Dielectric study of the cooperative order-disorder transition in aqueous solutions of schizophyllan, a triple-helical polysaccharide.

Schizophyllan exists in aqueous solution as a triple helix, which is intact at room temperature. Its aqueous solution forms some ordered structure at low temperatures but undergoes a sharp transition to a disordered structure as the temperature is raised. The transition temperature Tc is about 7 and 18 degrees C for H2O and D2O solutions, respectively. This transition was followed by time-domain reflectometry to investigate dynamic aspects of the transition. In addition to a major peak around 10 GHz, the dielectric dispersion curve of a 20 wt % schizophyllan in D2O exhibited a small peak around 100 MHz below Tc and around 10 MHz above Tc. The major peak is due to bulk water, whereas the 100 MHz peak is assigned to "bound" or "structured" water, and that around 10 MHz to side-chain glucose residues. However, unlike usual bound water reported for biopolymer solutions, this "structured" water disappears abruptly when the temperature becomes close to Tc without accompanying a conformational transition of the main chain. The above assignment is consistent with the structure of the ordered phase derived from previous static data that it consists of side-chain glucose residues along with nearby water molecules surrounding the helix core that are interacting with each other loosely through hydrogen bonds, and spreads radially only a layer of one or two water molecules but a long distance along the helix axis.