Structural determination of the capsular polysaccharide of Neisseria meningitidis group I: a two-dimensional NMR analysis.

The capsular polysaccharide antigen of Neisseria meningitidis group I was isolated by Cetavlon precipitation and purified by ion-exchange chromatography. The structure of the I polysaccharide was determined largely by comprehensive proton and carbon-13 nuclear magnetic resonance studies in which both one-dimensional and two-dimensional experiments were carried out directly on the I polysaccharide. The I polysaccharide is composed of the repeating unit----4)alpha-L-GulpNAcA(1----3)[4-OAc]beta-D-ManpNA-cA(-- --in which the former residue adopts the 4C1 (L) conformation and the latter residue adopts the 4C1 (D) conformation. The one-bond coupling between the anomeric carbon and proton (1J13C,H) of the 2-acetamido-2-deoxy-beta-D-mannuronopyranosyl residue is not consistent with its beta-D configuration. This anomalous value of 1J13C,H for this residue is due to through-space anisotropy effects on its anomeric proton, generated by the proximity of the carboxyl group of the neighboring 2-acetamido-2-deoxy-alpha-L-guluronopyranosyl residue. The O-acetyl substituents of the I polysaccharide are essential for its antigenicity to group I polysaccharide-specific antibodies.