Structural elucidation of the 3-deoxy-D-manno-octulosonic acid containing meningococcal 29-e capsular polysaccharide antigen using carbon-13 nuclear magnetic resonance

The capsular polysaccharide antigen from Neisseria meningitidis serogroup 29-e contains equimolar quantities of 2-acetamido-2-deoxy-D-galactose and 3-deoxy-D-manno-octulosonic acid (KDO), the latter of which is rarely found in biopolymers other than lipopolysaccharides. Carbon-13 nuclear mangetic resonance in conjunction with other chemical data indicated that the polysaccharide is composed of an alternating sequence of these two residues, the linkages being at C-3 of galactosamine and C-7 of KDO in the alpha-D and beta-D configuration, respectively. The native 29-e polysaccharide is O-acetylated, the O-acetyl groups being located at C-4 and C-5 of the KDO residues. Assignments of signals in the 13C nuclear magnetic resonance spectrum of the 29-e polysaccharide were made by consideration of those in the spectra of the monomer models, which necessitated the first recorded syntheses of methyl-alpha- and beta-D-3-deoxy-manno-octulopyranosonic acid. Like the methyl alpha- and beta-D-ketosides of sialic acid (Na+ salts), the equivalent methyl alpha- and beta-D-ketosides of KDO exhibit large chemical shift differences in the exocyclic C-8 position dependent on anomeric configuration. This can again be attributed to hydrogen bonding between the axial carboxylate group of the methyl beta-D anomer of KDO (C1 conformation) and the primary hydroxyl group at C-8. This phenomenon is also exhibited by the beta-D-linked KDO units of the 29-e polysaccharide.