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Roger A Sheldon - One of the best experts on this subject based on the ideXlab platform.

Martin Newcomb - One of the best experts on this subject based on the ideXlab platform.

  • spectra and kinetic studies of the compound i derivative of cytochrome p450 119
    Journal of the American Chemical Society, 2008
    Co-Authors: Xin Sheng, John H Horner, Martin Newcomb

    The Compound I derivative of cytochrome P450 119 (CYP119) was produced by laser flash photolysis of the corresponding Compound II derivative, which was first prepared by reaction of the resting enzyme with peroxynitrite. The UV−vis spectrum of the Compound I species contained an asymmetric Soret band that could be resolved into overlapping transitions centered at ∼367 and ∼416 nm and a Q band with λmax ≈ 650 nm. Reactions of the Compound I derivative with organic substrates gave epoxidized (alkene Oxidation) and hydroxylated (C−H Oxidation) products, as demonstrated by product studies and oxygen-18 labeling studies. The kinetics of Oxidations by CYP119 Compound I were measured directly; the reactions included hydroxylations of benzyl alcohol, ethylbenzene, Tris buffer, lauric acid, and methyl laurate and epOxidations of styrene and 10-undecenoic acid. Apparent second-order rate constants, equal to the product of the equilibrium binding constant (Kbind) and the first-order Oxidation rate constant (kox), we...

  • laser flash photolysis generation of high valent transition metal oxo species insights from kinetic studies in real time
    Accounts of Chemical Research, 2008
    Co-Authors: Rui Zhang, Martin Newcomb

    High-valenttransition metal−oxo species are active oxidizing species in many metal-catalyzed Oxidation reactions in both Nature and the laboratory. In homogeneous catalytic Oxidations, a transition metal catalyst is oxidized to a metal−oxo species by a sacrificial oxidant, and the activated transition metal−oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic Oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseco...

  • laser flash photolysis generation and kinetic studies of porphyrin manganese oxo intermediates rate constants for Oxidations effected by porphyrin mnv oxo species and apparent disproportionation equilibrium constants for porphyrin mniv oxo species
    Journal of the American Chemical Society, 2005
    Co-Authors: Rui Zhang, John H Horner, Martin Newcomb

    Porphyrin−manganese(V)−oxo and porphyrin−manganese(IV)−oxo species were produced in organic solvents by laser flash photolysis (LFP) of the corresponding porphyrin−manganese(III) perchlorate and chlorate complexes, respectively, permitting direct kinetic studies. The porphyrin systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP), and 5,10,15,20-tetrakis(4-methylpyridinium)porphyrin (TMPyP). The order of reactivity for (porphyrin)MnV(O) derivatives in self-decay reactions in acetonitrile and in Oxidations of substrates was (TPFPP) > (TMPyP) > (TPP). Representative rate constants for reaction of (TPFPP)MnV(O) in acetonitrile are k = 6.1 × 105 M-1 s-1 for cis-stilbene and k = 1.4 × 105 M-1 s-1 for diphenylmethane, and the kinetic isotope effect in Oxidation of ethylbenzene and ethylbenzene-d10 is kH/kD = 2.3. Competitive Oxidation reactions conducted under catalytic conditions display approximately the same relative rate constants as were found i...

John M Woodley - One of the best experts on this subject based on the ideXlab platform.

  • automated determination of oxygen dependent enzyme kinetics in a tube in tube flow reactor
    Chemcatchem, 2017
    Co-Authors: Rolf Hoffmeyer Ringborg, Asbjorn Toftgaard Pedersen, John M Woodley

    Enzyme-mediated Oxidation is of particular interest to synthetic organic chemists. However, the implementation of such systems demands knowledge of enzyme kinetics. Conventional wisdom holds that collecting kinetic data for biocatalytic Oxidations is fraught with difficulties such as limited oxygen supply and low oxygen solubility in water. We present here a novel method for the collection of such kinetic data using a pressurized tube-in-tube reactor, operated in the low-dispersed flow regime to generate time-series data. Experimental development and validation of the instrument reveal not only the high accuracy of the kinetic data obtained, but also the necessity of making measurements in this way to enable the accurate evaluation of high K_MO enzyme systems, with minimal material consumption. For the first time this paves the way to integrate kinetic data into the protein engineering cycle.

Elim Albiter - One of the best experts on this subject based on the ideXlab platform.

  • photosensitized Oxidation of 9 10 dimethylanthracene with singlet oxygen by using a safranin o silica composite under visible light
    Photochemical and Photobiological Sciences, 2015
    Co-Authors: Elim Albiter, Salvador Alfaro, M.a. Valenzuela

    The photosensitized Oxidation of 9,10-dimethylanthracene with singlet oxygen in acetonitrile was investigated using a safranin O/silica composite as an heterogeneous delivery system of the photosensitizer. The only detected product was the corresponding endoperoxide (9,10-endoperoxianthracene) and its formation rate depended on the initial concentration of DMA, the light intensity and the amount of the composite. The kinetics of this reaction was compared with that of the reported kinetic model of photosensitized Oxidations of organic compounds in homogeneous reactions. It was found that both reactions followed the same model, suggesting that the actual reaction between photoproduced singlet oxygen and 9,10-dimethylanthracene was performed in homogeneous media and the surface of the composite was not involved in the reaction.

Halina Wojtowicz - One of the best experts on this subject based on the ideXlab platform.

  • selenium promoted Oxidation of organic compounds reactions and mechanisms
    European Journal of Organic Chemistry, 2003
    Co-Authors: Jacek Mlochowski, Monika Brząszcz, Miroslaw Giurg, Jerzy Palus, Halina Wojtowicz

    Oxidation reactions are fundamental processes widely applied in organic synthesis. Elemental selenium and more often its compounds have been successfully used as stoichiometric reagents and catalysts for Oxidation of different organic substrates. Selenium(IV) oxide, areneseleninic acids and their anhydrides are widely used as stoichiometric oxidants or as oxygen-transfer agents for oxygen donors, particularly hydrogen peroxide and tert-butyl hydroperoxide. Organic diselenides (the precursors of seleninic acids) have been used as Oxidation catalysts while dimethyl and diphenyl selenoxides are stoichiometric oxidants. Selenenamides, such as 2-phenyl-1,2-benzisoselenazol-3(2H)-one (ebselen) and its analogues, known as glutathione peroxidase mimics acting via active hydroperoxide intermediates, are efficient and selective Oxidation catalysts. Selenium(IV) oxide and some organoselenium compounds have been successfully applied for various Oxidations useful in practical organic syntheses such as epOxidation, 1,2-dihydroxylation, and α-oxyfunctionalization of alkenes as well as for ring contraction of cycloalkanones, conversion of halomethyl, hydroxymethyl, or active methylene groups into formyl groups, Oxidation of aldehydes into carboxylic acids, sulfides into sulfoxides, and secondary amines into nitrones, regeneration of parent carbonyl compounds from their azomethine derivatives and for other reactions. The Oxidation mechanisms depend on the substrate and oxidant or catalyst used. The electrophilic center localized on the selenium atom or the nucleophilic center localized on the oxygen atom of the selenahydroperoxide group are involved in the reaction mechanism. In both cases the selenium-containing moiety is a good leaving group. Exceptionally Oxidation can proceed via free radical selenium species. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)