Characterisation of the intermediates produced upon one-electron oxidation of 4-, 5-, 6- and 7-hydroxyindoles by the azide radical

Abstract

One-electron oxidation of a series of monohydroxylated indoles (HI) by the azide radical in the pH range 5–9 has been studied using the technique of pulse radiolysis with spectrophotometric detection. One-electron oxidation of 4-, 5-, 6- and 7-hydroxyindoles results in the formation of the corresponding indoloxyl radicals, the optical absorption spectra of which are independent of pH (5–9). It is confirmed, using the N(1)-methyl substituted analogue of 6-HI, that deprotonation of the resulting radical cation of the hydroxyindoles occurs preferentially from the hydroxy group to yield the corresponding indoloxyl radical. Such deprotonation would be consistent with the resulting indoloxyl radical having a low pK_a.

With the exception of 4-HI, the indoloxyl radicals decay bimolecularly in the dose/pulse range of 1–30 Gy to yield semi-permenent products (2k= 2–4 × 10⁹ dm³ mol^–1 s^–1). With 4-HI, the decay of the indoloxyl radical changes from second-order to first-order kinetics on lowering the dose/pulse. At 1 Gy/pulse, the first-order kinetics are dependent upon the concentration of 4-HI. The second-order rate constant for reaction of the indoloxyl radical with 4-HI was determined to be 4.8 × 10⁷ dm³ mol^–1 s^–1. The decay of these semi-permanent products from the indoloxyl radicals is first order and depends upon the concentration of azide. The second-order rate constants determined for this reaction depend markedly on the hydroxyindole used (k= 159–821 dm³ mol^–1 s^–1). The semi-permanent product arising from the bimolecular decay of the indoloxyl radicals is discussed in terms of the formation of reactive quinone-methides and/or -imines.