A pulse radiolysis investigation of the oxidation of methoxylated metabolites of indolic melanin precursors

Abstract

The rate constants associated with the series of successive transient absorptions initiated by one-electron oxidation of 6-hydroxy-5-methoxyindole (6H5MI) and its isomer 5-hydroxy-6-methoxyindole (5H6MI) have been studied by pulse radiolysis. These close analogues of 5,6-dihydroxyindole (DHI) are metabolites of the oxidative melanogenic pathway. The species initially produced from N 3 . oxidation of both methoxyindoles at pH 7.2–7.4 are assigned as the corresponding sekiquinones. That from 6H5MI shows peaks at 500, 370 and 330 nm, very close to those of the semiquinone of DHI, whereas the semiquinone of 5H6MI shows no absorption at 500 nm but bands at 420 and 340 nm. These spectral differences are attributed to marked changes in the degrees of electron delocalisation for the two types of radical, both rings of the indole being involved for the 6H5MI radical but only the benzenoid moiety for the 5H6MI radical. In both cases, the radicals decayed, probably by disproportionation, into products which absorbed in the 400–420 nm region. For 6H5MI, the subsequent decay in this region was best fitted by two consecutive first-order processes which were both strongly base-catalysed. The first of these process is assigned to partial decay via deprotonation of the corresponding quinonoid cation to form an equilibrium mixture of this cation and the corresponding quinone methide. The second process is assigned to reaction of the quinone methide with water yielding hydroxylated product(s) which may subsequently react with remaining quinonoid cation or quinone methide to give dimeric product(s) with broad absorption centreing in the 550 nm region detected 0.5 s after the pulse. For 5H6MI, the decay at 430 nm fitted a single first-order process, which was weakly base-catalysed. This process is attributed to deprotonation of the corresponding quinonoid cation to the corresponding quinone imine absorbing below 350 nm, which was stable for at least tens of seconds. The current experiments suggest that our previous analogous observations (Lambert et al. (1989) Biochim. Biophys. Acta 993, 12–20) on the oxidation of the melanogenic precursors DHI and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) may be interpreted, as with 6H5MI, in terms of the corresponding indolequinones decaying into equilibrium mixtures of quinone imine and quinone methide. These decay via reaction of the methide with water generating hydroxylated species which proceed to give the coloured product(s) absorbing in the 550 nm region.