One-electron oxidation of 5,6-dihydroxy-2,3-dihydroindole: the influence of Zn2+

Abstract

One electron oxidation of 5,6-dihydroxy-2,3-dihydroindole (DDI) and its N-ethyl analogue by the azide radical, N₃˙, at pH 5 and 9 was studied using pulse radiolysis to further the understanding of chemical pathways leading to melanin formation. One electron oxidation of DDI yields the benzosemiquinone radical with a pK_a value of 5.3. With the N-ethyl analogue, the corresponding benzosemiquinone radical is formed, consistent with one of the hydroxy groups being the preferred site for deprotonation of the initially formed radical cation. The benzosemiquinone radicals of DDI disproportionate to yield a stable dopachrome-like product. In the presence of Zn²⁺ at pH 5.0, the benzosemiquinone radicals of DDI react to form, by an alternative route, a Zn ion complex of the o-semiquinone radical with a rate constant of 3.0 × 10⁶ dm³ mol^–1 s^–1. This Zn ion complex decays by second order kinetics to yield a Zn²⁺–quinone complex which has a lifetime of 3–4 ms. Auto-oxidation of DDI is suggested to lead to the formation of a dopachrome-like intermediate. From these studies, it is concluded that Zn²⁺ significantly influences the reactions involving the semiquinone radical of DDI and these alternative reaction pathways may help to clarify the initial biochemical stage of the free-radical pathway(s) leading to melanin formation.