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

The phase transition between the lamellar liquid crystal (Lα) phase and the inverted hexagonal (HII) 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 Lα/HII transition were compared with those of another phase transition found in the DOPE bilayer membrane, which is the transition from the lamellar crystal (Lc) phase to the Lα phase. The dT/dp value of the Lα/HII transition was about 3.5 times as large as that of the Lc/Lα transition while the thermodynamic quantities were significantly smaller than those of the latter to the contrary. Comparing the enthalpy and volume behavior of the Lα/HII transition with that of the Lc/Lα transition, we concluded that the Lα/HII transition can be regarded as the volume-controlled transition for the reconstruction of molecular packing.