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 T_c is about 7 and 18°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 T_c and around 10 MHz above T_c. The major peak is due to bulk water, whereas the 100 MH_z 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 T_c 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. © 1995 John Wiley & Sons, Inc.