Reactive oxygen species organic radicals

All aerobic organisms contain substances that help prevent injury mediated by free radicals, and these include antioxidants such as a-tocopherol and the enzymes superoxide dismutase and glutathione peroxidase. When the protective effect of the antioxidants is overwhelmed by the production of reactive oxygen species, the intracellular milieu becomes oxidative, leading to a state known as oxidative stress (Halliwell and Gutteridge, 1999). Thus the balance between the generated free radicals and the efficiency of the protective antioxidant system determines the extent of cellular damage. 

Already in 1982, it was suggested that the intermediate chromium(V) state is involved in the carcinogenic process.9 Reactive Cr(V) and Cr(IV) intermediates may be harmful in many ways acting as tyrosine phosphatase inhibitors, or by forming organic radicals upon reaction with cellular reductants, which in turn can react with O2 and lead to reactive oxygen species.10 Reaction of chromium(VI) with... 

As indicated in Fig. 16.2, in addition to energy transfer, chemical reactions of excited UCs ( UC, 3UC ) may lead to the formation of other reactive oxygen species (ROS) that may react with organic pollutants. Such ROS include DOM-derived oxyl- and peroxyl radicals (RO , ROO ), superoxide radical anions (02 ) that may be further reduced to H202, and hydroxyl radicals (HO ). In the case of HO , however, DOM is a net sink rather than a source. Finally, some of the 3UC may react directly with certain more easily oxidizable pollutants (see below). 

Other reactive species are or may be formed upon irradiation of CDOM, but their production rates were determined (or are considered) to be much lower than for the species represented in Scheme 1. Nevertheless, these species can be important for the transformation of organic contaminants that are not susceptible to reaction with the main reactive species. They comprise the hydroxyl radical, the carbonate radical and all the radicals derived from the DOM (carbon-centred, oxyl, peroxyl radicals). The following subsections describe in more detail the role that each reactive species plays in the transformation of aquatic organic contaminants. Hydrogen peroxide, although considered a reactive oxygen species (ROS) and an important player in aquatic photochemistry [7], is neglected because there is no evidence that... 

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