Dioxygen reduction mechanism

As seen from the above scheme, XO reduces dioxygen into hydrogen peroxide by two-electron reduction mechanism and into superoxide by one-electron reduction mechanism. The efficiency of superoxide production depends on the nature of the substrate (in addition to... 

Nitrobenzyl chlorides are also reduced by microsomes through one-electron reduction mechanism. Moreno et al. [47] suggested that p- and o-nitrobenzyl chlorides are reduced by rat hepatic microsomes to unstable radical anions, which are decomposed to form benzyl radicals under anaerobic conditions. However, in the presence of dioxygen the radical anions of these compounds participate in futile redox cycling yielding superoxide (Figure 24.2). In contrast to p- and o-nitrobenzyl chlorides, m-nitrobenzyl chloride was reduced by microsomes to a relatively stable m-nitrobenzyl radical anion. 

Malmslrom, B. G. The mechanism of dioxygen reduction in cytochrome c oxidase and laccase. In Oxidases and Related Redox Systems (King, T. E., Mason, H. S., Morrison, M., eds.), Oxford-New York, Pergamon Press, 1981... 

Even more controversy arose when, in the early 1990s, several research groups reported the formation of dihydrogen peroxide instead of water as the product of dioxygen reduction in the catalytic oxidation of DTBCH2 by the copper(II) complexes [35, 36]. In order to explain their experimental results, Chyn and Urbach proposed two different mechanisms for the catalytic cycle, as depicted in Scheme 5.1 [35]. 

In addition to the mechanism of dioxygen reduction, an understanding of how Cco pumps protons is also desirable. Models have been proposed that allow for linkage of the proton pumping to the dioxygen reduction reaction (50). One attractive model involves the CuA site and is shown in Figure 6 (10). In this mechanism, the CuA center is ligated to two histidines and two thiolates and receives the initial electron from... 

Cytochrome c is the sole physiological source of electrons for dioxygen reduction at cytochrome c oxidase 11). It is crucial to a discussion of the mechanism of dioxygen reduction to know whether the binding of cytochrome c to the oxidase is to a single site or to more than one site. This will determine whether the reduction reaction can occur symmetrically (e.g., two sites) or asymmetrically (one site). 

The mechanism of dioxygen reduction at the trinuclear cluster in MCO catalysis has been a strong focus for research on this class of copper oxidases. Dioxygen reduction has been most thoroughly investigated in Rhus laccase (a plant laccase, from the Japanese lacquer tree). The primary reason for using Rhus Lac is the availability of a metal-substituted form the enzyme, a TlHg form, in which the... 

Reduction of the heme iron of cytochromes P450 to the ferrous state 3 is necessary for the binding and subsequent activation of atmospheric dioxygen. Initially, two electrons are derived from NAD(P)H by flavin adenine dinucleotide (FAD)-containing proteins and then are used sequentially via one-electron transfers. AU cytochromes P450 can be divided into two main classes with respect to the reduction mechanism and the structure of their immediate redox partner. The first class includes most soluble... 

The vast majority of irreversibly adsorbed macrocycles promote dioxygen reduction via the sequence of steps specified in Scheme 3.7, known as electrochemical-chemical-electrochemical, or ECE mechanism. 

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... 

Scheme 3.8 Proposed mechanism for dioxygen reduction mediated by CoPI/OPG in alkaline solutions.

Scherson, Palenscar, Tolmachev and Stefan provide a critical review of transition metal macrocycles, in both intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. An introduction is provided to fundamental aspects of electrocatalysis, oxygen reduction, and transition metal macrocydes. Since the theoretical and experimental tools used for investigation of homogeneous and heterogeneous electrocatalysis are considerably different, these topics are given separate discussion. The influence of the electrode surface on adsorbed macrocydes, and their influence on mechanism and rates of 02 reduction is treated in detail. Issues related to pyrolyzed macrocydes are also described. 

Figure 1. Proposed mechanism for the catalytic cycle and dioxygen reduction site structure in the blue copper oxidase, laccase (after ref. 19, with permission).

A considerable body of results accumulated during earlier decades from activity studies of hCp now awaits a more meaningful analysis using the available 3D structure. Catalysis of amine oxidation by hCp, in particular biogenic ones present in plasma, cerebral, spinal, and intestinal fluids as well as of ferrous ions, which is probably physiologically relevant, has been studied extensively (68, 71). The mechanism of dioxygen reduction by hCp at the trinuclear center is of particular interest, as the presence of three distinct Tl sites raises the question of which centers are involved in internal ET to the single O2 reduction site. This mechanistic question prompted us to initiate ET studies by PR. 

As previously mentioned, laccase is very closely related to ascorbate oxidase. The principal molecular architecture and arrangement of the mononuclear and trinuclear copper centers are the same. Furthermore, spectroscopic and kinetic properties are similar in many circumstances. Therefore, the catalytic mechanism of the dioxygen reduction should be the same for both. Kinetic studies on fungal and tree laccases have been... 

The true metabolic role of HA per se (and not as mere precursor of other substances such as quinolinic acid and so on) is still awaiting a conclusive definition. However, HA toxicity seems to be related not to the compound itself, but rather to other substances, arising form its (auto)oxidation. As ever, one can speculate about the chemical nature of those species, therefore some evidence exists in favour of the profound involvement of reactive intermediates in dioxygen reduction, namely superoxide, peroxide and hydroxyl, whereas other indications suggest the participation of anthranilyl and/or HA quinoneimine in the toxicity mechanism. 

Instead of the dioxygen-reductant pair, one can employ oxo componds containing an oxygen atom which is already partly reduced H2O2 [68], ROOH [69], PhIO [70], NaOCl [71], KHSO. [72], amine Af-oxides [73], and magnesium monoperoxyphtalate [74] (see also Chapter X). One of the most efficient (in terms of reaction rate and turnover number) systems is the combination of ruthenium porphyrin and 2,6-dichloropyridine A-oxide [73]. A simplified mechanism of alkane oxidation with iodosylbenzene catalyzed by iron porphyrinate is demonstrated in Scheme XI. 17. 

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