Synopsis
The thermal conformational transitions of two sonicated samples of schizophyllan were studied in w a t e r -d i m e t h y l e (DMSO) mixtures by high-Sensitivity differential scanning calorimetry (DSC). Two transitions were o b r v e d over most of the range of solvent compositions. These were assigned to an internal change of the triple helix [T. Itou et al. (1986) Macromlec& 19, 1234-12401 and a triple-helix-single-coil transition [T. Sat0 et al. (1981) Carbohyd-. Re. 95, 195-2041, respectively. In water, the former transition observed at lower temperature for a low molecular weight sample, U-1, is centered at 3OC and characterized by the specific enthalpy, Ah, = 3.29 J g-l. A higher molecular weight sample, M-2, showed this transition at 7OC with Ah, = 4.39 J g-l. The transition temperature for both samples increased with increasing DMSO concentration up to about 50Β°C at 70 weight 8 DMSO, and then rapidly decreased with increasing DMSO concentration, with about 3Β°C higher for M-2 than for U-1 over the DMSO concentration. The transition was not observed when the concentration of DMSO exceeded 878.
It was found that Ah, for both samples was a linear function of + the temperature of half-completion in "C, Ah, = 0.177.t + 2.96. The triple helix-coil transtion was observed at around 127Β°C for U-1 and above 130Β°C for M-2 in the range of DMSO composition below about 708. The transition temperature decreased with increasing DMSO concentration at above 708, and the transition finally disappeared when the DMSO concentration exceeded 90%. The plot of Ah, vs. t,/* for the transition of both samples gave a linear relation, Ah,, = 0.253t -10.58. The reversibility of the transition at lower temperature was demonstrated by the reversibility of the curves when the first heating was stopped before the second transition. Once the heating was performed over the second transition, the reheating DSC curves showed several endothermic peaks, indicating the irreversibility of the transition and heterogeneity in the conformation of the heated schizophyllan.