Lipid phase transitions in model biomembranes: The effect of ions on phosphatidylcholine bilayers

Differential scanning calorimetry has been used to study the endothermic phase behaviour of some model biomembranes (i.e. phosphatidylcholine-water systems) in the presence of a wide range of alkaline, alkaline earth and heavy metal salts. Studies and comparisons were made of both cation and anion effects. Shifts occur in the temperatures of both the pre-transition and main transition endotherms. The observed shifts are smaller than those which have been reported for charged lipids, and no evidence has been found for the formation of specific complexes. Electron microscopic studies on freeze-fractured dispersions of phosphatidylcholine-water-salt systems show that with some salts the typical rippled surface observed with l-α-dimyristoyl phosphatidylcholine, when in the gel state, is replaced by a smooth surface.