Influence of acetyl and pyruvate substituents on the solution properties of xanthan polysaccharide

Abstract

Xanthan, an exocellular polysaccharide produced by the plant pathogenic bacterium Xanthomonas campestris has been the subject of considerable interest in recent years because of its unusual rheological properties in solution (‘weak gel’) and consequent range of applications. The polymer consists of a cellulosic backbone with trisaccharide side chains linked to alternate backbone residues; acetyl and pyruvate substituents are carried in variable amounts on these side chains. In this study a series of xanthans differing in the percentage of substituent groups and in molecular weigth range have been prepared by culturing a variety of different strains of X. campestris. All of the xanthans have been characteized by a range of physicochemical techniques. In particular, the intrinsic viscosities at low shear rates, and at a range of ionic strengths, have been determined and the geometric persistence lengths evaluated by the Smidsrød—Haug method. Intensity light scattering measurements have been made using the procedure of Coviello and co-workers to promote molecular dispersion. Despite significant differences in the acetyl and pyruvate contents, the molecular weight vs mean square radius behaviour of our samples did not differ substantially from each other or from those reported for other xanthan samples in the literature. The persistence length, determined by the method of Schmidt et al. (120 ± 8 nm) was also, within experimental error, the same for all the samples measured. These values differed considerably from those calculated from the ionic strength dependence of intrinsic viscosity (the Smidsrød—Haug method) as reported by Tinland and Rinaudo and calculated for our samples. We feel this illustrates the limitations of the latter method when applied to systems where the electrostatic contribution to the persistence length is only a small fraction of the geometrical contribution. The values obtained from the light scattering measurement support other recent conclusions that the inherent stiffness of the xanthan macromolecule is not greatly influenced by the pattern of acyl substitution.