The oxidation of 6-hydroxydopamine in aqueous solution. Part 1. The formation of three metastable quinones at low pH

Abstract

¹H and ¹³C NMR investigations have been carried out in order to elucidate the products of oxidation of 6-hydroxydopamine [5-(2-aminoethyl)benzene-1,2,4-triol, protonated form H₃LH⁺]. Stopped-flow, NMR kinetic experiments, and quantum mechanical calculations were employed as additional aids to the interpretation of the results. Evidence is provided that, at low pH, three quinones are produced that are in metastable equilibrium, namely the p- (pQ), o- (oQ), and triketo-quinones (tQ). Results obtained with sodium periodate and iron(III) are compared and discussed. At higher pHs (>2.5) the quinones reach rapid equilibrium because they start to deprotonate, all giving rise to the same species (Q⁻), which is the only species detectable above a pH of about 6 and which is stable over a large pH range.