Non-ideal mixing and fluid-fluid immiscibility in phosphatidic acid-phosphatidylethanolamine mixed bilayers.

Differential scanning calorimetry (DSC) was used to study the miscibility of phosphatidic acids (PAs) with phosphatidylethanolamines (PEs) as a function of chain length (n = 14, 16) and degree of ionization of PAs at pH 4, pH 7, and pH 12. Phase diagrams were constructed using temperature data for onset and end of the phase transition obtained from the direct simulation of the heat-capacity curves. The phase diagrams were analyzed by simulations of the coexistence curves utilizing a four-parameter regular solution model. For PA-PE mixtures, the non-ideality parameters are a function of composition indicating non-symmetric non-ideal mixing behavior. At pH 7, where the PA component is negatively charged, the systems DMPA:DMPE and DPPA:DPPE have positive non-ideality parameters ρ (1) in both phases, indicating a preferred aggregation of like molecules. In contrast, DMPA:DPPE and DPPA:DMPE mixtures had negative ρ (1) values. Measurements at pH 4 showed that mixed pair formation is favored when PA is protonated. At pH 12 where PA is doubly charged, highly positive ρ (l1) parameters are obtained for the liquid-crystalline phase except for the system DPPA:DPPE (ρ (1) < 0). This indicates clustering of like molecules and possibly domain formation in the liquid-crystalline phase. DPPA:DMPE at pH 12 even shows a miscibility gap in the liquid-crystalline phase. Obviously, despite the presence of doubly charged PA a fluid-fluid immiscibility is induced.