The influence of acetyl and pyruvate substituents on the helix-coil transition behaviour of xanthan

Xanthans (Na+ salt form) having various contents of acetyl and pyruvic acid groups were prepared by culturing different strains of Xanthomonas campestris and by deacetylation and depyruvation. Optical rotation ([α]365) was used to characterize the helix-coil transition behaviour of these polymers in deionized water. There were correlations between the acetyl and pyruvic acid contents and the mid-point temperature of the transition, between the pyruvic acid content and [α]365 in the high-temperature-plateau (coil) region of the curve, and between the content of pyruvic acid and the height of the transition. In deionized water, each of the polymers showed marked thermal hysteresis and a time-dependent fall in [α]365 at low temperatures. This behaviour, which was attributed to kinetic factors, was eliminated by the addition of NaCl. Salt also increased the melting temperature and reduced [α]365 in the low-temperature-plateau region of the curve in relation to the charge carried by the polymer. A high-pyruvate, low-acetyl xanthan exhibited unusual two-phase helix-coil transition behaviour in the presence of salt.