Mwn. Hember et al., NATIVE ORDERED STRUCTURE OF WELAN POLYSACCHARIDE - CONFORMATIONAL TRANSITIONS AND GEL FORMATION IN AQUEOUS DIMETHYL-SULFOXIDE, Carbohydrate research, 252, 1994, pp. 209-221
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.