superoxo

Superoxo refers to the monoatomic radical anion of dioxygen, commonly written as O2−• or O2−. It is the one-electron-reduced form of molecular oxygen and is classified as a reactive oxygen species. In solution and in many biological contexts, superoxide has a diatomic O–O framework with radical character, and its reactivity is influenced by how it is bound or solvated.

Superoxide is produced by one-electron reduction of O2. In biology this occurs in mitochondria during respiration, and in immune cells via NADPH oxidases to generate antimicrobial reactive oxygen species. It can undergo dismutation, a reaction accelerated by the enzyme superoxide dismutase: 2 O2−• + 2 H+ → H2O2 + O2. Hydrogen peroxide can then be further converted to water and oxygen by catalase or peroxidases. Superoxide also reacts with nitric oxide to form peroxynitrite, a potent oxidant. At low concentrations it can participate in redox signaling, whereas at higher levels it contributes to oxidative stress and cellular damage.

In inorganic and bioinorganic chemistry, superoxo can act as a ligand, binding to metal centers such as iron, copper, or manganese. Metal–superoxo species are important intermediates in enzymatic cycles and in models of dioxygen activation, with end-on or side-on binding modes influencing subsequent reactivity toward O–O bond cleavage or electron transfer.

Detection of superoxide often relies on electron paramagnetic resonance due to its unpaired electron, as well as spectroscopic methods and spin-trapping techniques. Overall, superoxide is a key intermediate in oxygen biology and chemistry, balancing roles in cellular signaling and oxidative damage.