PRESSURE EFFECTS ON THE LATERAL DISTRIBUTION OF CHOLESTEROL IN LIPID BILAYERS - A TIME-RESOLVED SPECTROSCOPY STUDY

The effects of hydrostatic pressure and temperature on the phase behav ior and physical properties of the binary mixture palmitoyloleoylphosp hatidylcholine/cholesterol over the 0-40 molar % range of cholesterol compositions, were determined from the changes in the fluorescence lif etime distribution and anisotropy decay parameters of the natural lipi d trans-parinaric acid (t-PnA), Pressurized samples were excited with a Ti-sapphire subpicosecond laser, and Fluorescence decays were analyz ed by the quantified maximum entropy method. Above the transition temp erature (T-T = -5 degrees C), at atmospheric pressure, two liquid-crys talline phases, alpha and beta, are formed in this system. At each tem perature and cholesterol concentration below the transition pressure, the fluorescence lifetime distribution pattern of t-PnA was clearly mo dulated by the pressure changes. Pressure increased the fraction of th e liquid-ordered beta-phase and its order parameter, but it decreased the amount of cholesterol in this phase. Palmitoyloleoylphosphatidylch oline/cholesterol phase diagrams were also determined as a function of temperature and hydrostatic pressure.