Anisotropic motion of cholesterol in oriented DPPC bilayers studied by quasielastic neutron scattering: the liquid-ordered phase.

Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.