Oxygen superoxide species

It is well established that aerobes constantly produce small amounts of oxygen-derived species, such as the superoxide radical (02 ), hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), the latter being generated by... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

Forms of oxygen-derived species produced by activated phagocytes can cause considerable tissue damage unless limited by copper-zinc superoxide dismutase (SOD) which destroys superoxide, 02, or catalase which destroys H202. However, at least two, if not three, other reactive forms of oxygen need to be considered70,71. ... 

A small number of surface sites are able to abstract hydrogen even from benzene and toluene. The occurrence of this surface process is revealed only by the subsequent interaction with oxygen to produce a small number of superoxidic species (257). 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

These results suggested that superoxide ion is indeed involved as a reactive intermediate, at least in the electron-transfer processes, leading to 1,4-diketones and oxiranes, whereas the ring-contracted derivatives arise from a process not requiring superoxide ion as the oxygen reactive species. In this regard, a comparative study on the sensitized photooxidation of the same substrates with... 

Transient quantities of 02 in aqueous solution can be generated by pulse radiolysis of 02 and by photolysis of H202 in aqueous media. In aprotic media stable solutions of OJ can be prepared by electrochemical reduction of molecular oxygen and by the base-induced decomposition of H202. Superoxide species can also be made from basic dioxygen-saturated solutions of aniline in dimethyl sulfoxide 34... 

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