The oxidation of 6-hydroxydopamine in aqueous solution. Part 3. Kinetics and mechanism of the oxidation with iron(III)

Abstract

The kinetics of the oxidation of 6-hydroxydopamine [5-(2-aminoethyl)benzene-1,2,4-triol, protonated form H3LH+] by iron(III) under anaerobic conditions are presented. A complex mechanism whereby the o- (oQ), p- (pQ), and triketo-quinones (tQ) are formed via parallel inner- and outer-sphere electron transfer mechanisms has been established. The outer-sphere mechanism is particularly fast (nearly diffusion limiting) and predominates. By following the dependence of the rate on ionic strength it has been shown that a deprotonated form of 6-hydroxydopamine reacts via an outer-sphere reaction with all species of iron. Like the other catecholamines [3,4-dihydroxy-1-(2-aminoethyl)benzenes], but to a much smaller extent, complex formation occurs by FeOH2+ reacting with the fully protonated form of 6-hydroxydopamine. Three different semiquinones are initially produced; two of them, the triketo- and p-semiquinones, are tautomers. The o- and triketo-semiquinones react quickly with another iron atom to form their respective quinones. The p-semiquinone, however, is seemingly stable, partly reacting with more iron and partly disproportionating to form pQ and reforming 6-hydroxydopamine. At pHs above 2.5, pQ and oQ are in equilibrium via a deprotonated quinone Q−. The biological implications of this mechanism are discussed.