High-pressure study on bilayer phase behavior of oleoylmyristoyl- and myristoyloleoyl-phosphatidylcholines

We investigated the thermotropic and barotropic bilayer phase behavior of 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC) and 1-oleoyl-2-myristoyl-sn-glycero-3-phosphocholine (OMPC) by means of the differential scanning calorimetry (DSC) and high-pressure light-transmittance technique. Water could be used as a solvent for measurements at high pressures because of the elevation of the transition temperatures above 0 °C by pressurization, whereas aqueous 50 wt.% ethylene glycol solution was used mainly for those at low pressures. Only one phase transition was observed in the DSC thermogram of the MOPC bilayer membrane as an endothermic peak, and also observed at high pressures as an abrupt change of the light-transmittance. The transition was assigned as a main transition between the lamellar gel (Lβ) and liquid-crystalline (Lα) phases on the basis of the values of enthalpy change (ΔH) and slope of the transition temperature with respect to pressure (dT/dP). The DSC thermogram of the OMPC bilayer membrane similarly showed a single endothermic peak but two kinds of phase transitions were observed at different temperatures in the light-transmittance profile at high pressures. The extrapolation of the lower-temperature transition in the high-pressure range to an ambient pressure coincided with the transition observed in the DSC thermogram. This transition was identified as a transition between the lamellar crystal (Lc) and Lα (or Lβ) phases from the ΔH and dT/dP values. The higher-temperature transition, appearing only at high pressures, was identified as the Lβ/Lα transition considering the topological resemblance of its temperature-pressure phase diagram as that of the dioleoylphosphatidylcholine bilayer membrane. The phase diagram of the OMPC bilayer membrane demonstrated that the Lβ phase cannot exist at pressures below ca. 190 MPa while it can exist stably in a finite temperature range at pressures above the pressure.