Effect of deuterium oxide on the phase transitions of phospholipid bilayer membranes under high pressure

Abstract

The present study demonstrates the substitution effect of hydrogen oxide (H 2 O) by deuterium oxide (D 2 O) on the phase transitions of dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC) bilayer membranes under high pressure. Substitution effect of H 2 O by D 2 O 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 2 O and D 2 O. 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 substitution of H 2 O by D 2 O. In H 2 O, a pressure-induced interdigitated gel (L β I) phase has been observed at pressures above 100 MPa for DPPC and 70 MPa for DSPC, respectively. It has been known that the L β I phase of DPPC bilayer in D 2 O 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 β I phases to be lowered in D 2 O, on the other hand, the transition temperature between L β ′ and L β I phases to be raised in D 2 O under high pressure. In other words, the pressure for appearance of the L β I phase was elevated by the substitution of H 2 O by D 2 O. The difference in transition temperatures by the water substitution is discussed by the difference of hydrophobic interaction in H 2 O and in D 2 O.