Thermodynamic characterization of bilayer- nonbilayer phase transitions of phospholipid membranes

We determined thermodynamic properties of phase transitions between bilayer and nonbilayer for phosphatidylcholines with saturated hydrophobic chains (C18:0-PC, O-C18:0PC) and phosphatidylethanolamines with unsaturated ones (C18:1(cis)-PE, C18:1(trans)-PE) by means of calorimetry under ambient pressure and optical measurements under high pressure. The thermodynamic quantities of the transitions between bilayer and nonbilayer were much smaller than those of the transition between bilayers (gel-liquid crystal or hydrated crystalliquid crystal transition) for the corresponding phospholipids. Although the nonbilayer formations correspond to a dynamic transformation between lamellar structure and nonlamellar structure, we can say that the order of the lipid molecule in both structures may not appreciably change judging from the smaller thermodynamic quantities. A notable feature of the bilayernonbilayer transitions is the large pressure dependence of the transition temperature as compared with that of the bilayer-bilayer transitions. Comparing the enthalpy and volume changes of the bilayer-nonbilayer transitions with those of the bilayer-bilayer transitions, we concluded that the former transitions can be regarded as the volume-driven transitions for the reconstruction of molecular packing.