Structure and conformations of heteropolysaccharides

Abstract

The various geometrical and molecular forces that can define the conformation adopted by a polysaccharide chain are reviewed. Specific characterised examples are used to illustrate the various general polysaccharide conformational types. The use of optical rotation, circular dichroism and nuclear magnetic resonance in the investigation of conformations adopted by polysaccharides in solution and the types of junctions that can be involved, are considered. Various solid state conformations adopted by hyaluronate have been studied by Xray diffraction techniques. A four-fold helical structure having a fibre repeat of 3.39 nm is the more readily obtained form. Initial computer model building studies showed that a double helical structure was one interpretation of the data. Subsequent calculations by colleagues have shovn that a single helical structure provides a more favourable interpretation of the results. The conformation of hyaluronate in solution has been examined by various physical techniques.One of these, nuclear magnetic resonance has demonstrated the presence of a novel ordered conformation in solution that is stable to changes in salt concentration, temperature and pH 2.5-7.0. The order can be disrupted by enzymic cleavage or the action of mild alkali. The relationship between such an orded conformation and the physical and biological properties of hyaluronate are considered. A preliminary interpretation of Xray diffraction patterns for two bacterial polysaccharides is given. The uncertainty surrounding their covalent structures is discussed and preliminary computer models corresponding to the various possibilities have been investigated. Part of the work described in Chapter 2 of this thesis has been published in collaboration with Drs. I.C.M.Dea, D.A.Rees, S. Arnott, J.M. Guss and E.A.Balazs. A reprint is included at the end of the thesis.