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

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Abstract
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. 13C NMR in conjunction with other chemical data indicated that the polysaccharide is composed of an alternating sequence of these 2 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 the signals in the 13C NMR spectrum of the 29-e polysaccharide were made by consideration of those in the spectra of the monomer models, which necessitated the first recorded synthesis 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 H 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.