Phase superdiagrams of two component phospholipid membranes : pH dependence

A model proposed to determine the phase diagrams of PC/PC, PE/PE and PC/PE mixtures (1, 2) has been considered to account for the phosphatidic acid (FA) mixtures too. PAs have two dissociable protons, the pK values of the phosphate group are 3.5 and 9. In the transition temperature (Tt) vs pH diagrams of the PAs the increasing ionisation results in a maximum value for Tt at pH = pK1. Small variations in Tt are observed in the regions between the pK values in spite of the large changes occurring on the surface charges of the membranes while a further pH increase abruptly decreases Tt. The proposed analysis is concerned with the intermediate pH region. Considering the free energy function of the one component system (PA) the energy term accounting for the surface contribution has been generalized and it has been observed that the pH dependence of the corresponding model parameter, L*, is of parabolic type. Other model parameters were determined comparing the experimental results of PC, PE and PA at different pH values and, for binary mixtures, the superdiagrams (SDs) could be constructed. Depending on the relationships between the chain lengths and that on the phase transition temperatures of the pure components it is possible, in these SDs, to predict the type of the phase diagrams. PC/PA and PE/PA mixtures, at different pH values, were analyzed. In the first case, increasing pH, an increase of area between the two fusion lines is found while an opposite situation is given by PE/PA mixtures. At the same pH, the region between the fusion lines is smaller in PC/PA than in PE/PA mixtures indicating a lower affinity of the components with prevalently eutectic and peritectic phase diagrams. 1) Sugar I.P. and Monticelli G. (1985) - Biophys. J. 48: 283-288. 2) Monticelli G. (1986) in "Biophysics of membrane transport" (Kuczera J. and Przestalski S. eds.), Wroclaw Univ. Publ., pp. 271-300.