All the temperatures of the main transition between lamellar gel (L_β) and liquid crystaline (L_α) phases for dilauroyl-(DLPE), dimyristoyl-(DMPE), dipalmitoyl-(DPPE), and distearoyl-phosphatidylethanolamine (DSPE) bilayer membranes were elevated by pressure. The temperature (T) ―pressure (p) phase boundary curve was best described by a smooth curve and not a linear function. The values of slope (dT/dp) at ambient pressure were 0.230, 0.251, 0.264 and 0.275 K MPa~(-1) for DLPE, DMPE, DPPE and DSPE, respectively. The volume change assoc ated with the main transition from L_β to L_α phase, which was estimated from the Clapeyron-Clausius equation, was 13.0, 19.5, 27.2 and 35.3 cm~3 mol~(-1) for DLPE, DMPE, DPPE and DSPE, respectively, which increased linearly with an increase in the acyl chain length of lipids. Thermodynamic properties for the main transition of phosphatidylethanolamine (PE) bilayer membranes were compared with those for phosphatidylcholine (PC) bilayer membranes. Regarding the enthalpy and volume changes, the values for PEs and PCs with the same acyl chain are similar in magnitude to each other. However, the main-transition temperatures for PE bilayer membranes are siginificantly higher than those for PC. The difference in the main-transition temperature between PE and PC bilayer membranes may be attributable to the head group interaction in the bilayers.
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