Determination of the configuration of ribitol in the C-polysaccharide of <i>Streptococcus pneumoniae</i> using a synthetic approach

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The C-polysaccharide is an antigen common to all known serotypes of Streptococcus pneumoniae bacteria and is a potential target for vaccine preparation. The final uncertainty in the structure of its repeating unit, a pentasaccharide phosphate containing two phosphorylcholine side chains, has been resolved by determining the configuration of ribitol. Assignment of configuration was performed by synthesis of the two trisaccharide phosphate fragments that have either D- or L-ribitol at their centers and comparison of their 1H and 13C NMR spectral data with that of the natural polysaccharide. The syntheses employed common synthons added in different orders to an asymmetrically substituted chiral ribitol derivative to obtain opposite chiralities in the ribitol segments. The data for the trisaccharide containing D-ribitol was almost identical to that of the natural material while that for the trisaccharide containing L-ribitol differed significantly. In particular, the chemical shift differences between the two protons of the primary carbons of ribitol units directly attached to the β-2-acetamido-2-deoxy-galactopyranosyl residue were 0.10, 0.12, and 0.33 ppm, in the natural polysaccharide, the D-ribitol-containing trisaccharide, and the L-ribitol-containing trisaccharide, respectively. The average difference between the 13C NMR chemical shifts of corresponding ribitol carbons from the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate was 0.034 ppm. This evidence indicates that ribitol in the C-polysaccharide has the D-configuration and that a very similar mixture of conformations in the ribitol portions is present for the natural polysaccharide and the D-ribitol-containing trisaccharide phosphate.Key words: C-polysaccharide, Streptococcus pneumoniae, ribitol, dibutylstannylene acetal, determination of configuration.