Change of motion and localization of cholesterol molecule during Lα-HII transition

Formation of the inverted hexagonal (H-II) phase from the lamellar (L-alpha) phase of bovine brain-extracted phosphatidylcholine (BBPC) and phosphatidylethanolamine (BBPE) was investigated using P-31-NMR with or without cholesterol. When the ratio of BBPC to BBPE was 1:1, the H-II formation was observed in the presence of 33 mol% cholesterol (i.e., BBPC:BBPE:cholesterol = 1:1:1) at 47 degrees C. The fraction of the H-II phase in the BBPC/BBPE/cholesterol system could be controlled by the addition of dioleoylglycerol. The change of molecular motion of cholesterol affected by the H-II formation was measured at various ratios of the L-alpha to H-II phase with the time-resolved fluorescence depolarization method, using dehydroergosterol as a fluorescent probe. It is observed that the motion of cholesterol became vigorous in the mixture state of the L-alpha, and the H-II phases compared to that in the L-alpha or the H-II phase only. These facts show that cholesterol has the strong ability to induce the H-II phase, probably by special molecular motion, which includes change of its location from the headgroup area to the acyl-chain area.