Nitric Oxide-Related Oxidative Stress and Redox Status in Health and Disease

The molecular oxygen or dioxygen (O2) molecule has 6 molecular orbitals (MO) in its triplet state, of which the two π 2p* MO contain each one electron. Thus, O2 has two unpaired electrons (indicated by a dot •); that is, O2 is an uncharged diradical molecule: •O–O•. An electron that is donated by another molecule is incorporated in one of the two π 2p* MO of the O2 molecule; that is, one π 2p* MO contains a single unpaired electron. Thus, 1-electron reduction of O2 yields a negatively charged radical species, that is, the superoxide radical anion (O2 −•). Subsequent 1-electron reduction of O2 −• yields a doubly negatively charged species, that is, the peroxide anion (O2 2−) which possesses a nonoccupied σ 2p* MO. Intake of two electrons by the peroxide anion does not form a stable molecule but the bond between the two O atoms breaks to yield two O2− ions which are protonated to form water.