Active oxygen species, nature

Wende R, F-H Bernhardt, K Pfleger (1989) Substrate-modulated reactions of putidamonooxin the nature of the active oxygen species formed and its reaction mechanism. Eur J Biochem 81 189-197. 

Although pathway 2 in the oxidation process (Scheme 2) may be considered analogous to mechanisms proposed for carbon hydroxylations catalyzed by cytochrome P-450, abstraction of an electron from the lone pair on nitrogen (pathway 1) would be a more likely first step in these types of reactions. It is reasonable to assume that the nature of substituents R, R2, and R3 would greatly influence the rate and path of reaction. The mechanistic possibilities in Scheme 2 are undoubtedly simplistic in their representation of the active oxygen species of cytochrome P-450 and are by no means comprehensive. However, these pathways do serve to illustrate.the role of radical substrate intermediates in cytochrome P-450-catalyzed reactions. More detailed analyses of mechanistic studies on these and other cytochrome P-450-mediated reactions can be found in recent reviews on the subject 49, 50, 60). 

The function of the metal site in the oxygen-dependent radical enzymes galactose oxidase, amine oxidases, ribonucleotide reductase, and cytochrome c oxidase is inter alia to bind 02 in their reduced forms and undergo the appropriate redox chemistry to generate a metal-bound, activated oxygen species of variable nature. 

In the years to follow, Gaffron s acceptor-activation mechanism was omitted from discussionf since all results obtained by different investigators were only in agreement with an oxygen-activation mechanism (see refs. 26,36,47 see also refs. 2 and 3 for discussion). However, the nature of the activated oxygen species remained obscure. In order to explain the activation process and the activated oxygen species, four different mechanisms were discussed. 

Even though the formation of an allylic intermediate has been relatively firmly established, the nature of the active oxygen species which interacts with the allylic intermediate is less clear. Nevertheless, much attention has been directed toward understanding the various oxygen species found on the surface of oxide catalysts. 

The extensive and controversial discussion about the nature of active oxygen species may benefit from a separation of two issues (1) the number and chemical nature of active oxygen species and (2) the location of active oxygen species. The present chapter refrains from reporting again the different views on both issues as this has been done several times in the reviews mentioned in Section 1.1. 

Much of the actual debate in the literature about the epoxidation process has centered on identifying the nature of the active oxygen species, i.e. the one that actuates the catalysis. This has been, and remains, a controversial issue. The older part of the literature was mostly concerned with... 

The issue of catalytic methane activation by active oxygen species associated with lattice oxygen has been the subject of many literature reports.Despite the fact that for a large number of oxidative coupling catalysts the involvement of lattice oxygen in the formation of Cj hydrocarbons is unquestionable, there is still a lack of convincing evidence that could resolve the issue of the nature of the active sites. 

P. Batdoni, J. P. Renaud,]. F. Bartoli, M. Reina-Artiles, M. Fort, D. Mansuy, Monooxygenase-like oxidation of hydrocarbons by hydrogen peroxide catalyzed by manganese porphyrins and imidazole selection of the best catalytic system and nature of the active oxygen species, /. Am. Chem. Soc. 110 (1988) 8462. 

The morphology, particle size and structure of the oxides were determined by using transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). BET surface area of the samples were measured by using Micromeritics ASAP-2000 instrument. The interaction of Ce with Mo and its effect on the nature of active oxygen species in the complex oxides were studied by using temperature-programmed reduction(TPR) and laser Raman spectroscopy (LRS). 

Fluorinated alcohols, TFE and HFIP, have often been used as catalytic solvents for epoxidation reactions under mild conditions [34]. The merit of these fluorinated alcohols is, surely, their resistant nature toward oxidation. Another merit of their use in oxidation reactions has often been said to be their catalytic activities via protonation of an active oxygen species, such as H2O2. Experimental and computational studies indicate that the protonated hydrogen peroxide, H3C>2+, which is generated by the action of H2O2 with a strong acid, is a very powerful oxidant. In contrast, weak acids such as TFE appear to... 

Finally, the similarity between the peroxidases, especially chloroperoxidase, and cytochrome P-450 in their reactivity with peroxide suggests that the active oxygen species in P-450 may also be a ferryl species, i.e., an oxo iron (TV) complex. Evidence both for [193] and against [194] the involvement of a Compound I-type intermediate in the reaction cycle of P-450 has appeared. As of yet, there exists no direct evidence for the nature of the intermediates beyond oxy-P-450 in the reaction cycle. 

The nature of the active oxygen species can in principle be elucidated by application of the Multitrack setup. It is possible to alter the concentration and nature of oxygen species by changing the temperature [3] or by addition of N2O [4]. Information on the reaction pathway can be obtained by examination of the... 

The nature of the active oxygen species could not be resolved by application of the Multitrack set-up, as under vacuum conditions the oxidation reaction is approximately zeroth order in oxygen. Oxidation products of propene over silver catalysts are already observed at low oxygen partial pressures (0.15 Pa). The observed time-scales of product formation suggest a fast formation of propene oxide and acrolein. As the amount of acrolein formed is rather constant... 

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