Molecular weight of xanthan polysaccharide

Abstract

The molecular weight ( M w ) and molecular-weight distribution of the extracellular polysaccharide xanthan, synthesized by the bacterium Xanthomonas campestris, have been determined from measurements of the sedimentation coefficient, s 20,itw, and the intrinsic viscosity, [η], with the aid of the Mandelkern-Flory-Scheraga equation. The sedimentation coefficient of native xanthan was measured by band-sedimentation of polysaccharide molecules that had been tagged with a fluorescent group; the fluorescent label permits the use of very low concentrations of polymer. A typical, native-xanthan sample has M w  15 x 10 6; the polydispersity index M w/M n is 2.8. Measurement of s and [η] for a homologous series of five xanthan samples having M w ranging from 0.40 to 15 X 10 6, prepared by sonication of native xanthan, shows that, for low molecular weight, the intrinsic viscosity [η] obeys the relation [η]  KM 1.35. The high value of the Staudinger exponent in this relation demonstrates that xanthan is a rod-like molecule having stiffness similar to that of native DNA, which has a Staudinger exponent of 1.32. Moreover, the absolute values of [η] suggest that xanthan has a mass per unit length of about 1900 daltons/nm, which is twice the mass per unit length of the single-stranded structure proposed from X-ray work.