Water permeability of bilayer lipid membranes: Sterol-lipid interaction.

Bilayer lipid membranes were generated in an aqueous medium from synthetic, egg or plant phosphatidyl choline (PC) or from plant monogalactosyl diglyceride (MG). The water permeability of the black membranes was determined by measuring the net volume flux produced by a NaCl gradient. The osmotic permeability coefficient,P os, was markedly affected by the number of double bonds in the fatty acid conjugates of the lipids: the greater the degree of unsaturation, the higher the value ofP os. The temperature dependence ofP os of the lipid membranes was studied over a range of 29 to 40°C. The experimental activation energy,E a , estimated from the linear plots of log (P os)versus 1/T, was significantly higher for MG membranes (17 kcal/mole) than for the various PC membranes (11 to 13 kcal/mole), probably owing to hydrogen bonding between MG and water molecules. In comparison with PC membranes, the membranes generated from PC and cholesterol (1∶1 molar ratio) had lowerP os but similarE a values. Likewise, either stigmasterol or β-sitosterol decreasedP os of MG membranes, whileE a was not affected by the sterols. MG-cholesterol membranes were specifically characterized by a unique value ofE a (-36 kcal/mole) thus indicating temperature dependent structural changes.