Nathaniel A. Anderson

Regulation of the Electric Charge in Phosphatidic Acid Domains

Although a minor component of the lipidome, phosphatidic acid (PA) plays a crucial role in nearly all signaling pathways involving cell membranes, in part because of its variable electrical charge in response to environmental conditions. To investigate how charge is regulated in domains of PA, we applied surface-sensitive X-ray reflectivity and fluorescence near-total-reflection techniques to determine the binding of divalent ions (Ca(2+) at various pH values) to 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA) and to the simpler lipid dihexadecyl phosphate (DHDP) spread as monolayers at the air/water interface. We found that the protonation state of PA is controlled not only by the pK(a) and local pH but also by the strong affinity to PA driven by electrostatic correlations from divalent ions and the cooperative effect of the two dissociable protons, which dramatically enhance the surface charge. A precise theoretical model is presented providing a general framework to predict the protonation state of PA. Implications for recent experiments on charge regulation by hydrogen bonding and the role of pH in PA signaling are discussed in detail.

Structure of biodegradable films at aqueous surfaces: X-ray diffraction and spectroscopy studies of polylactides and tyrosine-derived polycarbonates.

Three representative polymers of increasing modulus, poly(d,l-lactic acid), PDLLA, poly(desaminotyrosyl-tyrosine ethyl ester carbonate), PDTEC, and the same polymer with iodinated DTE segments, PI2DTEC, were characterized by surface-pressure versus area (Π-A) isotherms and surface sensitive X-ray diffraction techniques. Films of 10-100 Å thickness were prepared for these studies by spreading dilute polymer solutions at air-water interfaces. The general properties of the isotherms and the Flory exponents, determined from the isotherms, vary in accordance with the increasing modulus of PDLLA, PDTEC, PI2DTEC, respectively. The analysis of in situ X-ray reflectivity and grazing incidence X-ray diffraction (GIXD) measurements from films at aqueous surfaces provides a morphological picture that is consistent with the modulus of the polymers, and to a large extent, with their packing in their dry-bulk state. Large absorption of X-rays by iodine enabled X-ray spectroscopic studies under near-total-reflection conditions to determine the iodine distribution in the PI2DTEC film and complement the structural model derived from reflectivity and GIXD. These structural studies lay the foundation for future studies of polymer-protein interactions at aqueous interfaces.

Halide ions effects on surface excess of long chain ionic liquids water solutions.

The interfacial structure and composition of water solutions with alkylimidazolium ionic liquids varying in their halide anions ([C12mim][X], X = Cl and I) were investigated by X-ray near-total-reflection fluorescence spectroscopy and X-ray reflectivity measurements. We demonstrate that X-ray fluorescence and reflectivity techniques provide a more direct measurement of surface adsorption. Furthermore, we show that for [C12mim][Cl] and [C12mim][I] solutions with mixed inorganic salts (NaI, NaCl), I(-) ions replace Cl(-) above the critical micelle concentration (CMC) of [C12mim][Cl] at much lower concentrations of NaI, whereas NaCl concentrations a hundred times higher than the CMC of [C12mim][I] only partially replace the I(-) at the interface. Our surface-sensitive X-ray diffraction and spectroscopy provide two independent tools to directly determine the surface adsorption of ionic surfactants and the interfacial composition of the surface films.

Intercalated europium metal in epitaxial graphene on SiC

X-ray magnetic circular dichroism (XMCD) reveal the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nano-clusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M

Magnetite nano-islands on silicon-carbide with graphene

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Reverse-engineering of graphene on metal surfaces: a case study of embedded ruthenium

edges at

An X-ray fluorescence study on the segregation of Cs and I in an inverted organic solar cell

T = 15

Magnetic properties of Dy nano-islands on graphene

K, our samples show paramagnetic-like behavior with distinct anomaly at T

Magnetic properties of intercalated rare-earth metals under graphene

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Magnetic properties of intercalated Dy and Eu under Graphene

90 K which may be related to the N{\`e}el transition, T