Cation interactions in gellan: an X-ray study of the potassium salt

Gellan belongs to a new generation of nonsulfated, microbial, texturing polysaccharides of potential interest to the food industry. The influence of monovalent cations on its molecular geometry has been investigated by X-ray diffraction analysis of oriented fibers of the potassium salt. The molecule forms a parallel, half-staggered, double helix in which each polysaccharide chain is a left-handed, 3-fold helix of pitch 5.63 nm. The potassium ion is coordinated to the carboxylate group, which is in turn involved in interchain hydrogen-bonds to stabilize the duplex. There are two such duplexes, packed antiparallel to each other, cross-linked by a network of duplex-water-duplex interactions, in the trigonal unit cell, a = b = 1.575 nm, and c = 2.815 nm. The present study not only confirms the correctness of the basic structure of gellan reported previously for the lithium salt but also furnishes a clear insight into the critical interactions taking place between the polymer chains, cations, and water molecules which are of importance for industrial utilization.