DIRECT MEASUREMENT OF ION DISTRIBUTIONS BETWEEN LIPID-MEMBRANES WITH X-RAY-DIFFRACTION

A new and simple method is introduced, which allows the direct measure ment of the distribution of ions between lipid membranes with a conven tional X-ray source. It is based on a difference method which is combi ned with a swelling experiment. The presented method is applied to uno riented powder samples of 1,2-dipalmitoyl-sn-glycero-3-phosphorylglyce rol in different ionic solutions of RbCl and BaCl2. From these samples , results for the cation distributions with a resolution of 12 Angstro m were obtained. Analysis of the experimentally obtained distributions shows that the simple Gouy-Chapman theory is probably not able to des cribe the experimental data consistently. Instead a better corresponde nce between experiment and theory is obtained with a generalized linea r Gouy-Chapman model which takes into account the finite width of the lipid/electrolyte interface. Possible future improvements of the prese nted method with regard to the obtained resolution and the possibility to obtain ion densities on an absolute scale are discussed.