Radiation induced oxidation of hydroxy indoles by NO and Br radicals: effect of pH

The reactions of NO and Br radicals with 5‐hydroxyindole (HIn), 5‐hydroxytryptophol (HTpl), 5‐hydroxytryptophan (HTpn) and 5‐hydroxytryptamine (HTpe) were studied using pulse radiolysis. The rate constants for their reaction with NO radical were found to vary from 10⁵ to 10⁷ dm³ mol^?1 s^?1 in the pH range 5–9 but a higher value (k = 1.4 ± 0.01 × 10⁸ dm³ mol^?1 s^?1) was noticed in HTpe at pH 9. The gradual increase in reactivity with pH is due to the decrease in the reduction potentials of indoloxyl radicals with E = 0.55 V at pH 9. In contrast, the rate constants with Br radical were found to be diffusion controlled and remained unaffected by the pH. The transient spectra measured are attributed to the indoloxyl radical formed on oxidation with λ_max at 420 nm. The indoloxyl radicals further react with the parent hydroxy indole derivative forming the radical adduct and their decay was found to be pH dependent in derivatives containing an amino group. At pH 5, no decay of the radical adducts was seen in all derivatives up to 5 ms whereas those with the amino group decayed faster at pH 9. The total yields of the oxygen centred and carbon centred radicals formed in the reaction of NO radical with hydroxy indoles were found to be nearly equal to G(NO). Our results suggest that NO radical is inefficient in oxidizing hydroxy indoles under physiological conditions preventing the formation of toxic dimers of indole derivatives. Copyright © 2010 John Wiley & Sons, Ltd.