Phosphatidylcholine bilayers. A theoretical model which describes the main and the lower transitions

We present a theoretical model which describes both the main and the lower phase transition in phosphatidylcholine bilayers. The main transition involves a melting of the hydrocarbon chains while the lower transition is seen as a nematic to isotropic transition involving entire lipid molecules (which are rod shaped when projected onto the bilayer plane). This latter transition is consistent with experimental data which suggest the presence of long-axis rotation for temperatures below the main melting transition. The model is extended to mixtures of phosphatidylcholines and compared with experimental data.