STRUCTURE AND THERMOTROPIC BEHAVIOR OF MIXED CHOLINE PHOSPHOLIPID MODEL MEMBRANES

Phosphatidylcholines and sphingomyelins are among the most abundant li pids in mammalian cell membranes, being major components of platelets or erythrocytes, and of the lipid monolayer of lipoproteins. General e fforts have been devoted to the elucidation of the interaction of the ubiquitous membrane component cholesterol with these choline phospholi pids, but fewer studies have been reported on the interaction between the phospholipids themselves. A gel to liquid-crystalline phase transi tion was observed for pure sphingomyelin liposomes at physiological te mperature, while palmitoyloleoyl phosphatidylcholine adopts a liquid-c rystalline phase in the temperature range 273-323 K. The two phospholi pids are miscible at all molar ratios in the liquid-crystalline phase, characterized by very similar lamellar repeat distances for all binar y lipid mixtures. The gel phase of pure sphingolipid liposomes exhibit ed a markedly smaller lamellar repeat distance as compared to mixed li pid vesicles, which increased slightly with temperature for the pure s phingomyelin (66.9-69.2 Angstrom). Concomitantly, altered hydrocarbon chain packing was observed. Similar diffractograms were obtained in th e presence of 10 mol% phosphatidylcholine. However, in the composition range between 20 and 60 mol% phosphatidylcholine in the phosphatidylc holine-sphingomyelin admixture, the lamellar repeat distance in the ge l phase was markedly increased and remained almost constant (around 75 Angstrom) below the phase transition.