A statistical mechanical analysis of the effect of long-chain alcohols and high pressure upon the phase transition temperature of lipid bilayer membranes.

Long-chain n-alcohols decrease the main phase-transition temperature of lipid vesicle membranes at low concentrations but increase it at high concentrations. The nonlinear phenomenon is unrelated to the interdigitation and is analyzed by assuming that alcohols form solid solutions with solid as well as liquid phases. The biphasic response originates from the balance of the free energy difference of alcohols in the liquid and solid membranes (delta gA) and the alcohol-lipid interaction free energy difference (delta u) between the two phases. When delta gA less than 0 and delta u greater than 0, or delta gA less than delta u less than 0, the transition temperature decreases monotonously according to the increase in the alcohol concentration. When delta gA greater than 0 and delta u less than 0, or delta gA greater than delta u greater than 0, it increases monotonously. Biphasic response occurs with a minimum temperature when delta u greater than delta gA greater than 0, and with a maximum temperature when delta u less than delta gA less than 0. When the alcohol carbon-chain length becomes closer to the lipid carbon-chain length, delta u is equalized by delta gA, and the temperature minimum of the main transition is shifted to extremely low alcohol concentrations. Hence, long-chain alcohols predominantly elevate the main transition temperature and lose their anesthetic potency. High pressure decreased both delta gA and delta u. Presumably, high pressure improves the packing efficiency of liquid membranes and decreases the difference between the solid and liquid membrane properties.