Mechanism of the Oxidation of Dopamine by the Hydroxyl Radical in Aqueous Solution

Article abstract of DOI:10.1021/ja00354a041

The hydroxyl radical (HO*) reacts with dopamine (4-(2-aminoethyl)-1,2-benzenediol) via one-electron oxidation producing o-semiquinone and o-semiquinone anion radicals, k = 5.9 * 10⁹ M^-1 s^-1.Reaction of OH* with protonated dopamine (H⁺QH2) proceeds via addition to the aromatic ring, while reaction with deprotonated dopamine (H⁺QH^-) proceeds via direct, one-electron oxidation.The isomeric hydroxycyclohexadienyl radicals formed via addition of HO* to H⁺QH2 yield three kinetically distinguishable groups (1- and 2-, or 3- and 6-, or 4- and 5-hydroxycyclohexadienyl radicals) with regard to rates of water elimination to produce semiquinone radicals; dehydration is "very fast" (addition of OH* ipso to an OH group), "fast" or "slow".The fast and slow reactions are acid and base catalyzed, with minimum rates at pH 4-5.Acid catalysis appears to be specific, while base catalysis is general.Spectra were determined for the initial products of HO* reaction with H⁺QH2 at pH 3.1 and 4.7.Deprotonation of the HO* adducts is the rate-controlling step in the base-catalyzed water loss at pH 2-, HO^-, and PO4^3- were 1.7 * 10², 6.5 * 10⁸, 11, and 11 M^-1 s^-1, respectively; for the last reaction, rate constants for H2O and HPO4^2- were 1.1 * 10³ and 2.2 * 10⁸ M^-1 s^-1, respectively.