Miscibility, of binary phospholipid mixtures under hydrated and non-hydrated conditions. II. Phosphatidylethanolamines with different acyl chain length

Abstract

Phase diagrams for binary mixtures of diacylphosphatidylethanolamines (PE) with different acyl chain length ( n = 14, 16, and 18, where n represents the number of carbon atoms per chain) were constructed by differential scanning calorimetry in the presence of and absence of added water, in order to examine the miscibility of these phospholipid species under fully hydrated and non-hydrated conditions. The phase diagrams were analyzed according to a thermodynamic relationship derived by applying the Bragg-Williams approximation to the non-ideality of mixing. The non-ideality parameters of mixing, ϱ 0 were estimated for binary mixtures of PE species in hydrated liquid-crystalline ( ϱ 0 (L)) and gel ( ϱ 0 (S)) bilayers, and in non-hydrated liquid ( ϱ 0 (L)) and solid ( ϱ 0 (S)) phases, based on which the mixing behavior of PE species in various phases was discussed. For non-hydrated mixtures, ϱ 0 (L) = 0 was obtained regardless of the chain length difference between the two components, demonstrating that the two components form an ideal solution in non-hydrated liquid phase. With other phases, positive values were obtained for ϱ 0 Both ϱ 0 (L) and ϱ 0 (S) for hydrated mixtures were larger than the corresponding values for non-hydrated mixtures, which indicates that the miscibility of PE with different chain length is poorer in hydrated bilayer than non-hydrated bulk phase. This difference in the mixing behavior between hydrated and non-hydrated PE mixtures was interpreted qualitatively in terms of the difference in pair-interactions between the hydrated lipid bilayer and the bulk lipid phase.