Polysaccharides as well as the oligosaccharide components of glycoproteins and glycopeptides and glycolipids may be linear or branched, homo- or heteropolymer, neutral or charged. These oligo- and polysaccharides can assume tertiary (globular) structure, and in some cases the particular spacial presentation of a carbohydrate becomes critical in dictating the immunologic specificity. For instance, Jennings and associates have evidence that the capsular polysaccharide of group B streptococcus assumes a conformation in which short, sialic acid-terminated side-chains each form a hydrogen bond with an intra-chain GlcNAc nearby. That results in an immunologically specific conformation. This, even though the sialic acid itself is not a (contact) immunodetermin-ant, and apparently only functions as the vector in creating the correct conformational rigidity. However, it must be kept in mind that for carbohydrates, conformational changes on the polymeric level are easier to achieve (i.e. have lower energy barriers) than they are for proteins. Indeed, various conformational possibilities are probably populated in proportion to the energy differences between them. Critically important requirements such as in the example above do not (yet?) appear ubiquitous for carbohydrate antigens. It should be realized that, for proteins, loss of tertiary structure (i.e. denaturation), once it occurs, is often hard to reverse, even when conditions are changed, while denaturation of polysaccharides is unknown, or rare at best. Polysaccharides are quite heat stable - although auto-hydrolysis can occur if they are acidic polysaccharides - and their solution structure is less influenced by changes in electrolyte concentration unless they carry charged groupings.
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