The Influence of Sidechains on the Calculated Dimensions of Three Related Bacterial Polysaccharides

The effect of van der Waals interactions between sidechain and backbone on the shape of three bacterial polysaccharides in solution has been investigated. The three polymers, namely, gellan, welan, and rhamsan, share the same four-sugar backbone repeating-unit. Gellan is unbranched, whereas welan and rhamsan display comblike branching. Consequently, the effect of chain branching on backbone conformation may be investigated. Van der Waals repulsive interactions of sidechain and backbone serve to limit, somewhat, the range of conformational freedom of the welan backbone in comparison to that of gellan. Attractive side-chain-backbone interactions, which may be as significant as 2-3 kcal/mol, predominate over much of the accessible conformational space of the welan backbone. Despite the strength of these interactions, the unperturbed shape of welan in solution is calculated to be very similar to that of the unbranched gellan. Attractive sidechain-backbone interactions in rhamsan have a modest influence on the conformational characteristics of the rhamsan backbone. The calculated, unperturbed conformation in solution is slightly more extended than that of gellan and welan, but the fundamental shape of the chain is changed only slightly. Significant differences in the physical properties of these polymers seem not to arise from differences in their random-coil conformations provoked by van der Waals interactions of sidechain and backbone. Other contributions to the sidechain-backbone interaction, e.g., hydrogen bonding, could be involved; interchain interactions are also likely to be important.