Small-Angle Orientational Motions of Spin-Labeled Lipids in Cholesterol-Containing Bilayers Studied at Low Temperatures by Electron Spin Echo Spectroscopy

Abstract

Electron spin echo (ESE) study was performed for a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer containing an admixture of lipids (of 1:100 molar ratio) spin-labeled at the polar head or at different positions along the acyl chain and optionally containing additionally cholesterol that substituted POPC lipids in a 50:50 molar ratio. ESE signal was observed below 150-115 K, depending on the label position. Three-pulse stimulated ESE is sensitive to two types of orientational motion of spin labels at these temperatures. The first type of motion (which is also seen in a two-pulse primary ESE experiment) is fast stochastic librations, with a correlation time in the nanosecond scale. The second type of motion is slow millisecond rotations, which in the accessible microsecond time scale are seen as small-angle reorientations. (Restricted rotations also refer to this type of motion.) We found that in presence of cholesterol, fast librations are enhanced at all label positions. This effect may be related to the cholesterol ordering effect. Rotational motions in the presence of cholesterol are remarkably suppressed near the bilayer surface, which may be readily explained by the cholesterol condensing effect. In contrast to that, in the bilayer center rotations in the presence of cholesterol are remarkably enhanced. This effect may be attributed to topological properties of a cholesterol-containing membrane system in which beyond the steroid core, more freedom appears for small-angle motions, as compared with the cholesterol-free case. The found property may be important for physiological temperatures, as well. In particular, it may imply that cholesterol weakens interaction between two leaflets in the bilayer.