Lateral pressure change on phase transitions of phosphatidylcholine/diolein mixed membranes

In this work, the changes in the lateral pressure in mixed membranes of egg yolk phosphatidylcholine (EPC) and a nonlamellar-forming lipid diolein (DO) were investigated with respect to increasing DO content. Several fluorescence techniques were employed to probe transitions of EPC/DO lipid mixtures from lamellar to inverted hexagonal via bicontinuous cubic phases. Excimer fluorescence of dipyrenyl phospholipids revealed that the lateral pressure in the acyl chain region of the EPC/DO mixed membrane increased with the mole fraction of DO (X_DO) in the lamellar phase and further increased almost linearly up to X_DO=0.2 through the lamellar-to-cubic phase transition. Water penetration into the acyl chain region, as determined by fluorescence lifetime experiments, decreased linearly with increasing X_DO until the cubic phase was formed. These results suggest that the acyl chain packing becomes tighter by the incorporation of DO in the lamellar phase and that it does not alter during the lamellar-to-cubic phase transition. Only the packing of the headgroup region was found to increase during the transition to the cubic phase, as detected by the fluorescence lifetime of dansyl phosphatidylethanolamine, which localizes in this region. Through the cubic-to-hexagonal transition, headgroup packing was increased further, and the acyl chain packing was in turn loosened. These results suggest that the acyl chain packing stress induced by the incorporation of DO is released not by the lamellar-to-cubic phase transition but by the cubic-to-hexagonal transition.