Thermotropic and barotropic phase transitions of N-methylated dipalmitoylphosphatidylethanolamine bilayers

In order to understand the effect of polar head group modification on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidyl-N-methylethanolamine (DPMePE), dipalmitoylphosphatidyl-N,N-dimethylethanolamine (DPMe2PE) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were observed by differential scanning calorimetry and high-pressure optical methods. The temperatures of the so-called main transition from the gel (Lβ) or ripple gel (Pβ′) phase to the liquid crystalline (Lα) phase were almost linearly elevated by applying pressure. The slope of the temperature-pressure boundary, dT/dp, was in the range of 0.220-0.264 K MPa−1 depending on the number of methyl groups in the head group of lipids. The main-transition temperatures of N-methylated DPPEs decreased with increasing size of head group by stepwise N-methylation. On the other hand, there was no significant difference in thermodynamic quantities of the main transition between the phospholipids. With respect to the transition from the subgel (Lc) phase to the lamellar gel (Lβ or Lβ′) phase, the transition temperatures were also elevated by applying pressure. In the case of DPPE bilayer the Lc/Lβ transition appeared at a pressure higher than 21.8 MPa. At a pressure below 21.8 MPa the Lc/Lα transition was observed at a temperature higher than the main-transition temperature. The main (Lβ/Lα) transition can be recognized as the transformation between metastable phases in the range from ambient pressure to 21.8 MPa. Polymorphism in the gel phase is characteristic of DPPC bilayer membrane unlike other lipid bilayers used in this study: the Lβ′, Pβ′ and pressure-induced interdigitated gel (LβI) phases were observed only in the DPPC bilayer. Regarding the bilayers of DPPE, DPMePE and DPMe2PE, the interdigitation of acyl chain did not appear even at pressures as high as 200 MPa.