Phase behavior of the palmitic acid/palmitin system.: A 2H NMR study

The phase behavior of mixtures of palmitic acid (PA) and 1-monohexadecanoyl-rac-glycerol, palmitin, was studied by phase contrast microscopy and deuterium solid-state NMR. At pH 5, mixtures remained precipitated as lumps in solution. The NMR spectrum of the perdeuterated PA (PAd(31)) at 300 K exhibited a shape and quadrupolar splittings, Deltanu, characteristic of lipids embedded in a gel phase. The alkyl chains remained in a trans conformation with their long molecular axis oriented at about 15degrees with respect to the bilayer normal. However, gauche defects were shown to occur at the end of the alkyl chain. At 330 K, the system underwent a phase transition to a hexagonal phase followed by an isotropic phase at 340 K. Upon cooling to 330 K, the spectrum in the hexagonal phase was oriented at 0degrees showing that the cylinders were oriented with their long axis parallel to the field. Up to 11 positions (from 15) of PAd(31) could be assigned. At pH 7 and 9 at room temperature, the mixtures were fully dispersed in a viscous solution of vesicles. The system underwent a phase transition at 320 K from a gel phase to a fluid phase with the bilayer normal oriented at 90degrees with respect to the field. Analogous experiments performed with PA selectively labeled on carbon C-2 allowed for the assignment of Deltanu for that position and suggested different conformations of the headgroup in the gel and fluid or hexagonal phases. The implications of these findings for the bio-availability of these fatty acids, in the understanding of the contribution of hydroxyl and carboxyl groups in the membrane formation, and for the production of simple self-oriented systems are discussed.