The effect of cholesterol incorporation on the temperature dependence of water permeation through liposomal membranes prepared from phosphatidylcholines.

The permeation of water through liposomal membranes composed of phosphatidylcholine plus varying amounts of cholesterol was studied as a function of temperature. 1. Increasing amounts of cholesterol caused a gradual disappearance of the abrupt change in the rate of water permeation near the gel to liquid-crystalline phase transition temperature of dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine liposomes. At cholesterol concentrations above about 30 mol % there was no longer a discontinuity in the rate of water permeation. 2. The incorporation of cholesterol produces a steep change in the activation energy of the water permeation above the transition temperature of the saturated lecithin occurring at about 15 mol % of cholesterol. Below the transition temperature there was a gradual decrease in the activation energy of the water permeation in the region of 0 to 33 mol % of cholesterol. 3. In systems containing unsaturated phosphatidylcholines cholesterol also enhanced the activation energy of the water permeation although to a lesser extent. The results indicate that the position of the cis-double bond in the fatty acid chain is very important in this respect. 4. In systems in which cholesterol increased the temperature dependence of the water permeation there is also an enhancement of the temperature dependence of the isotonic glycerol and erythritol swelling by the same number of kcal/mol.