NONIDEAL MIXING AND PHASE-SEPARATION IN PHOSPHATIDYLCHOLINE PHOSPHATIDIC-ACID MIXTURES AS A FUNCTION OF ACYL-CHAIN LENGTH AND PH

The miscibilities of phosphatidic acids (PAs) and phosphatidylcholines (PCs) with different chain lengths (n = 14, 16) at pH 4, pH 7, and pH 12 were examined by differential scanning calorimetry. Simulation of heat capacity curves was performed using a new approach that incorpora tes changes of cooperativity of the transition in addition to nonideal mixing in the gel and the liquid-crystalline phase as a function of c omposition. From the simulations of the heat capacity curves, first es timates for the nonideality parameters for nonideal mixing as a functi on of composition were obtained, and phase diagrams were constructed u sing temperatures for onset and end of melting, which were corrected f or the broadening effect caused by a decrease in cooperativity. In all cases the composition dependence of the nonideality parameters indica ted nonsymmetrical mixing behavior. The phase diagrams were therefore further refined by simulations of the coexistence curves using a four- parameter approximation to account for nonideal and nonsymmetrical mix ing in the gel and the liquid-crystalline phase. The mixing behavior w as studied at three different pH values to investigate how changes in headgroup charge of the PA influences the miscibility. The experiments showed that at pH 7, where the PA component is negatively charged, th e nonideality parameters are in most cases negative, indicating that e lectrostatic effects favor a mixing of the two components. Partial pro tonation of the PA component at pH 4 leads to strong changes in miscib ility; the nonideality parameters for the liquid-crystalline phase are now in most cases positive, indicating clustering of like molecules, The phase diagram for 1,2-dimyristoyl-sn-glycero-3-phosphatidic cid:1, 2-dipalmitoyl-sn-glycero-3-phosphorylcholine mixtures at pH 4 indicate s that a fluid-fluid immiscibility is likely. The results show that a decrease in ionization of PAs can induce large changes in mixing behav ior. This occurs because of a reduction in electrostatic repulsion bet ween PA headgroups and a concomitant increase in attractive hydrogen b onding interactions.