Mechanisms of N-Demethylations Catalyzed by High-Valent Species of Heme Enzymes:  Novel Use of Isotope Effects and Direct Observation of Intermediates

1998 ◽  
Vol 120 (41) ◽  
pp. 10762-10763 ◽  
Author(s):  
Yoshio Goto ◽  
Yoshihito Watanabe ◽  
Shunichi Fukuzumi ◽  
Jeffrey P. Jones ◽  
Joseph P. Dinnocenzo
Keyword(s):  
Direct Observation ◽  
Isotope Effects ◽  
Heme Enzymes ◽  
High Valent

Computer-Generated High-Valent Iron-Oxo and Manganese-Oxo Species with Polyoxometalate Ligands: How do they Compare with the Iron-Oxo Active Species of Heme Enzymes?

2004 ◽  
Vol 116 (42) ◽  
pp. 5779-5783 ◽  
Author(s):  
Samu�l P. de Visser ◽  
Devesh Kumar ◽  
Ronny Neumann ◽  
Sason Shaik
Keyword(s):  
Active Species ◽  
Heme Enzymes ◽  
High Valent

scholarly journals Roles of multiple-proton transfer pathways and proton-coupled electron transfer in the reactivity of the bis-FeIV state of MauG

2015 ◽  
Vol 112 (35) ◽  
pp. 10896-10901 ◽  
Author(s):  
Zhongxin Ma ◽  
Heather R. Williamson ◽  
Victor L. Davidson
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Second Phase ◽  
Side Chains ◽  
Alternative Pathways ◽  
Proton Coupled Electron Transfer ◽  
Valent State ◽  
Two Phases ◽  
High Valent ◽  
Solvent Isotope

The high-valent state of the diheme enzyme MauG exhibits charge–resonance (CR) stabilization in which the major species is a bis-FeIV state with one heme present as FeIV=O and the other as FeIV with axial heme ligands provided by His and Tyr side chains. In the absence of its substrate, the high-valent state is relatively stable and returns to the diferric state over several minutes. It is shown that this process occurs in two phases. The first phase is redistribution of the resonance species that support the CR. The second phase is the loss of CR and reduction to the diferric state. Thermodynamic analysis revealed that the rates of the two phases exhibited different temperature dependencies and activation energies of 8.9 and 19.6 kcal/mol. The two phases exhibited kinetic solvent isotope effects of 2.5 and 2.3. Proton inventory plots of each reaction phase exhibited extreme curvature that could not be fit to models for one- or multiple-proton transfers in the transition state. Each did fit well to a model for two alternative pathways for proton transfer, each involving multiple protons. In each case the experimentally determined fractionation factors were consistent with one of the pathways involving tunneling. The percent of the reaction that involved the tunneling pathway differed for the two reaction phases. Using the crystal structure of MauG it was possible to propose proton–transfer pathways consistent with the experimental data using water molecules and amino acid side chains in the distal pocket of the high-spin heme.

Kinetics and mechanism of the oxidation of alcohols by ferrate ion

1993 ◽  
Vol 71 (9) ◽  
pp. 1394-1400 ◽  
Author(s):  
Donald G. Lee ◽  
Huifa Gai
Keyword(s):  
Transition Metal Oxides ◽  
Isotope Effects ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Reaction Rates ◽  
Radical Intermediates ◽  
Kinetic Isotope ◽  
Oxidation Of Alcohols ◽  
Aliphatic Ether ◽  
High Valent

A kinetic study of the reduction of ferrate ion under basic conditions has been completed. The observation that a typical aliphatic ether, tetrahydrofuran, is oxidized at a rate comparable to that of aliphatic alcohols, such as cyclopentanol, indicates that the reaction between ferrate and alcohols is likely initiated by attack of the oxidant at an α-C—H bond, a conclusion that is consistent with the occurrence of primary deuterium kinetic isotope effects (2.8–4.3 at 25 °C) when α-hydrogens are replaced by deuterium. Because only acyclic products are obtained from the oxidation of cyclobutanol by ferrate, it may be concluded that free radical intermediates are involved in the reaction. The insensitivity of the reaction rates to substituent effects during the oxidation of substituted mandelic acids indicates that substantial charges are not built up in the transition state. All of these observations are most readily accommodated by a mechanism in which the reaction is initiated by a 2 + 2 addition of an Fe=O bond to the α-C—H of an alcohol to give an organometallic intermediate that subsequently decomposes by homolytic cleavage of the resulting C—Fe bond. Comparisons are made with the reactions between alcohols and other high-valent transition metal oxides.

Kinetics and mechanism of the oxidation of alcohols by ruthenate and perruthenate ions

1990 ◽  
Vol 68 (10) ◽  
pp. 1774-1779 ◽  
Author(s):  
Donald G. Lee ◽  
Ligaya N. Congson
Keyword(s):  
Organic Chemistry ◽  
Free Radical ◽  
Transition Metal ◽  
Metal Ions ◽  
Mandelic Acid ◽  
Isotope Effects ◽  
Transition Metal Ions ◽  
Oxidation Of Alcohols ◽  
Deuterium Isotope Effects ◽  
High Valent

In addition to being important synthetic reagents, ruthenate and perruthenate are of interest because they are representative of a number of high valent transition metal ions that can be used as oxidants in organic chemistry. In their reactions with alcohols both ions exhibit several striking similarities — concave Hammett plots, primary deuterium isotope effects, small enthalpies of activation, and large negative entropies of activation. The most likely explanation of the concave upward Hammett plots is the involvement of free radical-like transition states that could be formed by the decomposition of organometallic intermediates. Keywords: oxidation, ruthenate, perruthenate, alcohols, mandelic acid.

Mechanisms of Sulfoxidation Catalyzed by High-Valent Intermediates of Heme Enzymes:  Electron-Transfer vs Oxygen-Transfer Mechanism

1999 ◽  
Vol 121 (41) ◽  
pp. 9497-9502 ◽  
Author(s):  
Yoshio Goto ◽  
Toshitaka Matsui ◽  
Shin-ichi Ozaki ◽  
Yoshihito Watanabe ◽  
Shunichi Fukuzumi
Keyword(s):  
Electron Transfer ◽  
Oxygen Transfer ◽  
Transfer Mechanism ◽  
Heme Enzymes ◽  
High Valent

Direct Observation of Heat Exchanger Deposits by Cross Sectioning

1967 ◽  
Vol 25 ◽  
pp. 364-365
Author(s):  
R. W. Anderson ◽  
D. L. Senecal
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Heat Exchanger ◽  
Direct Observation ◽  
Cross Sections ◽  
Preparation Method ◽  
Specimen Preparation ◽  
Flowing Water ◽  
Transmission Electron ◽  
Deposit Layer

A problem was presented to observe the packing densities of deposits of sub-micron corrosion product particles. The deposits were 5-100 mils thick and had formed on the inside surfaces of 3/8 inch diameter Zircaloy-2 heat exchanger tubes. The particles were iron oxides deposited from flowing water and consequently were only weakly bonded. Particular care was required during handling to preserve the original formations of the deposits. The specimen preparation method described below allowed direct observation of cross sections of the deposit layers by transmission electron microscopy.The specimens were short sections of the tubes (about 3 inches long) that were carefully cut from the systems. The insides of the tube sections were first coated with a thin layer of a fluid epoxy resin by dipping. This coating served to impregnate the deposit layer as well as to protect the layer if subsequent handling were required.

Application of a Simple Cold Stub to the Direct Observation of Frozen Hydrated Sand

1973 ◽  
Vol 31 ◽  
pp. 212-213
Author(s):  
John M. Wehrung ◽  
Richard J. Harniman
Keyword(s):  
United States ◽  
Surface Water ◽  
Direct Observation ◽  
Controlled Study ◽  
Clay Particles ◽  
Organic Debris ◽  
Rock Formations ◽  
Water Tables ◽  
Permeable Rock ◽  
Natural Means

Water tables in aquifer regions of the southwest United States are dropping off at a rate which is greater than can be replaced by natural means. It is estimated that by 1985 wells will run dry in this region unless adequate artificial recharging can be accomplished. Recharging with surface water is limited by the plugging of permeable rock formations underground by clay particles and organic debris.A controlled study was initiated in which sand grains were used as the rock formation and water with known clay concentrations as the recharge media. The plugging mechanism was investigated by direct observation in the SEM of frozen hydrated sand samples from selected depths.

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