Phase transitions in lipid bilayers. A theoretical model for phosphatidylethanolamine and phosphatidic acid bilayers.

We present an application of an earlier model to the description of phase transitions in phosphatidylethanolamine and phosphatidic acid bilayers. For the former system we modify the model to allow for very slightly closer chain packing. This small modification alone is sufficient to shift the main phase transition temperature to higher values, and to eliminate the lower transition. For the charged bilayers we incorporate electrostatic contributions into the free energy using double layer theory and we consider the excluded volume of counterions at the bilayer/water interface. Phase transition temperatures and the presence of absence of a lower transition are determined for various pH values, and the results are compared with experiment.