Kelco microbial polysaccharides S-130 (welan) and S-657 Display similar dilute aqueous solution behavior

Abstract

The comb-like, branched microbial polysaccharides S-130 (welan gum) from Alcaligenes ATCC 31555 and S-657 from Xanthomonas ATCC 53159, produced by the Kelco Division of Merck, have potential commercial applications as non-gelling water-soluble thickening and suspending agents. Each is structurally related to the Kelco linear copolysaccharide S-60 (gellan gum) from Auromonas ATCC 31461 which forms gels in aqueous salt solutions and has the repeating unit structure [→ 3)-β- d-Glc p-(1 → 4)-β- d-Glc pA-(1 → 4)-β- d-Glc p- (1 → 4)-α- l-Rha p (1 → ] . In S-130 this backbone is regularly glycosylated at C(3) of the second β- d-Glc p with α- l-Rha p (67%) or α- l-Man p (33%), while in S-657 regular glycosylation occurs at the same backbone site with α- l-Rha p-(1 → 4)-α- l-Rha p . Both S-130 and S-657 were purified, degraded to various extents by sonication and, for most samples, fractionally precipitated into moderately dispersed fractions differing in mean molecular weight. Potentiometric proton titration and investigations of the dependence of intrinsic viscosity on ionic strength suggest only very weak polyelectrolyte behavior. For neither polymer does the temperature or ionic strength dependence of optical activity or reduced specific viscosity present evidence of a conformational change in the temperature range 20≤ T≤62° C. Light scattering as a function of ionic strength discloses a modest tendency of both polymers to aggregate with increasing aqueous NaCl concentration in the range 0·001–0·100 m; intrinsic viscosities were large relative to the molecular weights and essentially constant over the same range of salt concentration. The mean chain extension per back-bone sugar residue measured by light scattering, as well as the molecular weight dependence of the intrinsic viscosity, suggest a highly extended chain configuration, similar for the two polymers and comparable to that of the most highly extended cellulose derivatives. This is found despite the periodic occurrence in S-130 and S-657 of the α-(1 → 3)-linkage, which is expected on elementary grounds to disrupt the essentially rectilinear propagation of the otherwise cellulose-like backbone and generate a less extended chain configuration.