HYDROLYSIS OF XANTHAN IN DILUTE ACID - EFFECTS ON CHEMICAL-COMPOSITION, CONFORMATION, AND INTRINSIC-VISCOSITY

The polysaccharide xanthan has been depolymerized by mild acid hydrolysis (pH 1-4) at 80-degrees. The conformational state was varied from fully ordered to partially disordered by varying the ionic strength and pH. Hydrolysis occurred mainly in the side chains, with the terminal beta-mannose as the most susceptible unit, yielding a continuous series of modified xanthans from the intact ''polypentamer'' to the ''polytetramer'', while retaining a high molecular weight. Depolymerization of the glucan backbone was analysed by monitoring the intrinsic viscosity ([eta]). For ordered xanthan the calculated changes in the degree of polymerization (X(w)) as a function of time deviate strongly from that expected for random depolymerization of a single-stranded, linear polymer, but the data are in qualitative agreement with the behaviour of such double-stranded polymers as DNA. The conformational properties of partly hydrolysed xanthan were investigated by optical rotation. For the series degraded at pH 2, the midpoint of the temperature-driven transition (T(m)) was > 95-degrees in all cases, but the absolute value of the specific optical rotation ([alpha]) increased by increasing hydrolysis time, a change partly ascribed to an increase in the fraction of disordered chains, but also to the loss of beta-mannose. This loss itself did not appear to influence the conformational state or the depolymerization rates of the xanthan backbone.