NATIVE ORDERED STRUCTURE OF WELAN POLYSACCHARIDE - CONFORMATIONAL TRANSITIONS AND GEL FORMATION IN AQUEOUS DIMETHYL-SULFOXIDE

Welan, in aqueous solution, has ''weak gel'' properties analogous to t hose of ordered xanthan but, unlike xanthan, shows no evidence of conf ormational change between 0 and 100 degrees C. When the polymer is dis served in dimethyl sulphoxide (Me(2)SO) rather than in water, however, there is a massive decrease in viscosity and total loss of gel-like c haracter. In mixtures of the two solvents, the change in rheology occu rs over a narrow range of composition (similar to 85-90% v/v Me(2)SO f or 0.5% welan). On heating and cooling in a solvent close to the lower end of the critical range (86% Me(2)SO) the polymer shows typical ord er-disorder and disorder-order transitions [as monitored by optical ro tation, differential scanning calorimetry, and temperature-course of t heological change]. When solutions of disordered welan in Me(2)SO are poured into excess water they form cohesive strings of gel. We interpr et these results as showing that: (1) the stable conformation of welan in water is the double helix structure identified by X-ray fibre diff raction in the solid state; (2) in native welan, as biosynthesised, th e strands are perfectly paired, and ordered along their full length; ( 3) on exposure to high concentrations of Me(2)SO, the native structure is dissociated into disordered coils; (4) rapid renaturation from the disordered state gives shorter helices, with exchange of partners to form a stable cross-linked network.