Richard A. Decréau

A Cytochrome c Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-Limiting Electron Flux

We studied the selectivity of a functional model of cytochrome c oxidases active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244. To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer–coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Using a functional enzyme model to understand the chemistry behind hydrogen sulfide induced hibernation

The toxic gas H2S is produced by enzymes in the body. At moderate concentrations, H2S elicits physiological effects similar to hibernation. Herein, we describe experiments that imply that the phenomenon probably results from reversible inhibition of the enzyme cytochrome c oxidase (CcO), which reduces oxygen during respiration. A functional model of the oxygen-reducing site in CcO was used to explore the effects of H2S during respiration. Spectroscopic analyses showed that the model binds two molecules of H2S. The electro-catalytic reduction of oxygen is reversibly inhibited by H2S concentrations similar to those that induce hibernation. This phenomenon derives from a weak, reversible binding of H2S to the FeII porphyrin, which mimics heme a3 in CcOs active site. No inhibition of CcO is detected at lower H2S concentrations. Nevertheless, at lower concentrations, H2S could have other biological effects on CcO. For example, H2S rapidly reduces FeIII and CuII in both the oxidized form of this functional model and in CcO itself. H2S also reduces CcOs biological reductant, cytochrome c, which normally derives its reducing equivalents from food metabolism. Consequently, it is speculated that H2S might also serve as a source of electrons during periods of hibernation when food supplies are low.

A functional nitric oxide reductase model

A functional heme/nonheme nitric oxide reductase (NOR) model is presented. The fully reduced diiron compound reacts with two equivalents of NO leading to the formation of one equivalent of N2O and the bis-ferric product. NO binds to both heme Fe and nonheme Fe complexes forming individual ferrous nitrosyl species. The mixed-valence species with an oxidized heme and a reduced nonheme FeB does not show NO reduction activity. These results are consistent with a so-called “trans” mechanism for the reduction of NO by bacterial NOR.

Metal corroles as electrocatalysts for oxygen reduction

The rotating ring disk electrode method has been used to study O2 electroreduction with metal corroles. Catalysis begins at potentials that are 0.5-0.7 V more positive than the expected potential of the M(III/II) couple based on studies in non-aqueous solutions. The path of O2 reduction depends on the nature of the metal ion. Cobalt corroles promote O2 reduction to H2O2. Iron corroles catalyse O2 reduction via parallel two- and four-electron pathways, with a predominate four-electron reaction. The rate constants for the individual O2 reduction paths are given at pH 7. Mechanisms are proposed on the basis of pH dependence, inhibition studies, and Tafel slopes. An imidazole-tailed iron corrole catalyses H2O2 disproportionation analogous to catalase.

Multiple active oxidants in competitive epoxidations catalyzed by porphyrins and corroles.

We demonstrate the existence of multiple active oxygenating species in porphyrin and corrole-catalyzed competitive epoxidations of styrene and cis-cyclooctene.

Electrochemical applications. How click chemistry brought biomimetic models to the next level: electrocatalysis under controlled rate of electron transfer

This tutorial review discusses the immobilization of alkyne-terminated cytochrome c oxidase models on azide-functionalized self-assembled monolayers (SAM) coated gold electrodes that was made possible by click chemistry. The rate of electron delivery from the electrode to the model could be tuned by changing the nature of the SAM. Biologically relevant electron transfer rates (2-4 s(-1)) were obtained on slow SAMs allowing the model to turn over catalytically under steady-state conditions. Hence, click chemistry was a crucial tool to demonstrate, through electrocatalytic studies: (1) the role played by several features present in the distal side of the model, such as the Cu(B)-Tyr244 pair, the distal pocket, and the stabilizing role of a distal water cluster; (2) the reversible inhibition of O(2) reduction by H(2)S.

Role of a distal pocket in the catalytic O2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

Five iron porphyrins with different superstructures were immobilized on self-assembled-monolayer (SAM)-coated interdigitated-array (IDAs) gold–platinum electrodes. The selectivity of the catalysts i.e., limited formation of partially reduced oxygen species (PROS) in the electrocatalytic reduction of dioxygen, is a function of 2 rates: (i) the rate of electron transfer from the electrode to the catalyst, which is controlled by the length, and conjugation of the linker from the catalyst to the electrode and (ii) the rate of bound oxygen (superoxide) hydrolysis, which correlates with the presence of a water cluster in the gas-binding pocket influencing the rate of oxygen binding; these factors are controlled by the nature of the porphyrin superstructure. The structurally biomimetic Tris-imidazole model is the most selective.

Catalytic Reduction of O2 by Cytochrome c Using a Synthetic Model of Cytochrome c Oxidase

Cytochrome c oxidase (CcO) catalyzes the four-electron reduction of oxygen to water, the one-electron reductant Cytochrome c (Cytc) being the source of electrons. Recently we reported a functional model of CcO that electrochemically catalyzes the four-electron reduction of O(2) to H(2)O (Collman et al. Science 2007, 315, 1565). The current paper shows that the same functional CcO model catalyzes the four-electron reduction of O(2) using the actual biological reductant Cytc in a homogeneous solution. Both single and steady-state turnover kinetics studies indicate that O(2) binding is rate-determining and that O-O bond cleavage and electron transfer from reduced Cytc to the oxidized model complex are relatively fast.

Intermediates Involved in the Two Electron Reduction of NO to N2O by a Functional Synthetic Model of Heme Containing Bacterial NO Reductase

Reaction of a functional biferrous heme/nonheme model complex at low temperature leads to the formation of a distal nonheme nitrosyl followed by a trans heme nonheme bis-nitrosyl intermediate. The EPR and Raman data on this intermediate indicate that the two nitrosyl centers are close. This complex gives off N2O and provides support for the trans mechanism proposed for NOR enzymes.

Microwave-assisted synthesis of corroles

The recently developed Grosss method for the synthesis of corroles has been modified and successfully applied to the preparation of new free base tris-aryl- and tris-pyrimidyl-corroles using solvent-free conditions and microwave irradiation. Compared to conventional heating, the microwave technique afforded an increase in corrole yields of ca. 30% and led to noticeably cleaner reaction mixtures. It is demonstrated that short reaction times and high temperatures are required to afford optimum yields.