Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)–pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T–p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127–0.145 K MPa−1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lα) phase for DOPC bilayer (0.131 K MPa−1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lβ) phase to the Lα phase for DOPC bilayer (0.233 K MPa−1) was significantly different from that of the Lc/Lα transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lα transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lα phase although they have been regarded as the main transition in the previous studies. The Lβ phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.