Abstract
Thermodynamic quantities of phase transitions between bilayer and nonbilayer for phospholipids, phosphatidylcholines with saturated acyl chains (C18:0-PC, O-C18:0-PC) and phosphatidylethanolamines with unsaturated acyl chains (C18:1-PE(cis), C18:1-PE(trans)), were determined by means of differential scanning calorimetry under ambient pressure and light-transmittance measurements under high pressure. The thermodynamic quantities of the nonbilayer formations were much smaller than those of the transition between bilayers (gel-liquid crystal or hydrated crystal-liquid crystal transition) for the corresponding phospholipids. Although the nonbilayer formations correspond to a dynamic transformation between lamellar structure and nonlamellar structure such as interdigitated or inverted hexagonal 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 bilayer-nonbilayer transitions is the large pressure dependence of the transition temperature as compared with that of the bilayer-bilayer transitions. This fact means that the transformation between bilayer and nonbilayer structures is remarkably influenced by pressure. 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 volume-controlled transitions for the reconstruction of molecular packing.
