Antigenic and structural relationships within group 19 Streptococcuspneumoniae: chemical characterization of the specific capsular polysaccharides of types 19B and 19C

The specific capsular polysaccharides of Streptococcuspneumoniae types 19B and 19C (American types 58 and 59) were investigated by a combination of 1 H, 13 C, and 31 P nuclear magnetic resonance spectroscopy and analytical methods based on mass spectrometry. The two polysaccharides were found to be high molecular weight polymers composed of L-rhamnose, 2-amino-2-deoxy-D-mannose, D-ribose, D-glucose, and phosphate. Homo- and heteronuclear chemical shift correlation techniques and nuclear Overhauser enhancement experiments led to the unambiguous assignment of the 1 H and 13 C resonances from the glycose residues and established their sequence within repeating oligosaccharide units. The oligosaccharide units are polymerized through phosphate diester linkages.[Formula: see text]Both polysaccharides share a common hexasaccharide structural unit and they differ only in the degree of substitution at the branched 2-acetamido-2-deoxy-β-D-mannopyranosyl residue: the 19C polysaccharide is O-6 linked by β-D-glucopyranosyl end groups to form a heptasaccharide repeating unit, while the 19B polysaccharide is unsubstituted at that position. The serologic cross-reactivity between S. pneumoniae serotypes 19B and 19C can now be related to the structural similarity of the antigenic capsular polysaccharides.