The Reduction of Dioxygen

A reduction to hydrogen peroxide is obtained with enzymes with one copper atom of type-2 (Table 1). A reduction to water requires enzymes with a dimeric site, with the three types of copper or with a four-metal center (2 heme, 2 Cu) (Table 1). 

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Porphyrinic co-complexes as novel multinuclear catalysts for the reduction of dioxygen directly to water 97ACR437. 

Complex IV consists of 13 peptides, two heme A groups (cytochrome a and a3> and two or three Cu atoms (Table 2). It spans the inner membrane and protrudes into the intermembrane space. Complex IV catalyzes the reduction of dioxygen by oxidized cytochrome c, and four protons derived from the matrix are consumed in the reaction. 

Le Mest Y, Inisan C, Laouenan A, L Her M, Talarmin J, El Khalifa M, Saillard J-Y. 1997. Reactivity toward dioxygen of dicobalt face-to-face diporphyrins in aprotic media. Experimental and theoretical aspects. Possible mechanistic implication in the reduction of dioxygen. J Am Chem Soc 119 6095. 

Ni CL, Abdalmuhdi I, Chang CK, Anson FC. 1987. Behavior of four anthracene-linked dimeric metalloporphyrins as electrocatalysts for the reduction of dioxygen. J Phys Chem 91 1158. 

Shigehara K, Anson EC. 1982. Electrocatal3dic activity of three iron porphyrins in the reduction of dioxygen and hydrogen peroxide at graphite cathodes. J Phys Chem 86 2776. 

Wan G-X, Shigehara K, Tsuchida E, Anson EC. 1984. Virtues of a copolymer containing pyr-rolidone and iron porphyrin groups in the catalysis of the reduction of dioxygen at graphite electrodes. J Electroanal Chem 179 239. 

Anson and co-workers have shown that two Co ions were not necessary for four-electron 02 reduction.266 The mew-substituted complex porphyrin Co(TPyP) (42) complex bears four active pyridyl donors which readily react with four equivalents of [Ru(NH3)5(OH2)]2+ to produce the tetra-ruthenated derivative. The four Ru centers are sufficiently remote that their RuIII/n potentials coincide. Under steady state conditions [Co(TPyP)] Ru(NH3)5 4]8+ (43) adsorbed onto a pyrolytic graphite working electrode catalyzes the reduction of dioxygen (Figure 6). 

D. Dolphin Collman and Anson have found with their five-atom bridged amide systems that the reduction of dioxygen is partitioned roughly one third undergoing a four-electron reduction to water and the rest a two-electron reduction to peroxide. We see the same result indeed the 5-5 systems of ours and that of Collman-Anson are indistinguishable. Neither type of cofacial dimers with chains greater than five show any four-electron reduction. 

Many organic and inorganic compounds, fibers, and particles are capable of damaging nucleic acids by generating reactive oxygen species via the reduction of dioxygen. These stimuli include different classes of organic compounds, classic prooxidants (anticancer antibiotics, various quinones, asbestos fibers, and so on), and even antioxidants, which can be oxidized in the presence of transition metal ions. 

The electrons, having passed through the transport chain to the terminal enzyme, are taken up by the reduction of dioxygen,... 

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

FIGURE 4.21. Catalysis of the reduction of dioxygen by a Ru(NH3)6 2+-GoIItetrakis(4-Af-methylpyridyl) porphyrin-Nafion film. 

FIGURE 4.23. Catalysis of the reduction of dioxygen by a Ru(NH3)g+-CoI1[tetrakis(4-A -methylpyridyl) porphyrin-Nafion film. Optimization of the film thickness in terms of catalytic efficiency. Adapted from Figure 5.12 of reference 17d, with permission from John Wiley Sons. 

The water-soluble Fe porphyrin, 3Na+ [Fe(III)(TPPS)] -12H20 [H2TPPS4- = tetra-anionic form of meso-tetrakis(7r-sulfonatophenyl)porphine], has recently been shown to be an effective catalyst for the electroreduction of nitrite to ammonia [419]. The Fe meso-tetrakis(A -methyl-4-pyridyl) porphyrin and/or the Fe meso-tetrakis (jr -sulfophenyl) porphyrin complex shows a catalytic activity for the reduction of dioxygen in aqueous solutions, leading to hydrogen peroxide [420]. 

The question arises as to the actual sequence of events that end up in the reduction of dioxygen into water. Even though complementary investigation is needed before a complete answer could be proposed, several observations are worth mentioning provisionally. The reoxidation of reduced heteropolyanions by dioxygen after the catalytic processes in which they participate is a popular practice. Its mechanism was generally and indistinctly explained to go through an intermediate adduct formation accompanied by inner sphere electron transfers [164]. 

The half-reaction expressing the reduction of dioxygen to water in acid solution (equation 5.2) has an value of + 1.23 V when the activity of the hydrogen ion is 1 mol dm. The Nernst equation for the dioxygen/ water couple is ... 

As mentioned above, in an earlier work, Nohl et al. [9] suggested that neutral ubisemi-quinone reduced dioxygen to superoxide (this suggestion was dropped in subsequent studies of these authors). Although the participation of neutral semiquinone in the reduction of dioxygen is impossible, the observation of these authors might be interpreted as the support of a role of ubihydroquinone in mitochondrial superoxide production. If neutral semiquinone is indeed formed in mitochondria via the protonation of semiquinone radical anion (Reaction... 

Scheme 1. The proposed mechanism for the reduction of dioxygen to hydrogen peroxide in the presence of excess [Co([14]aneN4)(OH2)2l3 (1).

The demonstration that PMNs formed O2- in the respiratory burst necessitated the consideration of all the species which result when dioxygen is reduced one electron at a time (Fig. 1). Superoxide, the result of the reduction of dioxygen by one electron, appears to act mainly as a mild reductant in aqueous solutions. But when it coexists with H2O2, its spontaneous dismutation product, O can initiate a number of potentially injurious events [reviewed by Fridovich The primary means by which cells deal with superoxide anions appears to be through the catalysis of their dismutation by a family of metalloenzymes collectively designated superoxide dismutases. 

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