Structural elucidation of the capsular polysaccharide from Streptococcus pneumoniae type 18B

The structure of the capsular polysaccharide from Streptococcus pneumoniae type 18B has been determined using NMR spectroscopy and methylation analysis as the principal methods. It is concluded that the polysaccharide is composed of pentasaccharide repeating units with a glycerol phosphate substituting the 3-position of the branch point residue. The carbohydrate backbone in type 18B is identical to that in S. pneumoniae type 18F but without the O-acetyl groups present in that type. In this structure, the absolute configuration of the glycerol phosphate moiety has not been determined but should be d, in analogy with that determined for the capsular polysaccharide from S. pneumoniae type 18A [T. Rundlöf, G. Widmalm, Anal. Biochem., 243 (1996) 228–233].