Reactions of indolic radicals produced upon one-electron oxidation of 5,6-dihydroxyindole and its N(1)-methylated analogue

Abstract

The reactions of indole semiquinone radicals produced following one-electron oxidation of 5,6-dihydroxyindole (DHI) and its N-methyl-substituted analogue (MeDHI) have been studied using pulse radiolysis with spectrophotometric detection in the pH range 5–10 using different dose/pulse values (1–20 Gy/pulse). Using a dose/pulse of 18.5 Gy the semiquinone radicals of DHI and MeDHI decay predominantly by second order kinetics. The second order rate constants for disappearance of the semiquinone radicals are dependent upon the pH. Values of rate constants for decay of the semiquinone radical of DHI (pK_a= 6.8) at pH 5.5 and 9.1 are 3.8 × 10⁹ and 1.0 × 10⁸ dm³ mol^–1 s^–1 respectively at room temperature. In contrast, at a lower dose/pulse of ≤2 Gy, the semiquinone radical decays predominantly by first order kinetics which are dependent upon the concentration of the indole when pH pK_a. of the semiquinone radical. The second order rate constants for interaction of the semiquinone radical of DHI and MeDHI with DHI and MeDHI at pH 8.8 were determined to be 1.6 × 10⁶ and 2.3 × 10⁶ dm³ mol^–1 s^–1 respectively. The decay of the semiquinone radical results in the formation of a semi-permanent product(s) with a lifetime of ca. 10 ms. These products are discussed in terms of the formation of a reactive quinone-methide and/or -imine.