Strain O139 of Vibrio cholerae, identified as a cause of epidemic cholera in Asia, is encapsulated, which raises serious public health concerns. This pathogen is antigenically distinct from O1 strains. The applicant has reported on the covalent chemical structure of the polysaccharide capsule and has initiated studies on the polysaccharide as a vaccine component. A working hypothesis concerning the polysaccharide is that based on analogy to similar epitopes found in Lewis blood group oligosaccharides, four of the six sugar residues in the repeating subunit, adopt a well defined fixed conformation with the linkages to the other two residues forming a flexible hinge. Tests of this idea will involve computer molecular modeling and NMR measurements involving isotope enrichment in 13C and 15N to facilitate approach to structural and dynamics issues. Long range carbon coupling constants, 3JHC and 3JCC, will be measured and correlated with glycosidic dihedral angles. Three-dimensional heteronuclear NMR spectroscopy will provide enhanced spectral resolution in NOESY experiments. Polysaccharide dynamics will be investigated by indirectly measured heteronuclear 13C and 15N relaxation rates, which will be interpreted using spectral density mapping methods as well as several forms of the model-free treatment augmented to include anisotropic tumbling and slower exchange processes. Oligosaccharide fragments will test the hypothesis of a conformational epitope that depends on molecular size. The oligosaccharides are to be used in development of chemically modified protein conjugates for vaccine development.
