Conformational transitions of schizophyllan in aqueous alkaline solution

Abstract

The conformational transitions of schizophyllan were studied in aqueous alkaline solutions by high-sensitivity differential scanning calorimetry (DSC) and optical rotation measurements. The temperature of half completion for reversible intramolecular conformational transition determined by DSC, centered at 7.4°C in water, increases to 37.2°C at 0.01M KOH with increasing alkaline concentration. The transition enthalpy per mole of the polysaccharide repeating unit is 2.62 ± 0.23 kJ mol−1 independent of the alkaline concentration. The cooperative unit size for the transition decreases with increasing alkaline concentration. Optical rotation was measured as a function of pH at 25 and 60°C. A sharp decrease in optical rotation was observed at pH = 13, which is ascribed to the triple helix-coil transition. From data obtained by DSC and optical rotation measurements, in combination with results reported previously, a phase diagram for the conformation of schizophyllan as a function of temperature and pH is proposed. The irreversibility of the triple helix to single coil transition, induced by strong alkali, was investigated as a function of polymer concentration by gel permeation chromatography and electron microscopy. The renatured samples at polymer concentrations < 1.0 mg/mL, which are prepared by dissolution in 0.25M KOH followed by neutralization with HCl, are observed as a mixture of globular, linear, and circular structures, and larger aggregates with less-defined morphology by electron microscopy. Higher concentrations lead to increased proportions of multichain clusters (aggregates). Subsequent annealing of the renatured samples at 115–120°C increases the proportion of circular species. The change in molecular weight distribution of samples that accompanies the renaturation and annealing mentioned above can be well interpreted in terms of the proportion of species having different morphology as observed by electron microscopy. © 1996 John Wiley & Sons, Inc.