DO THE LONG FATTY-ACID CHAINS OF SPHINGOLIPIDS INTERDIGITATE ACROSS THE CENTER OF A BILAYER OF SHORTER CHAIN SYMMETRICAL PHOSPHOLIPIDS

Novel cerebroside sulfate (CBS) spin labels containing long chain C24 or C26 fatty acids with a nitroxide spin label on the 22nd carbon were synthesized and used to investigate the ability of the long fatty aci d chains of glycosphingolipids to interdigitate across the center of a non-interdigitated bilayer of phospholipids formed of symmetric satur ated or unsaturated shorter fatty acid chain species, in the presence or absence of cholesterol. The motion of these long chain spin labels incorporated at 1 mole% in dimyristoylphosphatidylcholine (diC14-PC), dipalmitoylphosphatidylcholine (diC16-PC), distearoylphosphatidylcholi ne (diC18-PC), dibehenoylphosphatidylcholine (diC22-PC), sphingomyelin (SM), 1-stearoyl-2-oleoylphosphatidylcholine (18:0,18: I-PC), and dim yristoylphosphatidylethanolamine (diC14-PE) was compared to that of CB S spin labels containing stearic acid spin labeled at the 5th carbon a nd at the 16th carbon. The results indicated that the C26 chain is int erdigitated in the gel phase of diC14-PC, diC16-PC, SM, and possibly d iC18-PC, but not diC14PE, and the C24 chain may interdigitate in diC14 -PC but not in the other phospholipids. Thus in order to interdigitate across the center of gel phase bilayers, the long acyl chain of the s phingolipid probably must be long enough to nearly span the phospholip id bilayer. The inability to interdigitate in diC14-PE is likely due t o the close packing of this lipid in the gel phase. The C26 chain may also be interdigitated in these lipids in the presence of cholesterol at low temperatures. However, at physiological temperatures in the pre sence of cholesterol and in the liquid-crystalline phase of all the li pids, the results indicate that the long acyl chain of the glycosphing olipid is not interdigitated, but rather must terminate at the bilayer center. This may force the carbohydrate headgroup of the glycosphingo lipid farther above the bilayer surface, allowing it to be recognized better by various carbohydrate binding ligands and proteins.