Direct measurement of ion distributions between lipid membranes with X-ray diffraction

A new and simple method is introduced, which allows the direct measurement of the distribution of ions between lipid membranes with a conventional X-ray source. It is based on a difference method which is combined with a swelling experiment. The presented method is applied to unoriented powder samples of 1,2-dipalmitoyl-sn-glycero-3-phosphorylglycerol in different ionic solutions of RbCl and BaCl2. From these samples, results for the cation distributions with a resolution of 12 A degrees were obtained. Analysis of the experimentally obtained distributions shows that the simple Gouy-Chapman theory is probably not able to describe the experimental data consistently. Instead a better correspondence between experiment and theory is obtained with a generalized linear Gouy-Chapman model which takes into account the finite width of the lipid/electrolyte interface. Possible future improvements of the presented method with regard to the obtained resolution and the possibility to obtain ion densities on an absolute scale are discussed.