Barotropic phase transition between the lamellar liquid crystal phase and the inverted hexagonal phase of dioleoylphosphatidylethanolamine

The phase transition between the lamellar liquid crystal (L-alpha) phase and the inverted hexagonal (H-II) phase of dioleoylphosphatidylethanolamine (DOPE) in aqueous NaCl solutions was observed by means of differential scanning calorimetry (DSC) under ambient pressure and light-transmittance technique under high pressure. The pressure dependence of the transition temperature (dT/dp) and the thermodynamic quantities for the L-alpha/H-II transition were compared with those of another phase transition found in the DOPE bilayer membrane, which is the transition from the lamellar crystal (L-c) phase to the L-alpha phase. The dT/dp value of the L-alpha/H-II transition was about 3.5 times as large as that of the L-c/L-alpha transition while the thermodynamic quantities were significantly smaller than those of the latter to the contrary. Comparing the enthalpy and volume behavior of the L-alpha/H-II transition with that of the L-c/L-alpha transition, we concluded that the L-alpha/H-II transition can be regarded as the volume-controlled transition for the reconstruction of molecular packing. (C) 2006 Elsevier B.V. All rights reserved.