Ordered --fluid phase transitions in bilayers of charged lipids are accompanied by a decrease in electrostatic free energy mainly as a result of bilayer expansion. For uniform charge distribution the Gouy-Chapman theory of the electrical double layer predicts a decrease of the transition temperature with increasing charge density. We studied the effects of pH and monoand dlvalent cations on the phase transition of lecithin, cephalin, phosphatidylserine, and phosphatidic acid bi- (,7, 8). For dipalmitoyl-lecithin (C16-Lec) (Tj -410C), the molecular area, f, increases from 48 ,2 (T < T ) to 60-70 A2 (T > T,).Temperature has been used to trigger lipid phase transitions. Biological systems, however, are remarkably constant in temperature; hence conformational changes in vivo must be caused by variables other than temperature. We decided to test whether lipid phase transitions can be triggered at constant temperature by monovalent and divalent cations and by changes in pH. Such effects are to be expected for charged Abbreviations: Tj, transition temperature; PA, dimyristoyl-snglycerol-3-phosphate; PS, dipalmhtoyl-n-glycerol-3-phosphorylserine; C14-Lec, dimyristoyl-sn-glycerol-3-phosphorylcholine; C16-Lec, dipalmitoyl-sn-glycerol-3-phosphorylcholine; C-14-Ceph, dimyristoyl-sn-glycerol-3-phosphorylethanolamine; GP, glycerol-3-phosphate; poly(Lys), polylysine; NPN, N-phenylnaphthylamine; 214 lipid bilayers because the electrostatic free energy T, r, of the system, changes at T as a result of bilayer expansion. Therefore, all parameters affecting the value of r can be expected to alter the transition temperature. In a certain temperature range these variables may thus be used to induce the phase transition at a constant temperature.

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The movement of molecules across membranes is discussed in terms of thermal fluctuations in the hydrocarbon chains of the membrane lipids. The thermal motion of the hydrocarbon chains results in the formation of conformational isomers, so-called kink-isomers of the hydrocarbon chains. "Kinks" may be pictured as mobile structural defects which represent small, mobile free volumes in the hydrocarbon phase of the membrane. The diffusion coefficient of kinks is calculated to be 10(-5) cm(2)/sec; thus kinks diffusion is a fast process. Small molecules can enter into the free volumes of kinks and migrate across the membrane together with the kinks; thus kinks may be regarded as intrinsic carriers of lipid membranes. An expression is derived from this model for the flow of molecules through lipid membranes. The calculated value for the water permeability is compatible with measurements on lipid bilayers.

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