Phase Transition of Lipid Multilamellar Aqueous Suspension under High Pressure V. Ultrasonic Measurements in Temperature Range from 4th Transition to Main One under High Pressure

The ultrasonic velocity and absorption at single 5 MHz of the DPPC multilamellar containing 25 wt.% water in the temperature range from the 4th transition to the main one have been investigated under several pressures up to about 300 MPa. Four phase transitions were measured as follows: (1) A fluctuation of the DPPC bimembrane density in the vicinity of the main transition point was strongly restricted due to the effect of pressure, and an extremely unsymmetrical form of the absorption peak around the main transition with an increase in pressure beyond 150 MPa was observed, (2) the notable appearance of the absorption peak in the pretransition under 250 MPa was observed although no absorption peak could be detected under pressure of less than 250 MPa, and (3) the changes of the ultrasonic velocity and bulk modulus corresponding to the sub- (or melting-like behavior) and 4th transition were observed distinctly under 295 MPa.