Dioxygen reductive activation

Dioxygen reduction (oxidase activity) and activation for incorporation into organic substrates are catalysed by a number of mononuclear non-haem iron enzymes. We will first consider the intramolecular dioxygenases, in which both atoms of oxygen are introduced into the substrate, then the monoxygenases (in which we choose to include the pterin-dependent hydroxylases), the large family of a-hetoacid-dependent enzymes, and finally isopenicillin N-synthase. 

A polypyrrole film electrochemically deposited on gold electrodes from an MeCN-liCl04/Co(OAc)2 solution shows electrocatalytic activity in dioxygen reduction [404]. The catalytic electroreduction of dithio dipropionic acid (PSSP) with the water-soluhle cohalt(II I)tetrakis(4-trimethyl-ammonium phenyl) porphyrin (CoTMAP) has heen studied. The Co catalyst adsorbed on the glassy carbon electrode plays a major role in the electroreductive cleavage of the S—S bond [405]. 

Lien, H.-L. and Wilkin, R. Reductive activation of dioxygen for degradation of methyl ferf-butyl ether by bifunctional aluminum. Environ. Sci. Technol, 36(20) 4436-4440, 2002. 

A mechanism in which reductive activation of dioxygen precedes the C—H bond cleavage cannot explain such a tight coupling of oxygen and substrate consumption. (From Crespo et ah, 2006)... 

The enzymes reductively activate dioxygen using NADPH as an electron source. One oxygen atom is then reduced to water and the other atom is transferred to a substrate, resulting in the hydroxylation of alkenes and arenes, the epoxidation of alkenes and the formation of N-oxides and S-oxides from amino and sulphur compounds. Other P-450 reactions include N-dealkylation, O-dealkylation and reductase-like dehalogenation of halocarbons. Typical P-450 reactions are summarised in Ihble 5-4. 

The molecule is very stable and can be sublimed [i]. Numerous metal phthalocyanines can reversibly bind molecules like, e.g., dioxygen at the metal ion. This can result in activation of internal bonds and subsequent facilitation of chemical reaction, in this case of dioxygen -> electroreduction. Thus these molecules have attracted attention as catalysts for various reactions, in particular dioxygen reduction in, e.g., fuel cells [ii], in general -> electrocatalysis [iii] and in -> sensors [iv]. Their strong coloration, which can be modified electrochemically by reduction/oxidation, suggests use in -> electrochromic devices [v]. 

The active site metal center reductively activates dioxygen during the catalytic turnover cycle. Oxygenation of the ferrous [Fe(II)] metal center in the resting enzyme is... 

The dioxygen reduction site of the key respiratory enzyme, cytochrome c oxidase [E.C. 1.9.3.1], is a bimetallic catalytic center comprised of a heme iron adjacent to a Type 2 mononuclear copper center (see Cytochrome Oxidase). The recent solution of the X-ray crystal structure of this enzyme revealed an entirely unanticipated covalent modification of the protein structure, a cross-link between a histidine and tyrosine side chain (23) within the active site (Figure 2)." This extraordinary posttranslational modification has been confirmed by peptide mapping and mass spectrometry, and has been detected as a conserved element in cytochrome c oxidases isolated from organisms ranging from bacteria to cows. The role of the cross-linked structure in the function of cytochrome c oxidase is still controversial." " ... 

Matsuo T, Hayashi T, Hisaeda Y. Reductive Activation of dioxygen by a myoglobin reconstituted with a flavohemin. J. Am. Chem. Soc. 2002 124 11234-11235. 

Electrochemical data indicate that self-assembled monolayers of 5 and 6 catalyze the two-electron reduction of O2 to H2O2. The monolayer from 6 is a more effective electrocatalyst for the reduction of O2 than that from 5 [300]. The different reactivity results from different interfacial architecture this is confirmed by infrared, X-ray photoelectron, and visible spectroscopic measurements [300] which revealed coplanar, inclined t -7z stacking of the porphyrin ring in the monolayer of 5 and head-to-tail orientation of the porphyrin ring in the monolayer of 6. Treatment of the monolayer of 8 with Co(OAc)2 in methanol resulted in electrocatalytic activity in the reduction of O2 [300]. In contrast, a monolayer of 7 treated similarly failed to catalyze dioxygen reduction [300], although treatment of a mixed monolayer of 7 and CH3(CH2)3SH with Co(OAc)2 results in electrocatalytic activity similar to that of 6. 

NHA oxime or proton donation from protonated N to the heme species are suggested as likely mechanistic pathways [106], Regardless of the final mechanism, the L-Arg and NHA-bound structures of NOS imply that NOS catalysis selects between two different reductive activations of dioxygen. 

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

Among the plethora of solution phase, electrocatalytically active macrocycles for dioxygen reduction, only very few have been found to display no affinity for electrode surfaces. Attention in this Section will be mostly focused on the intrinsic electrochemical and electrocatalytic properties of two of such unique materials ... 

The macrocycles Co (111) (cyclam) and Fe( 111)TM PyP display high activity for dioxygen reduction and negligible affinity for carbonaceous surfaces providing close to ideal conditions to warrant analyses of electrochemical data within the strict homogeneous electrocatalysis framework. Their most salient features are summarized in the two sub-sections to follow. 

Yet another important aspect that can shed light on the mechanism of dioxygen reduction is the ability of the adsorbed catalyst to reduce and/or chemically decompose hydrogen peroxide. For example, CoTPP and CoTPyP (and certainly bare OPG) display no activity for hydrogen peroxide reduction (n = 2) in the potential region of... 

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