Investigating and Exploiting the Electrocatalytic Properties of Hydrogenases

This review regards the use of dynamic electrochemistry to study the mechanism of redox enzymes, with exclusive emphasis on the configuration where the protein is adsorbed onto an electrode and electron tranfer is direct.

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Direct Electrochemistry of Redox Enzymes as a Tool for Mechanistic Studies

Direct electron transfer of glucose oxidase promoted by carbon nanotubes

Synthetic Analogues and Reaction Systems Relevant to the Molybdenum and Tungsten Oxotransferases

Protein film voltammetry is a relatively new approach to studying redox enzymes, the concept being that a sample of a redox protein is configured as a film on an electrode and probed by a variety of electrochemical techniques. The enzyme molecules are bound at the electrode surface in such a way that there is fast electron transfer and complete retention of the chemistry of the active site that is observed in more conventional experiments. Modulations of the electrode potential or catalytic turnover result in the movement of electrons to, from, and within the enzyme; this is detected as a current that varies in characteristic ways with time and potential. Henceforth, the potential dimension is introduced into enzyme kinetics. The presence of additional intrinsic redox centers for providing fast intramolecular electron transfer between a buried active site and the protein surface is an important factor. Centers which carry out cooperative two-electron transfer, most obviously flavins, produce a particularl...

Enzyme electrokinetics: using protein film voltammetry to investigate redox enzymes and their mechanisms.

Protein film voltammetry of chicken liver sulfite oxidase (SO) bound at the pyrolytic graphite “edge” or modified gold electrodes shows that catalytic electron transport is controlled by the inherent electrochemical characteristics of the heme b domain and conformational changes that allow intramolecular electron transfer with the molybdenum active site. In the absence of sulfite, a single nonturnover electrochemical signal is observed at +90 mV (vs SHE) that is assigned to heme b. In the presence of sulfite, this signal transforms into a catalytic wave at similar potential. The shape and negligible pH dependence of this wave indicate that catalytic turnover is controlled by the one-electron transfers through heme b. The smaller turnover numbers obtained in this experiment (kcat ≈ 2−4 s-1, as compared to 100 s-1 in solution) suggest that only a small fraction of SO is bound at the electrode in a manner that permits the conformational change necessary for fast interdomain electron transfer.

A voltammetric study of interdomain electron transfer within sulfite oxidase.

Remote ligand substituent effects on the properties of oxo-Mo(V) centers with a single ene-1,2-dithiolate ligand

Complexes of the form (Tp * )MoOCl(p-OC 6 H 4 X) and (Tp * )MoO(p-OC 6 H 4 X) 2 (Tp * = hydrotris(3,5-dimethyl-1-pyrazolyl)-borate and X = OEt, OMe, Et, Me, H, F, Cl, Br, I, and CN) were examined by electrochemical techniques and gas-phase photoelectron spectroscopy to probe the effect of the remote substituent (X) on electron-transfer reactions at the oxomolybdenum core. Cyclic voltammetry revealed that all of these neutral Mo(V) compounds undergo a quasireversible one-electron oxidation (Mo V I /Mo V ) and a quasireversible one-electron reduction (Mo V /Mo I V ) at potentials that linearly depend on the electronic influence (Hammett σ p parameter) of X. The first ionization energies for (Tp * )MoO(p-OC 6 H 4 X) 2 (X = OEt, OMe, H, F, and CN) were determined by photoelectron spectroscopy. A nearly linear correlation was found for the Mo V I /Mo V oxidation potentials in solution and the gas-phase ionization energies. Calculated heterogeneous electron-transfer rate constants show a slight systematic dependence on the substituent.

Electrochemistry and Photoelectron Spectroscopy of Oxomolybdenum(V) Complexes with Phenoxide Ligands: Effect of Para Substituents on Redox Potentials, Heterogeneous Electron Transfer Rates, and Ionization Energies

Reaction of VO(acac)2 with 1,2-dithiols in the presence of triethylamine gives pentacoordinate oxovanadium complexes [HNEt3]2[VO(bdt)2] (1), [HNEt3]2[VO(tdt)2] (2), and [HNEt3]2[VO(bdtCl2)2] (3) (where H2bdt = 1,2-benzenedithiol, H2tdt = 3,4-toluenedithiol, and H2bdtCl2 = 3,6-dichloro-1,2-benzenedithiol). Compounds 1−3 have been characterized by IR, UV/visible, EPR, and mass spectroscopies. The X-ray crystal stuctures of 1 and 2 show hydrogen-bonding interactions between the terminal oxo atom and triethylammonium counterions and between ligand sulfur atoms and the counterions. These interactions are comparable with those found at the active sites of mononuclear molybdenum enzymes.

Anne E. Mcelhaney

Papers

A voltammetric study of interdomain electron transfer within sulfite oxidase.

Remote ligand substituent effects on the properties of oxo-Mo(V) centers with a single ene-1,2-dithiolate ligand

Electrochemistry and Photoelectron Spectroscopy of Oxomolybdenum(V) Complexes with Phenoxide Ligands: Effect of Para Substituents on Redox Potentials, Heterogeneous Electron Transfer Rates, and Ionization Energies

New oxovanadium bis(1,2-dithiolate) compounds that mimic the hydrogen-bonding interactions at the active sites of mononuclear molybdenum enzymes.

Electron transfer studies of dithiolate complexes: effects of ligand variation and metal substitution