FREE-RADICAL INTERMEDIATES AND STABLE PRODUCTS IN THE OXIDATION OF INDOLE-3-ACETIC-ACID

The free-radical intermediates and the stable products formed on one-electron oxidation of indole-3-acetic acid (IAA) in aqueous solution were investigated. The dibromine radical anion generated radiolytically reacted with IAA to yield the IAA radical cation. In acid solution, the latter decays by a first-order process (k = 1.8 x 10(4) s(-1)) to yield carbon dioxide and the skatole radical. At pH 7 it deprotonates (pK(a) = 5.09 +/- 0.02), giving the indolyl radical, which decays only by a bimolecular process (rate constant of first-order reactions <100 s(-1)) which yields indole-3-carbinol as one of the products. Under steady-state irradiation at pH 7 the free radicals had a sufficient lifetime to allow the small fraction of radical cation present in equilibrium to undergo decarboxylation, consistent with the observed formation of carbon dioxide. Glutathione reacted with the indolyl radical, regenerating IAA, and with the skatole radical to yield skatole. In the presence of oxygen the skatole radical is rapidly converted to a peroxyl radical, which appears to decay only by bimolecular reactions with indole-3-aldehyde as one of the products. No evidence was found for the reaction of the peroxyl radical with glutathione or for elimination of superoxide.