed transglycosylation for practical synthesis of oligosaccharides has been developed by using a thermostable CLONEZYMETM glycosidase library. Convenient syntheses of N-acetyllactosamine, xylose containing oligosaccharides, and galactobiosides by the use of regioselective transglycosylation were described. Furthermore, transglycosylation could be used to modify hydroxyalkyl polysaccharides.alpha-Galactosyl epitopes are carbohydrate structures bearing a Galalpha1 → 3Gal terminus. The interaction of these epitopes on the surface of animal cells with anti alpha-Gal antibodies in human serum is believed to be the main cause in antibody-mediated hyperacute rejection in xenotransplantation. An efficient chemoenzymatic approach based on the use of recombinant alpha1 → 3 galactosyltransferase has been developed to synthesize alpha-Gal epitopes. Simultaneously, chemical synthesis of alpha-Galactosyl epitopes was accomplished using thioglycoside chemistry.Conformational analysis of an N-linked alpha-Gal trisaccharide epitope was conducted in terms of each monosaccharide residue conformation, primary hydroxymethyl group configuration and interglycosidic conformations. Selective 2D J-delta INEPT experiments have been carried out at different temperatures to evaluate the three-bond long range 13C, 1H coupling constants for the alpha1 → 3 linkage. The NMR experimental data were complemented by theoretical calculations based on energy minimization, grid search, and Metropolis Monte Carlo simulations. The results indicated that the trisaccharide had a restricted flexibility around the crucial alpha1 → 3 linkage. The determination of this conformation set the foundation for the design of conformationally restricted alpha-Gal mimetics. The hydrogen bond between HO-2' and HO-4 in Galalpha1 → 3Gal disaccharide was replaced by a methylene bridge, thus introducing a highly rigid and preorganized conformation.Glycopolymer mediators bearing Galalpha1 → 3Gal termini as multivalent xenoactive antigens and alpha-mannosyl termini as specific multivalent ligands for bacterial cells were prepared through chemoenzymatic synthesis. The resulting glycopolymers binding to bacterial cells and human natural anti-Gal antibody were demonstrated. It holds the possibility of removing bacterial cells by redirecting human natural immunity through this alpha-Gal-mannose glycoconjugates.

Keywords

Chemistry, Biochemistry; Chemistry, Organic

Link to Full Text

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