Structural roles of calcium ions and side chains in welan: an X-ray study.

Welan is the first branched polymer in the gellan family of polysaccharides whose three-dimensional structure has been determined by X-ray diffraction analysis of polycrystalline and well oriented fibers of the calcium salt. The molecule exists as a half-staggered, parallel, double-helix, similar to that of gellan. The side chains fold back on the main chain to form hydrogen bonds with the carboxylate groups. This shielding enhances the stability of the double-helix. Three molecules are organized in a trigonal unit cell of dimensions a = 20.83 and c = 28.69 A with a lateral separation of 12.0 A in each pair; this is 2.9 A larger than in gellan. The double helices are in contact with each other through calcium ions and water molecules via COO-...Ca2+...COO- and COO-...W...Ca2+...COO- interactions, and through side chain-side chain hydrogen bonds. These structural features enable us not only to explain how the side chains in welan are responsible for the enhanced molecular stability relative to gellan, but also to show how essential they are for the associative properties which control the rheology of the polymer.