The changeable solution structures of the 197 kDa de-O-acetyl capsular polysialic acid of Neisseria C

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have been measured with small angle neutron scattering (SANS) where different solution conditions cause large structural changes in the polysaccharide. The nominal molecular mass of material used is 197 kDa. A number of possible structures can fit the data under the varied conditions, but straightforward chemical arguments lead to the following results. In the absence of calcium, each molecule forms a bundle of parallel polymer chain segments in a tight structure. However, in the presence of calcium the best chemical inference indicates structures of significantly swollen, solitary Gaussian chains, and, in one case, agglomeration of the swollen chains into larger structures. This open, swollen-chain structure of Men C differs from that of α2 → 8 polysialic acid of Men B, which has tight structures in the presence of calcium. This difference in structure plausibly contributes to the recognized greater immunogenicity of non-conjugated Men C compared to non-conjugated Men B.