Compression of lipid membranes as observed at varying membrane positions

We have measured the microscopic isothermal compressibility of dioleoyl- and dimyristyl-phosphatidylcholine multilayers and bilayers as a function of membrane depth by the pressure dependence of the polarization of a series of anthroyloxy fatty acids. In both systems, within experimental error, the compressibility did not change with membrane depth. The magnitudes of the compressibilities matched those of organic solids and those reported for dipalmitoylphosphatidylcholine multilayers from neutron diffraction measurements (Braganza, L. F., and D. L. Worcester. 1986. Biochemistry, 25:7484-7488). The bilayer compressibility decreased with temperature and this decrease was similar with membrane depth consistent with the isotropic thermal expansion of membranes previously observed (Scarlata, S. 1989. Biophys. J. 55:1215-1223). The vertical compressibility in the z direction is much lower than the horizontal (xy planes) for probes that lie parallel to the hydrocarbon chains which is consistent with an increase in bilayer thickness. The compressibility for probes that lie perpendicular to the hydrocarbon chains is more isotropic due to their limited spatial access to the z plane.