X-ray Standing-Wave Fluorescence study of the transverse distribution of cholesterol within a supported phospholipid bilayer.*

Abstract:

Standing wave x-ray fluorescence (SWXF) was used to measure the distribution of cholesterol transverse to the support plane of a phospholipid bilayer. In order to monitor the position of the cholesterol, the molecule was brominated near the region of the hydrophilic head group and the fluorescence intensity from the bromine was monitored as a function of the phase of the x-ray standing wave maxima relative to the support surface. The supporting surface was comprised of a Si/Mo multilayer with a repeat spacing chosen to be approximatley the bilayer thickness. Results revealed that in the gel phase, fluorescence is best described if the cholesterol is uniformly distributed throughout the bilayer rather than localized near the hydrophilic interfaces, which is in agreement with neutron scattering results of Marquardt³.