The structure and orientation of water molecules at a highly ordered Au(111) electrode surface in perchloric acid have been investigated in-situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. This newly developed infrared spectroscopy technique enables the observation of the electrode/electrolyte interface at a very high sensitivity without interference from the bulk solution. The spectrum of the interfacial water significantly differs from that of bulk water and drastically changes in peak frequencies and band widths around the potential of zero charge (pzc) of the electrode and at about 0.3 V positive from the pzc. The interfacial water molecules are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded ice-like structure at potentials slightly above the pzc. The ice-like structure is broken at more positive potentials due to the specific adsorption of perchlorate ion, where one OH moiety of water is non-hydrogen-bonded and the other OH moiety is hydrogen-bonded to another water molecule. The intensities of the fundamental modes of water are also a strong function of applied potential. They are very weak around the pzc and increase as the potential changes in both positive and negative directions. These results are explained in terms of the potential-dependent reorientation of water molecules from oxygen-up to oxygen-down as the surface charge changes from negative to positive. The adsorption of hydronium and perchlorate ions on gold is also discussed.

No abstract

Infrared transmission spectra of molecules adsorbed on silver island films evaporated on CaF2 have been investigated. The spectra are remarkably simple compared with those of the molecules in the solid state (KBr pellets). Only the vibrational modes which give dipole changes perpendicular to the metal surface are infrared active. In addition, their intensities are about 200 times larger than those of the free molecule. These results can be fully accounted for if the electric field which excites the surface molecule is perpendicular to the local surface of the metal islands and is stronger than the incident electric field. The origin of the absorption enhancement and the surface selection rule is discussed theoretically by using a classical electromagnetic model.

Potential-dependent reorientation of a water molecule, adsorption of sulfate, and interactions between water and sulfate on a highly ordered Au(111) electrode surface in sulfuric acid solutions have been investigated in situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. The spectrum of the water layer at the interface changes in both intensity and frequency as the applied potential changes due to the reorientation of water molecules. The orientations deduced from infrared spectra are in good agreement with the predictions made by molecular dynamics simulations at potentials below and around the potential of zero charge (pzc) of the electrode where sulfate adsorption is negligible. At potentials above the pzc, sulfate anion is adsorbed at 3-fold hollow sites on the (111) surface via three oxygen atoms. When the potential is increased and the fractional coverage of sulfate reaches to about one-half of full coverage, adsorbed sulfate anions start to form short-ranged domains and greatly change the water layer structure. Water molecules stabilize the sulfate domains by bridging neighboring sulfate anions via hydrogen bonding. The weak tunneling spots observed in the reported scanning tunneling microscopy images of the well-ordered (√3×√7) sulfate adlayers on (111) metal surfaces are attributed to water molecules that bridge adjacent adsorbed sulfate anions.

Abstract:A novel concept is introduced for the oriented incorporation of membrane proteins into solid supported lipid bilayers. Recombinant cytochrome c oxidase solubilized in detergent was immobilized on a chemically modified gold surface via the affinity of its histidine-tag to a nickel-chelating nitrilo-triacetic acid (NTA) surface. The oriented protein monolayer was reconstituted into the lipid environment by detergent substitution. The individual steps of the surface modification, including (1) chemical modification of the gold support, (2) adsorption of the protein, and (3) reconstitution of the lipid bilayer, were followed in situ by means of surface-enhanced infrared absorption spectroscopy (SEIRAS) and accompanied by normalmode analysis. The high surface sensitivity of SEIRAS allows for the identification of each chemical reaction process within the monolayer at the molecular level. Finally, full functionality of the surface-tethered cytochrome c oxidase was demonstrated by cyclic voltammetry after binding of the natural electron donor cytochrome c.

Electrochemically induced infrared difference spectra of cytochrome c on various chemically modified electrodes (CMEs) are recorded by exploiting the surface-enhancement exerted by a granular gold film. We have recently developed surface-enhanced infrared difference absorption spectroscopy (SEIDAS), which provides acute sensitivity to observe the minute enzymatic change of a protein on the level of a monolayer. By these means, we demonstrate that the relative band intensities in the potential-induced difference spectra of adsorbed cytochrome c are significantly dependent on the type of CME used (mercaptopropionic acid, mercaptoethanol, 4,4'-dithiodipyridine, or L-cysteine). These differences are attributed to the altered interaction of cytochrome c with the headgroup of the various CMEs leading to variations in surface orientation and relative distance from the surface. Nevertheless, the peak positions of the observed bands are identical among the CMEs employed. This implies that the internal conformational changes induced by the redox reaction of the adsorbed cytochrome c are not disturbed by the interaction with the CME and that full functionality of the protein is retained. Finally, we critically discuss our results within the framework of the different models for cytochrome c adsorption on CMEs.

No abstract

a b s t r a c t[Fe]-hydrogenase is one of three types of enzymes known to activate H 2 . Crystal structure analysis recently revealed that its active site iron is ligated square-pyramidally by Cys176-sulfur, two CO, an ''unknown" ligand and the sp 2 -hybridized nitrogen of a unique iron-guanylylpyridinol-cofactor. We report here on the structure of the C176A mutated enzyme crystallized in the presence of dithiothreitol (DTT). It suggests an iron center octahedrally coordinated by one DTT-sulfur and one DTToxygen, two CO, the 2-pyridinol's nitrogen and the 2-pyridinol's 6-formylmethyl group in an acyliron ligation. This result led to a re-interpretation of the iron ligation in the wild-type.