Fluorescence quenching in lecithin and lecithin/cholesterol liposomes by parmagenetic lipid analogues. Introduction of a new probe approach.
1. Perylene, whether incorporated into lecithin or lecithin/cholesterol (1:1) liposomes, exhibits identical fluorescence spectra, but fluorescence in the presence of cholesterol is enhanced by 30-50%. 2. The fluorescence of perylene in pure dipalmitoyllecithin vesicles increases sharply at the transition temperature (Tt equals 41 degrees C). No such fluorescence jump is observed in lecithin/cholesterol (1:1) micelles. 3. In lecithin liposomes maximal quenching of perylene fluorescence at 25 degrees C is effected by cholestane spin label (80%) followed by androstane spin label (70%), 5-nitroxide stearate (60%) and 16-nitroxide stearate (50%). 4. In liposomes containing 5  mol % cholesterol these differences are reduced; however, the sequence of quenching efficiencies is the same except for the nitroxide stearates, which interchange their positions. 5. 5. Paramagnetic quenching of perylene fluorescence is stable below 35 degrees C and above 45 degrees C, but decreases sharply about the phase-transition temperature of dipalmitoyllecithin. 6. In lecithin/cholesterol (1:1, molar ratio) lipsomes fluorescence quenching diminishes linearly, but only slightly, with increasing temperature. 7. Cholestane spin label and androstane spin label at concentrations of greater than 20 mol % themselves suppress the quenching discontinuity at Tt, indicating a cholesterol-like structural effect. 8. The quenching phenomena observed are attributed to a non-random accommodation of fluorophore and quencher molecules (co-clustering) below the phase transition and a statistical distribution of both impurities above Tt. 9. In the presence of cholesterol the clustering tendencies are reduced or even  eliminated; this is compatible with the concept that cholesterol fluidizes the phosphatide acyl chains below the transtion temperature.
D000731:Androstanes; D001665:Binding Sites; D002784:Cholesterol; D004578:Electron Spin Resonance Spectroscopy; D008081:Liposomes; D008433:Mathematics; D008567:Membranes, Artificial; D008954:Models, Biological; D008968:Molecular Conformation; D010713:Phosphatidylcholines; D013050:Spectrometry, Fluorescence; D013113:Spin Labels; D013229:Stearic Acids; D013696:Temperature
D000731:Androstanes; D008081:Liposomes; D008567:Membranes, Artificial; D010713:Phosphatidylcholines; D013113:Spin Labels; D013229:Stearic Acids; D002784:Cholesterol