Solvent stabilizing effects on the order–disorder transition of schizophyllan in aqueous mixtures of carboxylic acids

Schizophyllan is a triple helical β-1,3-_D-glucan, and shows the cooperative order–disorder transition in the aqueous solution at the triple helix state. In this paper, the solvent stabilizing effects of two carboxylic acids, acetic acid and citric acid, on the cooperative order–disorder transition of aqueous schizophyllan solution were investigated from DSC and SEC-MALS measurements. The transition temperature (T_r) was shifted to higher temperature with increasing the molar fraction of carboxylic acid in the mixture (x). The transition enthalpy (ΔH_r) was increased with increasing x. These solvent stabilizing effects indicate that these carboxylic acid molecules were selectively associated with the branched side chains of schizophyllan to stabilize the ordered state. The composition dependencies of T_r and ΔH_r were analyzed by the linear cooperative transition theory to estimate the association parameters between the side chains and carboxylic acid. The theoretical parameters obtained were compared with those for the other active substances for the transition to discuss the molecular interactions between the triple helix and carboxylic acid.