ONE-ELECTRON OXIDATION OF METHOXYLATED AND HYDROXYLATED INDOLES BY N-3. .1. CHARACTERIZATION OF THE PRIMARY INDOLIC RADICALS

The one-electron oxidation of a series of hydroxylated and methoxylated indoles by the azide radical in aqueous solution at pH 5-9 (k ∼ (4-9) × 109 dm3 mol-1 s-1) has been studied with the use of the technique of pulse radiolysis with spectrophotometric detection. One-electron oxidation of 5,6-dihydroxyindole (DHI) and its N-methyl-substituted analogue (MeDHI) results in the formation of the corresponding indole semiquinone radical with pKa value of 6.8 ± 0.2. Methylation of one of the hydroxy groups of DHI (or MeDHI) results in formation of the corresponding indoloxyl radical. In contrast, methylation of both hydroxy groups of DHI yields 5,6-dimethoxyindole (DMI), which, upon one-electron oxidation, forms the indolyl radical with a pKa of 6.0 ± 0.2. Further methylation at N(l) of DMI causes the radical cation to be stabilized over the pH range studied. From a comparison of the spectral characteristics of these radicals, the preferred site of deprotonation of the radical cation of DHI is identified to be from the 6-hydroxy group. With the exception of DHI (or MeDHI), the radicals produced decay bimolecularly in the dose/pulse range of 1-18 Gy used in these studies. It is concluded that the types of radical produced upon one-electron oxidation of methoxylated and hydroxylated indoles are critically dependent upon the sites of methylation of these indoles. © 1990 American Chemical Society.