Molecular recognition of antigenic polysaccharides: a conformational comparison of capsules from Streptococcus pneumonicæ serogroup 9

Aqueous solution conformations of three antigenic bacterial capsular polysaccharides (CPS) from S. pneumoniæ serogroup 9 were determined using a combination of NMR data (NOE build-up rates and conformation-dependent chemical shifts), simulated annealing, and molecular dynamics simulations. Each polymer adopts a flexible extended ribbon conformation in solution. Conformations of structural elements shared by each PS are indistinguishable. Differences in conformations are minor and localised at the sites of structural variations; there is no evidence of long-range stabilisation of a secondary structure. It is likely that antigenic specificity of Group 9 PS is dominated by local structural variation rather than by conformational differences.