X-Ray Phase Contrast Imaging of Freestanding Lipid Model Membranes

Abstract

Membranes are considered as the most important interfaces in biology, and can be visualized under physiological conditions by optical techniques such as phase contrast and fluorescence light microscopy. While the contour lines and large lateral domains of biological membranes can be imaged, the density profile of the membrane and associated changes cannot be resolved by visible light. We report on hard x-ray phase contrast imaging of black lipid membranes (BLMs), which are freely suspended over a micro machined aperture in an aqueous solution. This new way of membrane structure analysis allows investigating bio molecular and organic sub-stances in aqueous environments by parallel and divergent beam propagation imaging, using partially coherent multi-keV x-ray radiation. The width of the thinning film is significantly smaller than the detector pixel size, but can be resolved from quantitative analysis of the intensity fringes in the Fresnel diffraction regime down to its native thickness of about 5nm. We have put forward a simplified but extendable model, which enables the theoretical description of image formation and charac-terization of membrane thickness and its decrease during the thinning process from a bulk to a bimolecular film. The structural changes can be obtained from both the loss of contrast and the asymmetry of the detected Fresnel fringes. On the basis of the recent experiments, future investigations will be performed to study the inte-ractions of membranes, as they are for example known from synaptic fusion, with high spatial resolution.