Structure and phase behavior of hydrated mixtures of L-dipalmitoylphosphatidylcholine and palmitic acid. Correlations between structural rearrangements, specific volume changes and endothermic events

Abstract

Several new features of the phase diagram of L-dipalmitoylphosphatidylcholine (DPPC)/palmitic acid mixtures in excess water were established by means of static and time-resolved X-ray diffraction, densitometry and differential scanning calorimetry (DSC). At low temperatures, palmitic acid has a biphasic effect on the lamellar subgel phases: at concentrations below 5–6 mol%, it prevents formation of the DPPC subgel phase (L c ), while at higher contents (between about 40 and 90 mol%) another subgel phase (L c com) is formed as a result of lipid co-crystallization at 1 DPPC : 2 palmitic acid stoichiometry. A crystalline palmitic acid phase separates from L c com above 70–80 mol% of fatty acid. The L c com phase transforms into a lamellar gel phase (L β) in an endothermic transition centered at 38°C. At high temperatures, the mixtures form hexagonal liquid-crystalline phase (H II) in the region of 60–70 mol% and an isotropic phase (I) at 90–100 mol% of palmitic acid. No coexistence of H II phase with the fluid lamellar phase of DPPC was observed at intermediate compositions (20 and 50 mol% of palmitic acid) but rather formation of a complex phase with non-periodic geometry characterized by molten chains and a broad, continuous small-angle scattering band. No evidence for fluid phase coexistence was found also at compositions between H II and I phases. The L β-H II transition at 60–70 mol% of palmitic acids is readily reversible and two-state in both heating and cooling modes. It is characterized by the coexistence of initial and final phases with no detectable intermediates by time-resolved and static X-ray diffraction. The crystalline-isotropic transition in palmitic acid is two-state only in heating direction. On cooling, it is characterized by strong undercooling and gradually relaxing lamellar crystalline structures. The slowly reversible L c com-L β transition proceeds continuously through intermediate states. Although clearly discernible by both DSC and X-ray diffraction, it is not accompanied by specific volume changes.