Dioxygen chemical reduction

Scheme 3.3 Generalized mechanism for the macrocyclic-mediated reduction of dioxygen. Chemical and electrochemical processes are labeled in lower and upper case, respectively.

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

The Functional Model. Synthesis of a chemically similar or dissimilar molecule that will mimic the desired function of the enzyme be it dioxygen transport, electron transport, isomerization, or small molecule reduction. 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

Although reduction potentials may be estimated for half-reactions, there are limits for their values that correspond to both members of a couple having stability in an aqueous system with respect to reaction with water. For example, the Na+/Na couple has a standard reduction potential of -2.71 V, but metallic sodium reduces water to dihydrogen. The reduced form of the couple (Na) is not stable in water. The standard reduction potential for the Co3 + / Co2 + couple is +1.92 V, but a solution of Co3+ slowly oxidizes water to dioxygen. In this case the oxidized form of the couple is not stable in water. The standard reduction potential for the Fe3T/Fe2+ couple is +0.771 V, and neither oxidized form or reduced form react chemically with water. They are subject to hydrolysis, but are otherwise both stable in the aqueous system. The limits for the stability of both oxidized and reduced forms of a couple are pH dependent,... 

Electrochemical (24) and chemical (25, 26) techniques have been utilized to investigate the kinetics and the mechanisms of the addition of dioxygen to a metal center, and to follow its subsequent reduction to hydrogen peroxide when catalyzed by cobalt(III) complexes of macro-cyclic amine ligands. Such complexes have also been involved in the general investigation of dioxygen addition to cobalt complexes (27,28). 

Owing to lanthanide contraction, niobium and tantalum have virtually identical atomic rad (1.47 A) and close ionization energies (Nb6.67, Ta7.3eV), and usually display very similt chemical behavior. Some definite differences can however be noted these can usually be trace to the lower sensitivity of tantalum to reduction and to its higher affinity for dioxygen. lb tantalum-element multiple bonds are usually stabler, while MfiXg arrangements are so ft known only for niobium. 

In this mechanism, fresh metal oxide, Mn(OH)2, is electrochemically reduced and reformed by a chemical oxidation of the surface by the dioxygen in the solution. The scheme satisfies the requirement that the reduction potential and rate of reaction be dependent on the electrode material, M, and pH. The net ... 

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 focus of this article will be upon those aspects of the structure and function of cytochrome c oxidase that contribute particularly to an understanding of the chemical events that lead to the reduction of dioxygen to water. This important function is, however, only one aspect of its physiological role. The functioning enzyme is provided with electrons from the electron transport chain by cytochrome c, uses these electrons to reduce dioxygen bound at the active site, communicates the energy released in this reduction to the site of oxidative phosphorylation,... 

The chemical inertness of dioxygen at first seems surprising because the transformation to water is so strongly thermodynamically favorable ( 80 kcal) (Fig. 1) (28, 29). However, on the basis of the standard redox potentials, the simplest reduction step, the one-electron step to... 

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