Ultrasonic studies of lipid bilayer. Phase transition in synthetic phosphatidylcholine liposomes.

The ultrasonic velocity at 3 MHz and the density in the nonsonicated and sonicated liposomes of dipalmitoylphosphatidylcholine have been measured in the temperature range from 0 degrees C to 55 degrees C. The results indicate that nonsonicated multilamellar vesicles undergo a weak first order transition which is analogous to the nematic-isotropic transition of liquid crystals. A sharp change in the ultrasonic velocity associated with the first order transition disappears when the multilamellar vesicles are sonicated. The bulk modulus of the lipid bilayer calculated from the ultrasonic velocity and the density of sonicated liposomes has a value of 3.0 X 10(10) dyne/cm2 at 20 degrees C, reaches a minimum value of 2.1 X 10(10) dyne/cm2 at its transition temperature and increases slightly to 2.2 X 10(10) dyne/cm2 at 50 degrees C.