Theory of the effects of protons and divalent cations on phase equilibriums in charged bilayer membranes: comparison with experiment

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Abstract
The concepts in a recently developed statistical mechanical theory of the effects of proton binding and divalent cation binding on phase equilibria in bilayer membranes composed of acidic phospholipids are summarized. The theory is used to calculate membrane phase transition temperatures for different aqueous concentrations of protons, divalent cations, and monovalent salt. Methods are discussed for calculating transition temperatures even for systems in which there is not an excess of protons or divalent cations relative to lipids. The results are compared with existing experimental data for a number of lipids. There is good agreement between calculated transition temperature vs. pH curves and experimental data for dimyristolmethylphosphatidic acid, dimyristoyl-phosphatidylglycerol, dipalmitoylphosphatidylglycerol, dipalmitoylphosphatidylserine, and dimyristoylphosphatidic acid. General thermodynamic considerations are used to derive a Clapeyron-like equation for the rate of variation in membrane transition temperature with divalent cation concentration. This equation and some available experimental data are used to argue that the large increase in solid to fluid phase transition temperature that is observed experimentally as the divalent cation concentration is increased is the result of a metastable solid phase that exists at low but not high divalent cation concentration. A calculated coexistence diagram is compared with existing experimental data for transition temperatures of dimyristoylphosphatidylglycerol membranes at different total Ca concentrations. Good agreement is obtained when the existence of a metastable solid phase is assumed.

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