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 threedimensional conformation of the polysaccharide. It contains six residues in the repeating

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... unit with the unusual feature of a 4,6 cyclic phosphate on a -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 -galactopyranoside residue is in contact with the colitose residue linked to the -GlcNAc. 1 Abbreviations: NOESY, nuclear Overhauser effect spectroscopy; HMQC, heteronuclear multiple quantum coherence; DQF COSY, double quantum filtered coherence spectroscopy; CT, cholera toxin; LPS, lipopolysaccharide; FID, free induction decay; ELISA, enzymelinked immunosorbent assay; CPS, capsular polysaccharides. FIGURE 1: Schematic representation of capsular polysaccharide structure of V. cholerae O139. Tetrasaccharide epitope homologous to Le b histo-blood group antigen is shown in boldface type. Labeling a-f is according to Preston et al. (7).