Ultrasonic relaxation of multilamellar liposomes formed from dipalmitoylphosphatidylcholine was measured near the gel-to-liquid crystal transition by a differential ultrasonic resonator. The relaxation time and strength increased remarkably near the transition temperature, indicating a pseudocritical phenomenon. A quantitative analysis of the relaxation in terms of thermodynamic relationships between specific heat, thermal-expansion coefficient, and compressibility showed that more than 90% of the total endothermic heat of the transition arises from the latent heat. The temperature dependence of the ultrasonic relaxation parameters was also analyzed by the Landau theory; we obtain a small but finite difference, 0.6 degree C, between the pseudocritical temperature and the transition temperature. These results provide a quantitative description of both the first-order and second-order characters of the gel-to-liquid crystal transition.

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A differential ultrasonic resonator was developed for the measurements of ultrasonic velocity as well as absorption in dilute aqueous suspensions of biological materials. The accuracy of the resonator was improved by analyzing the resonance peak shape with the aid of a microcomputer; the accuracy of 1×10-5 for velocity and 5×10-5 for absorption per wavelength was obtained in the frequency range between 0.8 and 8 MHz. An aqueous suspension of multilamellar lipid bilayers was measured by this apparatus, and a marked slowing down of the relaxation time was observed in the vicinity of a gel-to-liquid crystal phase transition.

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Ultrasonic properties of bacteriorhodopsin which is a light-driven proton pump in purple membrane were studied in the dark as well as under illumination. A xenon lamp was added as a light source to a previously developed ultrasonic resonator. Bacteriorhodopsin showed no ultrasonic relaxation in the frequency range of a few megahertz. The adiabatic bulk modulus of bacteriorhodopsin estimated from ultrasonic velocity was about 4×1010 dyn/cm2 in the dark. When bacteriorhodopsin was illuminated, the ultrasonic velocity increased significantly in proportion to the intensity of light.