ALTERED MOLECULAR-MOTION IN PHOSPHOLIPID HEADGROUPS DURING L-ALPHA-H-II TRANSITION INDUCED BY CHOLESTEROL

Cholesterol is highly concentrated in the synaptic membranes which und ergo frequent membrane fusion and is believed to play an important rol e in the lipid phase transition that accompanies fusion. To better und erstand the phosholipid phase transition and associated molecular moti on of phospholipids during the lamellar to inverted hexagonal transiti on, two major methods were used. First, sn-1,2-dioleoylglycerol was ad ded to synthetic vesicles, and inverted hexagonal formation was detect ed using P-31-NMR. Multilamellar sheets were composed of 1,2-dioleoyl- 3-sn-phosphatidylcholine, 1,2-dioleoyl-3-sn-phosphatidylethanolamine a nd cholesterol (1:1:1 in molar ratio) and 5 mol%-sn-1,2-dioleoylglycer ol was added to transform lamellar to inverted hexagonal structures. H owever, the transition also required cholesterol. Multilamellar vesicl es were composed of phosphatidylcholine, phosphatidylethanolamine and cholesterol and were investigated using a time-resolved fluorescence d epolarization method with dansyl-phosphatidylethanolamine as a fluores cent probe. The molecular motion of phospholipid headgroups decreased during the lamellar to inverted hexagonal phase transition, suggesting that the headgroups of phospholipid molecules are more tightly packed in inverted hexagonal structure than in lamellar form of membrane.