Thermal denaturation and renaturation of a double-helical polysaccharide xanthan in acidic and basic solutions

The double helix of a polysaccharide, xanthan was unwound by heating (denaturation) and rewound by cooling (renaturation) in acidic (0.01 M HCl) and basic (0.01 M NaOH) solutions. Circular dichroism spectra of xanthan renatured in the acidic and basic solutions were similar to those of native xanthan, which suggests that the local helical structures of the renatured xanthan were recovered by renaturation. The changes in the molar mass and intrinsic viscosity of xanthan induced by renaturation in the acidic solution can be explained by the renaturation scheme in the neutral solution, as reported previously. (Matsuda et al. Polym. J. 41, 526-532, 2009). Both the molar mass and radius of gyration of xanthan in the basic solution were decreased by denaturation and renaturation, and the molar mass dependence of the radius of gyration was similar to that of native xanthan. This change can be explained by the formation of a hairpin structure during renaturation in the basic solutions. Structural changes induced by thermal denaturation and renaturation of a double-helical polysaccharide xanthan in acidic and basic solutions were investigated mainly by light scattering measurements and circular dichroism spectroscopy. In acidic solution, the renatured components were almost the same as those in native components, while small amounts of aggregates and a hairpin structure were produced via denaturation and renaturation processes. In basic solution, the double helices were dissociated upon heating into the single coils, and high molar mass sample produced a hairpin structure after subsequent renaturation.