CONFORMATIONAL TRANSITIONS OF SCHIZOPHYLLAN IN AQUEOUS ALKALINE-SOLUTION

The conformational transitions of schizophyllan were studied in aqueou s alkaline solutions by high-sensitivity differential scanning calorim etry (DSC) and optical rotation measurements. The temperature of half completion for reversible intramolecular conformational transition det ermined by DSC, centered at 7.4 degrees C in water, increases to 37.2 degrees C at 0.01M KOH with increasing alkaline concentration. The tra nsition enthalpy per mole of the polysaccharide repeating unit is 2.62 +/- 0.23 kJ mol(-1) independent of the alkaline concentration. The co operative unit size for the transition decreases with increasing alkal ine concentration. Optical rotation was measured as a function of pH a t 25 and 60 degrees C. A sharp decrease in optical rotation was observ ed at pH = 13, which is ascribed to the triple helix-coil transition. From data obtained by DSC and optical rotation measurements, in combin ation with results reported previously, a phase diagram for the confor mation of schizophyllan as a function of temperature and pH is propose d. The irreversibility of the triple helix to single coil transition, induced by strong alkali, was investigated as a function of polymer co ncentration 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 struc tures, and larger aggregates with less-defined morphology by electron microscopy. Higher concentrations lean to increased proportions of mul tichain clusters (aggregates). Subsequent annealing of the renatured s amples at 115-120 degrees C ina eases the proportion of circular speci es. The change in molecular weight distribution of samples that accomp anies the renaturation and annealing mentioned above can he well inter preted in terms of the proportion of species having different morpholo gy as observed by electron microscopy. (C) 1996 John Wiley & Sons, Inc .