Superoxide

Superoxide is the radical anion O₂⁻, formed when molecular oxygen gains one electron. It is a reactive oxygen species found in chemical oxidation and biological processes.

Molecular oxygen: Molecular oxygen is the diatomic molecule O₂, a stable form of elemental oxygen. Superoxide forms when this molecule accepts one electron.
Free radical: A free radical is an atom, molecule, or ion with one or more unpaired electrons. Superoxide's unpaired electron gives it radical character.
Respiratory burst: The respiratory burst is the rapid production of reactive oxygen species by activated immune cells. Phagocytes generate superoxide as an early step in this antimicrobial response.
Hydroperoxyl radical: The hydroperoxyl radical is the protonated form of superoxide, with formula HO₂•. It is the conjugate acid of superoxide and becomes more prevalent at low pH.
Electron transfer: Electron transfer is the movement of an electron between atoms, molecules, or larger systems. A single transferred electron converts molecular oxygen into superoxide.
Radical anion: A radical anion is a negatively charged species containing an unpaired electron. Superoxide is specifically the radical anion of molecular oxygen.
NADPH oxidase: NADPH oxidase is an enzyme complex that transfers electrons to oxygen, producing superoxide. It is a major regulated source of superoxide in immune cells.
Peroxide: A peroxide contains an oxygen–oxygen single bond in which each oxygen has oxidation state −1. Peroxide is a two-electron-reduced oxygen form, unlike radical superoxide.
Dismutation: Dismutation is a redox reaction in which one chemical species is simultaneously oxidized and reduced. Two superoxide ions can dismutate into oxygen and hydrogen peroxide.
Oxidation state: Oxidation state is a formal accounting value for electron distribution in a chemical species. It tracks the one-electron reduction of oxygen in superoxide formation.

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