Dopamine oxidation to aminochrome at physiological pH. Dissociated dopamine can be oxidized to a dopamine o-semiquinone radical by the reduction of one molecule of oxygen to form a superoxide radical. The dopamine o-semiquinone radical can then disproportionate with another dopamine o-semiquinone radical, generating one molecule of dopamine and one molecule of dopamine o-quinone. Alternatively, the dopamine o-semiquinone radical can undergo one-electron oxidation to dopamine o-quinone by reducing one molecule of molecular oxygen to a superoxide radical. Dopamine o-quinone immediately cyclizes to form aminochrome, which is only stable in environments below pH 2.
