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Query: UMLS:C1862200 (RHE)
 1,093 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Antigen-combining site arises by noncovalent association of the variable domain of the immunoglobulin heavy chain (VH) with that of the light chain (VL). To analyze the invariant features of the binding region (VL-VH domain interface), we compared the known immunoglobulin three-dimensional structures by a variety of methods. The interface forms a close-packed, twisted, prism-shaped "beta-barrel" characterized by cross-sectional dimensions 1.04 X 0.66 nm and a top-to-bottom twist angle of 212 degrees. The geometry of the interface is preserved via invariance of some 15 side chains, both inside the domains and on their surface. Buried polar residues form a conserved hydrogen-bonding network that has a similar topological connectivity in the two domain types; two hydrogen bonds contributed by invariant side chains extend across the interface and anchor the beta-sheets in their relative orientation. Invariant aromatic residues close-pack at the bottom of the binding-site beta-barrel with their ring planes oriented perpendicularly in the characteristic "herringbone" packing mode. Electrostatic computations that implicitly include solvent effects show the domains to be stabilized by large electrostatic forces. However, structures that were crystallized at lower pH have their electrostatic energies appropriately lowered, implying that full ionization of carboxyl side chains is essential for efficient electrostatic stabilization. The unusual mode of domain-domain association in the VL-VL dimer RHE correlates with its overall repulsive electrostatic energy (+54 kJ/mol), as opposed to negative (i.e., stabilizing) energy values (-263 to -543 kJ/mol) found in the domains of the other structures. The VL-VL dimer REI mimics closely the interface geometry of VL-VH dimers although its domain-domain contact area is lower by 18%.
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PMID:Structural invariants of antigen binding: comparison of immunoglobulin VL-VH and VL-VL domain dimers. 392 86

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.
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PMID:Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys. 1281 15

We have reinvestigated the behavior of a Cu(111) electrode in pure and cinchonidine containing aqueous 0.1 M HClO4 solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (STM). In contrast to previous publications by Wan et al. (Langmuir 2000, 19, 1958-1962 and references cited therein) on Cu(111) in pure 0.1 M HClO4 which claimed an adsorbate-free Cu(111) surface in the entire potential range, we have found a highly ordered hexagonal adsorbate structure with a (4 x 4) unit cell, which is stable in the potential range from hydrogen evolution at -350 to -150 mV (RHE). The adsorbate-free (1 x 1) Cu(111) surface is only visible in a fairly small potential range from -150 to +50 mV. A disordered surface structure is formed at more positive potentials which is interpreted by adsorption of an oxygen-containing species. Furthermore, the formation of a highly ordered cinchonidine adlayer on Cu(111) in 0.1 M HClO4 as reported by Wan et al. (J. Am. Chem. Soc. 2002, 124, 14300-14301) could not be reproduced here. In fact, the similarity of all structures reported by Wan et al. for a great variety of different organic adlayers on Cu(111) in HClO4 solution including cinchonidine with the (4 x 4) superstructure found here already in pure HClO4 solution (i.e., without organic solute) casts serious doubts on the validity of those previous results by Wan et al. in general.
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PMID:On the existence of ordered organic adlayers at the Cu(111)/electrolyte interface. 1583 56

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.
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PMID:CO oxidation on Pt-modified Rh(111) electrodes. 1603 23

Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 V(RHE) in 0.1 M H(2)SO(4). In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 x 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.
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PMID:Preparation and electrochemical behavior of ordered rh adlayers on Pt(100) electrodes. 1604 77

Surface structure of a stepped surface of Pt, Pt(311) (=2(100)-(111)), has been determined under potential control in 0.1 M HClO4 with the use of in situ surface X-ray scattering (SXS). The crystal truncation rods (CTRs) are reproduced well with the (1x2) missing-row model. Relaxation of surface layers, which is observed on the low-index planes of Pt, is not found on Pt(311) in the "adsorbed hydrogen region". CTRs at 0.10 (RHE) have the same feature as those at 0.50 V, showing that the surface layers of Pt(311) have no potential dependence. Scanning tunneling microscopy (STM) also supports the (1x2) structure of Pt(311) in 0.1 M HClO4.
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PMID:In situ surface X-ray scattering of stepped surface of platinum: Pt(311). 1790 17

Electrochemical measurements were performed to characterize the kinetics of adsorbed CO oxidation on the surface of the stepped Pt(s)-[4(111)x(100)][triple bond, length half m-dash]Pt(335) single crystal electrode. For CO adsorbed to full coverage at 0.1 V (versus the reversible hydrogen electrode, RHE) in 0.5 M H(2)SO(4) at ambient temperature (23 degrees C), oxidation of the layer gave 7.6 x 10(14) +/- 0.3 CO/cm(2) as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 10(14) +/- 0.6 CO/cm(2)). In potential step measurements carried out between 0.75 and 0.9 V, the peak region in the current-time transient was consistent with the surface reaction between adsorbed CO and adsorbed oxide as rate limiting. Plotting the log of the rate constant for the surface reaction versus potential gave a Tafel slope of 79 mV per decade, consistent with responses for CO electrochemical oxidation on structurally related stepped Pt electrodes. For CO coverages below saturation, current-time transients were more stable in 0.05 M H(2)SO(4) than in the higher concentration electrolyte. Numerically solving the rate equations to the Langmuir-Hinshelwood model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients measured at 0.7 V in 0.05 M H(2)SO(4) for sub-saturation CO coverages. The results provide new insights into CO oxidation on Pt at sub-saturation coverage and confirm that anions play a role in CO surface chemistry.
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PMID:Kinetic studies of adsorbed CO electrochemical oxidation on Pt(335) at full and sub-saturation coverages. 1856 26

The electrochemical oxidation behaviors of the surfaces of platinum nanoparticles, one of the key phenomena in fuel cell developments, were investigated in situ and in real time, via time-resolved hard X-ray diffraction and energy dispersive X-ray absorption spectroscopy. Combining two complementary structural analyses, dynamical and inhomogenous structural changes occurring at the surfaces of nanoparticles were monitored on an atomic level with a time resolution of less than 1 s. After oxidation at 1.4 V vs RHE (reversible hydrogen electrode) in a 0.5 M H(2)SO(4) solution, longer Pt-O bonds (2.2-2.3 A that can be assigned to OHH and/or OH species) were first formed on the surface through the partial oxidation of water molecules. Next, these species turned to shorter Pt-O bonds (2.0 A, adsorbed atomic oxygen), and atomic oxygen was incorporated into the inner part of the nanoparticles, forming an initial monolayer oxide that had alpha-PtO(2)-like local structures with expanded Pt-Pt bonds (3.1 A). Finally, quasi-three-dimensional oxides with longer Pt-(O)-Pt bonds (3.5 A, precursor for beta-PtO(2)) grew on the surface, at almost 100 s after oxidation. Despite the very complex oxidation mechanism on the atomic level, XANES analysis indicated that the charge transfer from platinum to the adsorbed oxygen species was almost constant and rather small, that is, about 0.5 electrons per oxygen, up to two monolayers of oxygen. This means that ionic polarization hardly develops at this stage of the surface platinum's "oxide" growth.
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PMID:In situ and real-time monitoring of oxide growth in a few monolayers at surfaces of platinum nanoparticles in aqueous media. 1935 77

A single-source heterobimetallic complex Ni2Ti2(OEt)2(mu-OEt)6(acac)4 (1) (acac=2,4-pentanedionate), having a low decomposition temperature and sufficient solubility in organic solvents, was synthesized by simple chemical techniques in high yield and analyzed by melting point, FTIR, single crystal X-ray analysis and thermal analysis. The TGA analysis proved that complex (1) underwent facile thermal decomposition at 500 degrees C to give NiTiO3 residue. In-house designed aerosol assisted chemical vapor deposition equipment was used to deposit high quality thin films of NiTiO3 on a SnO2 coated conducting glass substrate at 500 degrees C. An XRD analysis of the thin films proved the formation of crystalline NiTiO3 with average grain size 42 nm. Scanning electron microscopic studies (SEM) show that the thin films consist of flat, plate-like nanoparticles. The current-potential characteristics recorded under AM1.5 illumination indicate that NiTiO3 thin films are anodic and the photocurrent density at 1.23 V vs RHE (Reversible Hydrogen Electrode) is about 40 microA cm(-2).
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PMID:Photooxidation of water by NiTiO3 deposited from single source precursor [Ni2Ti2(OEt)2(micro-OEt)6(acac)4] by AACVD. 1941 32

The ultra-small silicon nanoparticle was shown to be an electrocatalyst for the electrooxidation of glucose. The oxidation appeared to be a first order reaction which involves the transfer of 1 electron. The oxidation potential showed a low onset of -0.4V vs. Ag/AgCl (-0.62 V vs. RHE). The particle was used as the anode catalyst of a prototype hybrid biofuel cell, which operated on glucose and hydrogen peroxide. The output power of the hybrid cell showed a dependence on the enzymes used as the cathode catalyst. The power density was optimized to 3.7 microW/cm(2) when horseradish peroxidase was replaced by microperoxidase-11 (MP-11). Comparing the output power of the hybrid cell to that of a biofuel cell indicates enhanced cell performance due to the fast reaction kinetics of the particle. The long-term stability of the hybrid cell was characterized by monitoring the cell voltage for 5 days. It appeared to that the robustness of the silicon particle resulted in more cell stability compared to the long-term performance of a biofuel cell.
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PMID:A hybrid biofuel cell based on electrooxidation of glucose using ultra-small silicon nanoparticles. 1942 31


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