Phase behavior of hydrated lipid bilayer composed of binary mixture of phospholipids with different head groups

Phase behavior of hydrated lipid bilayer was investigated for the mixtures of two phospholipid species chosen from phosphatidic acid (PA), phosphatidy lcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG) with the same acyl chains. The pseudo-binary phase diagrams constructed by a differential scanning calorimetry (DSC) were analyzed based on a thermod ynamic model applying the Bragg-Williams approximation for non-ideality of mixing. The interchange energy parameters, rho(0), derived from this approa ch were positive for all mixture systems in both gel and liquid-crystalline phase bilayers, and increased in the order PG/PE < PC/PA < PC/PE < PC/PA w ith a few exception. This suggests that the energetical disadvantage for th e mixed-pair formation relative to the like-pair formation in the hydrated bilayer increases in this order. In addition, the rho(0) values increased w ith the increase in the acyl chain length of the phospholipids. These exper imental results were discussed in terms of an intermolecular interaction of the phospholipid species in hydrated bilayer. (C) 1999 Elsevier Science Ir eland Ltd. All rights reserved.