X-ray Standing-Wave X-ray Fluorescence study of the interaction of an anti-microbial peptide with a supported phospholipid bilayer

We have used standing wave x-ray fluorescence (SWXF) to measure the configuration of the anti-microbial peptide indolicidin within a solid supported phospholipid bilayer. To facilitate the SWXF measurements the indolicidin was labeled with a 2 nm diameter gold nanoparticle and a Si/Mo multilayer was used as the support with a repeat spacing chosen to be close to the bilayer thickness. The indolicidin was incubated into negatively charged liposomes formed from mixtures of (97.5 mole %:2.5 mole %) DMPC/DMPE at concentrations of 2.5, and 10 M and then deposited onto the crystalline supports. We find that for concentrations below 10M the indolicidin sits primarily at the top and bottom surfaces of the bilayer, while for 10M concentrations the indolicidin penetrates the interior of the bilayer.