Solid- and liquid-phase equilibria in phosphatidylcholine/phosphatidylethanolamine mixtures. A calorimetric study.

Phase diagrams have been determined for mixing of binary mixtures of phosphatidylethanolamines (PE) with phosphatidylcholines (PC), using high-sensitivity differential scanning calorimetry and allowing extensive incubation times to equilibrate samples in the solid phase. All of the PE-PC systems examined, which contained saturated or trans-unsaturated PC components, showed limited solid-phase miscibility, chiefly because the PC component can adopt more solid phases than the PE component. For the dielaidoyl PE-PC system, the lamellar-to-hexagonal II transition endotherm seen at 63.5 degrees C for the pure PE is shifted to considerably higher temperatures upon incorporation of even low mole fractions of PC. All of the PE-PC systems examined here reveal a complete miscibility in the liquid phase, including the dipalmitoyl PE-dielaidoyl PC system for which limited liquid-phase miscibility had previously been suggested (Wu, S-H. and McConnell, H.M. (1975) Biochemistry 14, 847-854). However, PE-PC mixing appears to be less nearly ideal than the mixing of either PE or PC with anionic phospholipids. Our results demonstrate that calorimetry can be useful in determining accurate phase diagrams for lipid mixtures of this type, but only if proper attention is given to the existence and the proper equilibration of multiple solid phases in these systems.