Structure of the complex group-specific polysaccharide of group B Streptococcus

The group-specific antigen was isolated from a type Ia group B streptococcal strain and is a complex polysaccharide composed of alpha-L-rhamnopyranosyl, alpha-D-galactopyranosyl, 2-acetamido-2-deoxy-beta-D-glucopyranosyl, D-glucitol, and phosphate residues. The complexity of the group B polysaccharide antigen is evident from the fact that when depolymerized by basic hydrolysis it yielded three structurally related, but nevertheless significantly different, oligosaccharides. These oligosaccharides were obtained in different molar quantities as their monophosphate esters. This evidence strongly suggests that they are linked by phosphodiester bonds in the original group B antigen. If these oligosaccharides are in fact randomly situated throughout the linear polysaccharide, then this type of heterogeneous repeating unit is unusual for a polysaccharide of bacterial origin. However, this structural arrangement of the oligosaccharides has yet to be unambiguously established because the alternate explanation of there being three different polysaccharides in the group B antigen cannot be discounted in the evidence presented here. The oligosaccharides were enzymatically dephosphorylated, and the structures of two of the three oligosaccharides are (formula: see text) Despite their structural differences, the two oligosaccharides are related by the smaller being an integral part of the larger. In the structural analysis of the group B antigen, methylation analysis, periodate oxidation, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, fast atom bombardment mass spectrometry, and various specific chemical and enzymatic degradations were the principal methods used. Of particular interest was the use of an alpha-rhamnosidase to selectively degrade the larger oligosaccharide. This facilitated the assignment of signals in its 1H and 13C NMR spectra.