Influence of head-group interactions on the miscibility of synthetic, stereochemically pure glycolipids and phospholipids.

Phase diagrams of binary mixtures of the glycoglycerolipids 1,2-di-O-tetradecyl-3-O-beta-D-galactosyl-sn-glycerol (14-Gal) and 1,2-di-O-tetradecyl-3-O-beta-D-glucosyl-sn-glycerol (14-Glc) with the phospholipids L-dimyristoylphosphatidylcholine (DMPC) and L-dimyristoylphosphatidylethanolamine (DMPE) were recorded by high-sensitivity differential scanning calorimetry and used for determination of the glycolipid-phospholipid miscibility in solid and liquid-crystalline states. As a consequence of a metastable behavior of both glycolipids and DMPE, the solid-state glycolipid/phospholipid miscibility was strongly dependent on the temperature prehistory of the samples. While DMPC and 14-Glc mix continuously, the other three binaries display extended regions of solid-solid-phase separation in the equilibrium low-temperature states. The DMPE/glycolipid phase diagrams were of clearly expressed eutectic type. Continuous solutions were formed in the liquid-crystalline and in the metastable solid phases of the mixtures. Simulations of the shape of the phase diagrams using the Bragg-Williams approximation showed certain deviations from ideal mixing in the liquid-crystalline continuous solutions. Since both glycolipids and phospholipids contain fully saturated fatty acids of equal chain length, their mixing properties were predominantly determined by the interactions between the lipid polar moieties, assuming the influence of ester or either linkages of the alkyl chains on the mixing parameters to be negligible. The clearly expressed differences in the mixing of 14-Glc and 14-Gal with phospholipids are most probably due to different hydrogen-bond networks formed by the glucosyl and galactosyl residues.