Figure 2.

Reactive Oxygen Species (ROS): their structure and formation. The process begins with molecular oxygen (O₂), which has two unpaired electrons in the outer orbital. Thus, it can acquire an electron to form the superoxide anion (O₂⁻), which is a radical (i.e., with an unpaired electron). Superoxide dismutase (SOD) converts superoxide into hydrogen peroxide (H₂O₂) and oxygen. Catalases can further break down H₂O₂ into water and oxygen. The Haber–Weiss reaction generates hydroxyl radicals (HO^•) and hydroxyl ions (OH⁻) from superoxide and H₂O₂. The Fenton reaction also produces hydroxyl radicals and hydroxyl ions by reducing H₂O₂ in the presence of iron ions. O: oxygen atom. H: hydrogen atom. H₂O: water molecule. O₂⁻: superoxide anion. H₂O₂: hydrogen peroxide. HO^•: hydroxyl radical. OH⁻: hydroxyl ion. NO: nitric oxide. Fe²⁺: iron in reduced state. Fe³⁺: iron in oxidized state. SOD: superoxide dismutase.