Proton-coupled electron transfer in a model for tyrosine oxidation in photosystem II

Theoretical calculations of a model for tyrosine oxidation in photosystem II are presented. In this model system, an electron is transferred to ruthenium from tyrosine, which is concurrently deprotonated. This investigation is motivated by experimental measurements of the dependence of the rates on pH and temperature (Sj?din et al. J. Am. Chem. Soc. 2000, 122, 3932). The mechanism is proton-coupled electron transfer (PCET) at pH < 10 when the tyrosine is initially protonated and is single electron transfer (ET) for pH > 10 when the tyrosine is initially deprotonated. The PCET rate increases monotonically with pH, whereas the single ET rate is independent of pH and is 2 orders of magnitude faster than the PCET rate. The calculations reproduce these experimentally observed trends. The pH dependence for the PCET reaction arises from the decrease in the reaction free energies with pH. The calculations indicate that the larger rate for single ET arises from a combination of factors, including the smaller solvent reorganization energy for ET and the averaging of the coupling for PCET over the reactant and product hydrogen vibrational wave functions (i.e., a vibrational overlap factor in the PCET rate expression). The temperature dependence of the rates, the solvent reorganization energies, and the deuterium kinetic isotope effects determined from the calculations are also consistent with the experimental results.     

Centrality dependence of charged-hadron transverse-momentum spectra in d+Au collisions at sqrt[s(NN)]=200 GeV

We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25相似文献   

Synthesis of cubic ZrWMoO8 by a melt quenching method

We report a novel approach for fabrication of cubic ZrWMoO₈ applying a melt quenching method. The stoichiometric mixture of ZrO₂, WO₃ and MoO₃ in a 1:1:1 M ratio was melted and quenched at room temperature by pouring the melts between two metal plates. The quenched sample was characterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) analysis. As quenched material is polyphase contains above 70% ZrWMoO₈. The obtained composite is thermal stable up to 930 °C. The ZrWMoO₈ was indexed as cubic with a lattice parameter a = 9.1263 (3) Å calculated by the “PowderCell” program. By SEM observation it was established that the microstructure of bulk composite material is built up of one-direction orientated agglomerates with cuboid crystal morphology.     

Influence of heat treatment on the phase transition of ZrMo<Subscript>2</Subscript>O<Subscript>8</Subscript> and photocatalytic activity

ZrMo₂O₇(OH)₂·2H₂O was obtained from ZrOCl₂·2H₂O and Na₂MoO₄·2H₂O by a coprecipitation method. The phase and structural changes occurred during the heat-treatment of ZrMo₂O₇(OH)₂·2H₂O were investigated by XRD, IR and XPS analysis. The sequence of phase transformation can be divided into three stages: (1) transformation of ZrMo₂O₇(OH)₂·2H₂O to orthorhombic LT-ZrMo₂O₈ up to 300°C; (2) obtaining of mixture of both polymorphs of ZrMo₂O₈: cubic and trigonal at 400°C; (3) conversion to single trigonal (α) ZrMo₂O₈ above 450°C. The microstructure of the obtained trigonal (α) ZrMo₂O₈ was observed by scanning electron microscopy (SEM). The particle sizes were below 0.5 μm. The specific surface area was measured by modified BET method. The photocatalytic activity of the obtained trigonal (α) ZrMo₂O₈ powders was investigated by degradation of a model aqueous solution of Malachite Green (MG) upon UV-light irradiation.     

The Influence of Nd and Sm on the Structure and Properties of Sol-Gel-Derived TiO2 Powders

TiO₂ nanopowders modified by Nd and Sm were prepared using the sol-gel technique. It was found by XRD analysis that the samples containing Sm are amorphous up to 300 °C, while those with Nd preserve a mixed organic-inorganic amorphous structure at higher temperatures (400 °C). The TiO₂ (rutile) was not detected up to 700 °C in the presence of both modified oxides. TiO₂ (anatase) crystals found at about 400 °C in the Sm-modified sample exhibited an average crystallite size of about 25–30 nm, while doping with Nd resulted in particles of a lower size—5–10 nm. It was established by DTA that organic decomposition is accompanied by significant weight loss occurring in the temperature range 240–350 °C. Photocatalytic tests showed that the samples heated at 500 °C possess photocatalytic activity under UV irradiation toward Malachite green organic dye. Selected compositions exhibited good antimicrobial activity against E. coli K12 and B. subtilis.     

Glass formation in the MoO3-CuO-PbO system

Glasses in the MoO₃-CuO-PbO system are obtained at high cooling rates (10⁴-10⁵ K/s) and characterized using X-ray diffraction (XRD), differential thermal analysis (DTA), infrared (IR) and X-ray photoelectron spectroscopy (XPS). Two glass formation regions are determined: one with compositions having a high MoO₃ content (50-80 mol%) and the other in the PbO-rich compositions (65-80 mol%). In the region of MoO₃-rich compositions the building units of the amorphous network are МоО₆, МоО₄ and CuO₄ groups. For these high MoO₃ contents and respectively low PbO concentrations, the lead oxide is supposed to act as a network modifier while at high content PbO is found to be the main glass network former. In latter case the structure of glasses is formed by chains of PbO_n (n = 3, 4) polyhedra, between which there are isolated MoO₄ and CuO₄ complexes. IR and XPS data reveal the existence of Mo-O-Mo, Mo-O-Me(Me’) (where Me = Cu²⁺, Cu¹⁺ and Me’ = Pb) and Me(Me’)-O-Me(Me’) bonds in the amorphous network. Surprising result is found for low PbO content (10 mol%) where the lead oxide acts as glass network modifier: the actual MoO₃ content drops strongly which is accompanied with a significant increase of the actual CuO content with respect to their nominal MoO₃-CuO composition. Such effect is not observed in PbO-rich composition (70 mol%) where PbO has a role of network former.     

Centrality dependence of charged hadron transverse momentum spectra in Au+Au collisions from sqrt[s(NN)]=62.4 to 200 GeV

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt[s(NN)]=62.4 GeV. The spectra are presented for transverse momenta 0.25相似文献   

Theoretical study of electron, proton, and proton-coupled electron transfer in iron bi-imidazoline complexes.

A comparative theoretical investigation of single electron transfer (ET), single proton transfer (PT), and proton-coupled electron transfer (PCET) reactions in iron bi-imidazoline complexes is presented. These calculations are motivated by experimental studies showing that the rates of ET and PCET are similar and are both slower than the rate of PT for these systems (Roth, J. P.; Lovel, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122, 5486). The theoretical calculations are based on a multistate continuum theory, in which the solute is described by a multistate valence bond model, the transferring hydrogen nucleus is treated quantum mechanically, and the solvent is represented as a dielectric continuum. For electronically nonadiabatic electron transfer, the rate expressions for ET and PCET depend on the inner-sphere (solute) and outer-sphere (solvent) reorganization energies and on the electronic coupling, which is averaged over the reactant and product proton vibrational wave functions for PCET. The small overlap of the proton vibrational wave functions localized on opposite sides of the proton transfer interface decreases the coupling for PCET relative to ET. The theory accurately reproduces the experimentally measured rates and deuterium kinetic isotope effects for ET and PCET. The calculations indicate that the similarity of the rates for ET and PCET is due mainly to the compensation of the smaller outer-sphere solvent reorganization energy for PCET by the larger coupling for ET. The moderate kinetic isotope effect for PCET arises from the relatively short proton transfer distance. The PT reaction is found to be dominated by solute reorganization (with very small solvent reorganization energy) and to be electronically adiabatic, leading to a fundamentally different mechanism that accounts for the faster rate.     

Hydrogen Degree of Dissociation in a Low Pressure Tandem Plasma Source

ABSTRACT

Hydrogen dissociation degree in an inductively-driven tandem plasma source, operating at low pressure, is measured by applying optical actinometry method with an argon gas as actinometer. The gas temperature, a parameter in the investigations, is obtained from the rotational temperature, analyzing the intensity distribution of the Fulcher-α band. Several actinometric pairs are used for examining the dissociation degree and its pressure dependence. The influence?on the results?of the difference in the excitation cross sections for argon, commonly accepted, is analyzed. Two suitable actinometric pairs are proposed, one of which can also be applied for fast monitoring of the dissociation degree.