title: Native ordered structure of welan polysaccharide: conformational transitions and gel formation in aqueous dimethyl sulphoxide.
abstract: Welan, in aqueous solution, has "weak gel" properties analogous to those of ordered xanthan but, unlike xanthan, shows no evidence of conformational change between 0 and 100 degrees C. When the polymer is dissolved in dimethyl sulphoxide (Me2SO) rather than in water, however, there is a massive decrease in viscosity and total loss of gel-like character. In mixtures of the two solvents, the change in rheology occurs over a narrow range of composition (approximately 85-90% v/v Me2SO for 0.5% welan). On heating and cooling in a solvent close to the lower end of the critical range (86% Me2SO), the polymer shows typical order-disorder and disorder-order transitions [as monitored by optical rotation, differential scanning calorimetry, and temperature-course of rheological change]. When solutions of disordered welan in Me2SO are poured into excess water they form cohesive strings of gel. We interpret these results as showing that: (1) the stable conformation of welan in water is the double helix structure identified by X-ray fibre diffraction in the solid state; (2) in native welan, as biosynthesised, the strands are perfectly paired, and ordered along their full length; (3) on exposure to high concentrations of Me2SO, 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.
authors.affiliation: Department of Food Research & Technology, Cranfield University, Silsoe College, United Kingdom.
authors.forename: M W
authors.initials: MW
authors.lastname: Hember
authors.forename: R K
authors.initials: RK
authors.lastname: Richardson
authors.forename: E R
authors.initials: ER
authors.lastname: Morris
mesh_terms: D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D005782:Gels; D006358:Hot Temperature; D008956:Models, Chemical; D008969:Molecular Sequence Data; D009904:Optical Rotation; D011135:Polysaccharides, Bacterial / Q000737:chemistry*; D012212:Rheology; D014783:Viscosity; D014867:Water / Q000737:chemistry
chemical_list: D005782:Gels; D011135:Polysaccharides, Bacterial; D014867:Water; C068570:welan; D004121:Dimethyl Sulfoxide