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 mu eV) was employed to study high-frequency cholesterol motion in the liqui d ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatid ylcholine 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 respe ct to the incident neutron beam. This and the two energy resolutions for ea ch orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separate ly at two different time scales in the GHz range. We find a surprisingly hi gh, model-independent motional anisotropy of cholesterol within the bilayer . The data analysis using explicit models of molecular motion suggests a su perposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined m otion within the bilayer plane. The rather high amplitude of the out-of-pla ne diffusion observed at higher temperatures (T greater than or equal to 36 degrees C) strongly suggests that cholesterol can move between the opposit e leaflets of the bilayer while it remains predominantly confined within it s host monolayer at lower temperatures (T = 20 degrees C). The locally conf ined in-plane cholesterol motion is dominated by discrete, large-angle rota tional jumps of the steroid body rather than a quasicontinous rotational di ffusion by small angle jumps. We observe a significant increase of the rota tional jump rate between T = 20 degrees C and T = 36 degrees C, whereas a f urther temperature increase to T = 50 degrees C leaves this rate essentiall y unchanged.