XM-6: A new gel-forming bacterial polysaccharide

A new extracellular microbial polysaccharide, XM-6, has been isolated from cultures of an Enterobacter species and shows unusual gelation properties of potential technological significance. The polysaccharide contains d-glucose, l-fucose and d-glucuronate in the approximate molar ratio 3:1:1. No significant amounts of acetate or pyruvate were detected. d-Glucuronate and some d-glucose are destroyed on periodate oxidation, but l-fucose and some d-glucose may be recovered intact, indicating the presence of some 1,3 linkages in the primary structure. The major oligosaccharide isolated from autohydrolysates was an aldobiuronic acid containing equal amounts of d-glucose and l-fucose.Thermally-reversible gels are formed on addition of salt to solutions of the polysaccharide. A preliminary investigation of the mechanism of gelation by optical rotation, circular dichroism, high resolution n.m.r. and mechanical spectroscopy suggests interchain association through conformationally ordered ‘junction zones’, with specific incorporation of site-bound cations within the ordered structures. In the sol state the polysaccharide shows the shear-rate and temperature dependence of viscosity typical of a disordered (‘random coil’) polymer solution. Divalent cations are, in general, more effective than monovalent cations in promoting gelation of XM-6, while trivalent cations normally cause precipitation. Within Groups I and II, optimum gelation is achieved with Na⁺ and Ca²⁺ (ionic radius ? 0·1 nm), with larger and smaller ions becoming progressively less effective. Both gel strength and melting temperature increase with increasing salt concentration.XM-6 forms gels of reasonable strength at unusually low concentrations of the polysaccharide. For example, gels comparable to those required for normal industrial or food applications may be obtained using 0·3% w/v XM-6 and 1% w/v NaCl. Gel strength increases with increasing polymer concentration but there is no systematic variation in melting point. The sol-gel transition of XM-6 is unusually sharp and, by suitable adjustment of salt concentration, can be made to occur just below body temperature (e.g. 30–35°C), with obvious implications for biomedical or food applications.