Properties of xanthan gum in aqueous solutions: Role of the conformational transition

Abstract

Xanthan is shown to exist in native (I) and denatured (II) forms, both of which are ordered by the criterion of optical activity, and both of which undergo a temperature-driven, conformational transition to a common, disordered form (III). Changes in optical activity and conductivity are identical for the transformations I → III and II → III. The denatured, ordered form II displays a considerably larger viscosity increment than the native, ordered form I under given conditions of added salt at temperatures below the transition temperature (Tm). Light-scattering measurements yield the same molecular weight for forms I and II; consequently, the observed difference in viscosity increment appears to reflect inherent differences in the chain extension and conformation of forms I and II. Measured persistence lengths for forms I and II are consistent with earlier reports of the persistence length of single-stranded, ordered xanthan in solution. In the present viscosity studies, the absence of any dependence of measured properties of the native form I and the denatured form II on the thermal and concentration history strongly suggests that forms I and II are not aggregated species. Similar comparisons of the chain extension of disordered form III with the ordered forms I and II are more difficult, owing to the absence of conditions under which ordered and disordered forms are simultaneously stable. It is possible, nevertheless, to conclude that the viscosity increment of form II exceeds that of form III, whereas those of forms I and III are similar. Measurements of the molecular weight of form III, needed to convert this observation reliably into information about chain extension, are lacking. These observations are interpreted in terms of a model for xanthan in which the native, single-stranded, ordered form I is stabilized by side chain-backbone interactions that are shown, by experiments reported here, to be strong. These strong interactions are disrupted upon temperature-induced conversion into the disordered form III above the Tm. On subsequent cooling, there is established an alternative pattern of side chain-backbone interactions, presumably dictated by kinetic rather than thermodynamic factors, which stabilizes a more-extended backbone conformation in the denatured, ordered form II. The assumption is made that these strong, side chain-backbone interactions can bridge some backbone breaks which can appear during partial degradation of forms I and II.