MISCIBILITY OF BINARY PHOSPHOLIPID MIXTURES UNDER HYDRATED AND NON-HYDRATED CONDITIONS .4. PHOSPHATIDYLGLYCEROLS WITH DIFFERENT ACYL-CHAIN LENGTH

The miscibility property of homologous diacylphosphatidylglycerol (PG) with different acyl chain length (n = 14, 16, and 18, where n is the number of carbon atoms per chain) under hydrated and non-hydrated cond itions was examined in terms of the binary phase diagram constructed b y differential scanning calorimetry. The phase diagrams were analyzed based on thermodynamic equations derived by evaluating the excess free energy of mixing within a framework of the Bragg-Williams approximati on, and the non-ideality parameter of mixing, rho(0), was estimated fo r various phases of PG mixture. The rho(0) values obtained for hydrate d liquid-crystalline and gel phases are larger than those obtained for non-hydrated liquid and solid phases, respectively. This demonstrates that the miscibility is poorer in hydrated lipid bilayer than in non- hydrated bulk lipid phase. The tendency in the variation of rho(0) val ues with respect to the lipid phases is similar to that obtained previ ously for homologous diacylphosphatidylethanolamines (PE). However, th e rho(0) values for PG are smaller than the corresponding values for P E, which indicates a better miscibility of PG species compared to PE s pecies. According to an interpretation of the mixing behavior in terms of the difference in the pair-interaction energies between like-pair and mixed-pair formed in the mixture, this suggests that the inter-hea dgroup interaction acting in a PG mixture is stronger than that in a P E mixture.