Pro-oxidant activity of vitamin C in drinking water ... - Ã bo Akademi

Review of the literatureOH • + H 2O 2 → H 2O + H + + O 2• − Eq. 4O 2• − + Fe 3+ → Fe 2+ + O 2 Eq. 5HO 2• + Fe 2+ + H + → Fe 3+ + H 2O 2 Eq. 6OH • + Fe 2+ → Fe 3+ + OH − Eq. 7HO 2• + Fe 3+ → Fe 2+ + H + +O 2 Eq. 8For example, if hydroxyl radicals are formed and they do not have anything to react with,they can react with hydrogen peroxide, or alternatively, take one electron from Fe 2+ andoxidize it to Fe 3+ . However, if hydroxyl radicals are formed in vivo many target moleculeswill be available.In the laboratory, hydroxyl radicals can be easily generated by UV induced homolyticfission of hydrogen peroxide (Equation 9) or by metal-catalyzed (copper or iron) reductionof oxygen to superoxide that reacts with hydrogen peroxide (Equation 10). 140, 141UVH 2O 2 −−−→ OH • + OH • Eq. 9H 2O 2 + O 2• − → O 2 + OH • + OH − (Haber-Weiss reaction) Eq. 102.4. The reactivity of hydrogen peroxide and hydroxyl radicalsThe formation of hydroxyl radicals has been studied using the spin trap technique, 142aromatic hydroxylation of target molecules 143-145 and by studying radical attacks on targetmolecules such as DNA 4 and deoxyribose 146 . Generally, standards for the final products(e.g. 8-hydroxyguanosine to demonstrate hydroxyl radical attack on guanosine) have beenused in order to identify the radicals (ROS or RNS) that are generated. In addition, radicalscavenger molecules have been used to confirm the presence of a specific radical. Thishowever, can sometimes be misleading, as some hydroxyl radical generating reactions canbe site-specific. In this case radical scavengers fail to scavenge the radicals, but this doesnot mean that hydroxyl radicals do not form.Of the ROS, hydroxyl radicals are very harmful due to their high reactivity. 134 Hydroxylradicals are short-lived and extremely reactive and easily attack any nearby molecule by19