Order‐Disorder Transition of Triple Helical β‐1,3‐_d‐Glucans in Aqueous Mixtures of Dimethyl Sulfoxide and Imidazole: Schizophyllan and its Chemically Modified Derivatives

Abstract

Schizophyllan is a high molecular weight 𝜷-1,3-D -glucan having three 𝜷-1,3-D -glucoses and one branched 𝜷-1,6-D -glucose in the repeating unit. It dissolves as a triple helix in aqueous solutions but exhibits two separate order-disorder transitions: a low temperature cooperative order-disorder transition (CODT) related mainly to the side chains, and a high temperature transition involving triplex dissociation. The branched side chain can be oxidized selectively by periodate oxidation to form a dialdehyde. Subsequent reactions may produce different kinds of derivatives: a dicarboxylated derivative (sclerox), or a Smith-degraded (reduced dialdehyde) derivative. These derivatives still dissolve in water as trimers but the CODT may be strongly modified, depending on degree of chemical modification. The CODT behavior is analyzed with linear cooperative transition (LCT) theory. These theoretical analyses indicate that only unmodified units of sclerox and Smith degraded derivatives contribute to the transition enthalpy. With increasing degree of chemical modification, the transition curve broadens and the transition temperature shifts to lower temperatures. This happens because contiguous sequences of ordered structures gradually shorten with increasing degree of chemical modification. Dimethyl sulfoxide and imidazole are further employed to investigate solvent effects of CODT for both schizophyllan and the chemically modified derivatives. These compounds are shown to favorably associate with the unmodified units to stabilize the ordered structures.