Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS) In general pro-oxidants/oxidants are termed as ROS/RNS. The most important free radicals produced during meta- bolic reactions are radicals derived from oxygen, ROS. Both the ROS and RNS can be classified into two groups of compounds namely; radicals and non-radicals (see Table 1). Radicals are the species which contain at least one unpaired electron in the shells around the atomic nucleus and are capable of independent existence. The oxygen molecule itself is a radical, and because of the presence of two unpaired electrons it is referred as biradical. The examples for the radicals include Superoxide (O2À), Oxygen radical (O22), Hydroxyl (OH2), Alkoxyradical (RO2), Peroxyl radical (ROO2), Nitric oxide(nitrogen monoxide) (NO2) and nitrogen dioxide (NO2) [17]. The high reactivity of these radicals is due to the presence of one unpaired electron which tends to donate it or to obtain another electron to attain stability. The non radical species include hydrogen peroxide (H2O2), hypochlorous acid (HOCl), hypobromous acid (HOBr), ozone (O3), singlet oxygen (1O2), nitrous acid (HNO2), nitrosyl cation (NO?), nitroxyl anion (NO-), dinitrogen trioxide (N2O3), dinitrogen tetraoxide (N2O4), nitronium (nitryl)cation (NO2?), organic peroxides (ROOH), aldehydes (HCOR) and peroxynitrite (ONOOH) [16, 17]. These non radical species are not free radicals but can easily lead to free radical reactions in living organisms [18].
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