A study on phosphatidylcholine and phosphatidylethanolamine homologues binary mixtures

The complexity of the biological membrane lipid matrix has conducted both experimental and theoretical investigation from one-component lipid bilayers to the study of two component systems. Based on Landau phenomelogical theory (1) of phase transitions, we propose a model to describe both phosphatidycholine (PC) and phosphatidylethanolamine (PE) series main phase transitions. Taking into account the Priest (2) phenomelogical model for lecithin bilayers, we constructed a new excess free energy term in the state function to account for the thermodynamic properties of the mixtures where the chain lenghts and the polar heads of the components can be simultaneously different. The phase diagrams of all the possible PC/PE mixtures were calculated and the results were compared with experimental data taken from different sources. We obtained complete miscibility both in the liquid crystalline and in the gel phase if the chain lenghts of the components were the same. If the chain lenght of the PE component was longer than that of the PC component, we got a peritectic system. An eutectic system was obtained in the inverse case. The calorimetrical spectra and the cooperative unit sizes (N) of lecithin homologues binary mixtures have been determined. The shapes of these spectra depend strongly on the values of the cooperative unit sizes. N decreases drastically mixing the components. 1. L. D. Landau and E. M. Lifshitz – Statistical Physics (Addison-Wesley, Reading, Mass. 1969), pp.429-433. 2. R. G. Priest – Mol. Cryst. Liq. Cryst., 60, 167-184 (1980).