title: Anomalous x-ray reflectivity characterization of ion distribution at biomimetic membranes.
abstract: 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, approximately 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+.
authors.affiliation: Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: David
authors.initials: D
authors.lastname: Vaknin
authors.forename: Peter
authors.initials: P
authors.lastname: Krüger
authors.forename: Mathias
authors.initials: M
authors.lastname: Lösche
mesh_terms: D040761:Biomimetic Materials / Q000737:chemistry*; D008563:Membrane Lipids / Q000737:chemistry*; D008567:Membranes, Artificial; D008956:Models, Chemical; D010712:Phosphatidic Acids / Q000737:chemistry*; D012542:Scattering, Radiation; D013052:Spectrometry, X-Ray Emission / Q000379:methods; D014965:X-Rays
chemical_list: D008563:Membrane Lipids; D008567:Membranes, Artificial; D010712:Phosphatidic Acids