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 i ntact at room temperature. Its aqueous solution forms some ordered str ucture at low temperatures but undergoes a sharp transition to a disor dered structure as the temperature is raised. The transition temperatu re T-c is about 7 and 18 degrees C for H2O and D2O solutions, respecti vely. This transition was followed by time-domain reflectometry to inv estigate dynamic aspects of the transition. In addition to a major pea k around 10 GHz, the dielectric dispersion curve of a 20 wt % schizoph yllan in D2O exhibited a small peak around 100 MHz below T-c and aroun d 10 MHz above T-c. The major peak is due to bulk water; whereas the 1 00 MHz peak is assigned to ''bound'' or ''structured'' water, and that around 10 MHz to side-chain glucose residues. However, unlike usual b ound water reported for biopolymer solutions, this ''structured'' wate r disappears abruptly when the temperature becomes close to T-c withou t accompanying a conformational transition of the main chain. The abov e assignment is consistent with the structure of the ordered phase der ived 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, a nd spreads radially only a layer of one or two water molecules but a l ong distance along the helix axis. (C) John Wiley & Sons, Inc.