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 polysaccha rides whose three-dimensional structure has been determined by X-ray d iffraction analysis of polycrystalline and well oriented fibers of the calcium salt. The molecule exists as a half-staggered, parallel, doub le-helix, similar to that of gellan. The side chains fold back on the main chain to form hydrogen bonds with the carboxylate groups. This sh ielding enhances the stability of the double-helix. Three molecules ar e organized in a trigonal unit cell of dimensions a = 20.83 and c = 28 .69 Angstrom with a lateral separation of 12.0 Angstrom in each pair; this is 2.9 Angstrom larger than in gellan. The double helices are in contact with each other through calcium ions and water molecules via C OO- ...Ca2+ ...COO- and COO- ...W ...Ca2+ COO- interactions, and throu gh side chain-side chain hydrogen bonds. These structural features ena ble us not only to explain how the side chains in welan are responsibl e for the enhanced molecular stability relative to gellan, but also to show how essential they are for the associative properties which cont rol the theology of the polymer.