Conformation of the hexasaccharide repeating subunit from the Vibrio cholerae O139 capsular polysaccharide

In the past decade, several outbreaks of cholera have been reported to be caused by Vibrio cholerae O139, a strain which differs from the more common O1 strain in that the former is encapsulated. The hexasaccharide repeating subunit has been isolated from the V. cholerae O139 capsular polysaccharide by digestion with a recently discovered polysaccharide lyase derived from a bacteriophage specific for this serogroup. It specifically cleaves at a single position of the 4-linked galacturonic acid producing an unsaturated sugar product in quantities for conformational studies by (1)H and (13)C NMR spectroscopy. We report conformational studies on this oligosaccharide by molecular modeling and NMR spectroscopy including nuclear Overhauser effects and residual dipolar coupling of a sample weakly oriented in liquid crystalline solution. The structure contains a tetrasaccharide epitope homologous to the human Lewis(b) blood group antigen, which adopts a relatively well-defined single conformation. Comparison of these results with those of a previously published study of the intact capsular polysaccharide indicates that the conformations of the epitope in the two cases are identical or at least closely similar. Thus, this epitope, which may be essential for the pathogenicity of this V. cholerae strain, is not a "conformational epitope" requiring a certain critical size for antigenicity as has been reported for several other bacterial capsular antigens.