Influence of Cholesterol on Molecular Motions in Spin-Labeled Lipid Bilayers Observed by Stimulated ESE

Electron spin echo (ESE) study was performed for spin-labeled lipids 1-palmitoyl-2-stearoyl-(5-d)-sn-glycero-3-phosphocholine in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine phospholipid bilayer. Recently (Isaev and Dzuba in J Phys Chem B 112:13285–13291, 2008), three-pulse stimulated ESE was shown to be sensitive to two types of orientational motion of spin labels in phospholipid bilayers at low temperatures (~100–150 K). The first one is fast stochastic libration, with correlation time on the nanosecond time scale. The second one is slow rotational motion, developing on the accessible for measurements microsecond time scale in a small range of reorientation angles, ~0.1°–1°. These two types of motions may be easily discriminated by dependences of the echo decay rates on the time delays between the pulses. The presence of cholesterol in lipid bilayers is found to suppress remarkably rotational motions, while on the contrary stochastic librations seem to become somewhat enhanced. These results evidence that cholesterol increases the long-time stability of lipid orientations in the bilayer, with simultaneous increase of fast fluctuations of these orientations. The former may be related to the known condensing effect of cholesterol and to raft formations, while the latter to the ordering effect.