Anomalous X-Ray Reflectivity Characterization of Ion Distribution at Biomimetic Membranes

Anomalous x-ray reflectivity measurements provides detailed information on ion binding to biomembrane surfaces. Using a monochromatic beam tuned to various x-ray energies at the Argonne National Laboratory Advanced Photon Source and utilizing a newly commissioned x-ray liquid surfaces reflectometer, measurements at and away from ion absorption edges allow determination of the distribution of these ions as they accumulate near lipid membranes. As a model, the interaction of Ba²⁺ ions with DMPA^- (1,2-dimyristoyl-$sn$-glycero-3-phosphatidic acid) monolayers at the aqueous surface is studied. We find an unexpectedly large concentration of barium at the interface, ≈1.5 per DMPA^-, forming a Stern layer of bound ions and a cloud of less densely bound ions near the lipid headgroups. This result can be understood only if one assumes that bound cations are partially speciated, e.g., as BaOH⁺.