The present study demonstrates the substitution effect of hydrogen oxide (H_2O) by deuterium oxide (D_2O) on the phase transitions of dipalmitoylphosphatidyl-choline (DPPC) and distearoylphosphatidylcholine (DSPC) bilayer membranes under high pressure. Substitution effect of H_2O by D_20 was as follows. With respect to the main transition from the ripple gel (P_β′) phase to the liquid crystalline (L_α) phase for DPPC and DSPC bilayer membranes, there was no significant difference in the main transition temperature in H_2O and D_2O. On the other hand, the pretransition temperature between lamellar gel (L_β′) phase and P_β′ phase for both lipids was significantly raised by the substitutic n of H_2O by D_2O. In H_2O, a pressure-induced interdigitated gel (L_βI) phase has been observed at pressures above 100 MPa for DPPC and 70 MPa for DSPC, respectively. It has been known that the L_βI phase of DPPC bilayer in D2O is observed at higher pressure beyond 150 MPa by the method of neutron diffraction. In the present study, the temperature-pressure phase diagrams for DPPC and DSPC bilayers showed the transition temperature between P_β′ and L_βI phases to be lowered in D_2O, on the other hand, the transition temperature between L_β′ and L_βI phases to be raised in D_2O under high pressure. In other words, the pressure for appearance of the L_βI phase was elevated by the substitution of H_2O by D_2O. The difference in transition temperatures by the water substitution is discussed by the difference of hydrophobic interaction in H_2O and in D_2O.
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