LASER INDUCED ULTRASONIC STUDIES OF MODEL BIOMEMBRANES

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We describe here the use of a new laser-induced ultrasonic technique for the study of structural and mechanical properties of planar phospholipid bilayers. The technique, called Laser-Induced Phonon Spectroscopy (LIPS), uses the optical interference pattern generated by two crossed, picosecond laser pulses to launch acoustic waves whose wavelength and orientation match the interference pattern geometry. The acoustic velocity and attenuation are monitored by observing Bragg diffraction of a variably delayed probe pulse due to periodic variations in the sample density associated with the sound wave propagation. The advantages of LIPS over other ultrasonic methods which have been applied to membrane research include its frequency tunability (3 MHz-30 GHz) and inherently high signal-to-noise. The oriented phospholipid samples we will study are optically clear and contain up to 100,000 stacked bilayers, separated by layers of water. These samples can be prepared with a variety of phospholipids, including DLPC, DMPC, DPPC, DSPC etc., containing variable concentrations of water or other ingredients, such as cholesterol or inorganic ions. LIPS is capable of probing both static and dynamic properties of membranes. For example, analysis of the sound velocity as a function of propagation direction yields the anisotropic compressibilities, and how these change with temperature and chemical environment. The sensitivity of the acoustic velocity and attenuation to the physical state of the membrane can be used to investigate the lipid-water and lipid-cholesterol-water phase diagrams, in particular the phenomenon of lateral phase separation. LIPS can also be used to explore the dynamic processes occurring in the gel-liquid crystalline phase transition, as well as the mechanical coupling between bilayers in the multibilayer array. Finally, we can compare the mechanical properties of lipid bilayers with those of well-characterized liquid crystal of the DPPC type. Preliminary LIPS data on 125 Mu thick samples of DPPC (25,000 bilayers) containing 7% H2O are presented. Implications of these data for the mechanical properties of the gel and liquid crystalline phases of the bilayers are discussed.