A model for the electrostatic contribution to the pH-dependent nonideal mixing of a binary charged-zwitterionic lipid bilayer.

Nonideal mixing provides the physical basis of domain formation and macroscopic phase separation in lipid bilayers. We present a model for the electrostatic contribution to the nonideality of a two-component acidic-zwitterionic lipid membrane. Our model is based on the mean-field Poisson-Boltzmann approach; it includes a protonation/deprotonation equilibrium applicable to acidic lipids such as phosphatidic acid or phosphatidylglycerol. It also includes an electrostatic model for zwitterionic lipids such as phosphatidylcholine or phosphatidylethanolamine that accounts for the spatial separation of the two headgroup charges and the orientational freedom of the headgroup. Modeling the nonelectrostatic contribution to the free energy using the Bragg-Williams approximation of a binary lattice gas enables us to compute binodal lines that reflect the influence of membrane electrostatics on the nonideal mixing properties of the bilayer. If we neglect the zwitterionic lipid's electrostatic contribution, then increasing pH is predicted to always oppose demixing, stabilizing the membrane due to the repulsion between the charged lipids. If the electrostatic properties of the zwitterionic lipids are accounted for, this opposing tendency weakens and may even reverse. In this case, increasing the fraction of charged lipids through increasing pH would, somewhat unexpectedly, promote domain formation. We discuss the corresponding physical mechanism.