Comprehensive characterization of temperature- and pressure-induced bilayer phase transitions for saturated phosphatidylcholines containing longer chain homologs.

Complete elucidation of the phase behavior of phospholipid bilayers requires information on the subtransition from the lamellar crystal (Lc) phase to the gel phase. However, for bilayers of saturated diacylphosphatidylcholines (CnPCs), especially longer chain homologs, equilibration in the Lc phase is known to be very slow. In this study, bilayer phase transitions of three CnPCs with longer acyl chains, C19PC, C20PC and C21PC, were observed by differential scanning calorimetry under atmospheric pressure and by light-transmittance measurements under high pressure. Using lipid samples treated by thermal annealing enabled the observation of the sub-, pre- and main transitions of the C19PC and C20PC bilayers under atmospheric pressure. Only the pre- and main transitions could be observed for the C21PC bilayer due to very slow kinetics of the Lc phase formation for lipids with long acyl chains. The temperature and pressure phase diagrams constructed and phase-transitions quantities (enthalpy, entropy and volume changes) evaluated for these bilayers were compared with one another and with those of bilayers of the CnPC homologs examined in previous studies. These results allowed us (1) to clarify the temperature- and pressure-dependent phase sequence and phase stability of the CnPC (n=12-22) bilayers as a function of the hydrophobicity of the molecules, (2) to prove the presence of a shorter and a longer limit (n=13 and 21) in the acyl chain length for the pressure-induced bilayer interdigitation and (3) to reveal the chain-length dependence of the thermodynamic quantities of the subtransitions including the volume change.