The crystal structure of gellan

Abstract

Gellan is a nonsulfated, anionic, extracellular polytetrasaccharide secreted by the bacterium Auromonas elodea. It is potentially useful in the food industry because of its gel-forming properties. The molecular basis of these properties had been investigated by X-ray diffraction analysis of oriented fibers, but an exhaustive study by Upstill et al. in 1986 produced no molecular model with a remotely acceptable fit to the observed X-ray intensities. We describe here a successful re-examination of the crystal structure of gellan; the gellan chains have backbone conformations different from those previously considered. Two left-handed, 3-fold helical chains are organized in parallel fashion in an intertwined duplex in which each chain is translated half a pitch ( p = 5.64 nm) with respect to the other. The duplex is stabilized by interchain hydrogen bonds at each carboxylate group. There are two molecules in each trigonal unit cell ( a = 1.56 nm and c = 2.82 nm).