Abstract
Ultrasonic measurements of membranes and membrane-associated proteins are reviewed. Ultrasonic properties reflect the modulus of membranes and reveal the magnitude and relaxation time of their structural fluctuation. The measurements of lipid bilayers indicated the critical softening as well as the critical slowing down in the vicinity of the gel-to-liquid crystal transition, leading to the conclusion that this phase transition is very near to the second order transition. The relaxation time of the critical structural fluctuation was as long as 100 nsec. The bulk modulus of membranes increased linearly with the increase of the protein content of membranes. The extrapolation of the bulk modulus to 100% of the protein content was 4X1010 dyn/cm2, which has to be the bulk modulus of membrane protein. This value is in good agreement with the bulk modulus of soluble globular proteins. The frequency dependence of the ultra sonic properties indicated that there is no structural relaxation in the ultrasonic frequency region.
