CHOLESTEROL-INDUCED VARIATIONS IN THE VOLUME AND ENTHALPY FLUCTUATIONS OF LIPID BILAYERS

The sound velocity and density of suspensions of large unilamellar lip osomes from dimyristoylphosphatidylcholine with admired cholesterol ha ve been measured as a function of temperature around the chain melting temperature of the phospholipid. The cholesterol-to-phospholipid mola r ratio x(c) has been varied over a wide range (0 less than or equal t o x(c) less than or equal to 0.5). The temperature dependence of the s ound velocity number, of the apparent specific partial volume of the p hospholipid, and of the apparent specific adiabatic compressibility ha ve been derived from the measured data. These data are particularly di scussed with respect to the volume fluctuations within the samples. A theoretical relation between the compressibility and the excess heat c apacity of the bilayer system has been derived. Comparison of the comp ressibilities (and sound velocity numbers) with heat capacity traces d isplay the close correlation between these quantities for bilayer syst ems. This correlation appears to be very useful as it allows some of t he mechanical properties of membrane systems to be calculated from the specific heat capacity data and vice versa.