Conformation of a rigid tetrasaccharide epitope in the capsular polysaccharide of Vibrio cholerae O139.

A newly reported strain of Vibrio cholerae, known as strain O139 Bengal, is the first instance of an encapsulated strain that has caused epidemic cholera. The O-antigenic capsule is the critical antigen for protective immunity. Since mapping of the antigenic epitopes will assist in the development of a protein conjugate vaccine based on the capsular polysaccharide, we have undertaken a study of the three-dimensional conformation of the polysaccharide. It contains six residues in the repeating subunit with the unusual feature of a 4,6 cyclic phosphate on a beta-galactopyranoside. A structural epitope composed of four of the residues is somewhat similar to the Lewis(b) blood group tetrasaccharide. Polysaccharide samples enriched in (13)C have been prepared by growth of the bacteria in (13)C-enriched medium. Multidimensional heteronuclear NMR and molecular modeling studies are reported, which show that the O139 tetrasaccharide adopts a compact and tightly folded conformation that is relatively rigid and similar to the Le(b) conformation. The cyclic phosphate on the beta-galactopyranoside residue is in contact with the colitose residue linked to the beta-GlcNAc.