Miscibility of binary phospholipid mixtures under hydrated and non-hydrated conditions. IV. Phosphatidylglycerols with different acyl chain length

Abstract

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 conditions was examined in terms of the binary phase diagram constructed by 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 approximation, and the non-ideality parameter of mixing, ϱ 0, was estimated for various phases of PG mixture. The ϱ 0 values obtained for hydrated 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 ϱ 0 values with respect to the lipid phases is similar to that obtained previously for homologous diacylphosphatidylethanolamines (PE). However, the ϱ 0 values for PG are smaller than the corresponding values for PE, which indicates a better miscibility of PG species compared to PE species. 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-headgroup interaction acting in a PG mixture is stronger than that in a PE mixture.