Electrochemical and peroxidase O-2-mediated oxidation of indole-3-acetic acid at physiological pH

Indole-3-acetic acid (IAA) or an oxidative metabolite is believed to be a growth hormone in plants. IAA is also found in the mammalian central nervous system although its biological roles, if any, are presently unknown. 19 this investigation the electrochemically driven and peroxidase/O-2-mediated oxidation chemistry of IAA at physiological pH has been studied with the primary goal of identifying the major reaction products. Based upon the nature of these products it has been concluded that, at pH 7.4, the anion of IAA is initially oxidized (one-electron) to an acetoxy radical that in part undergoes a second one-electron oxidation/decarboxylation to a carbocation precursor of 3-hydroxymethyl-2-oxindole (1), indole-3-carbinol (13) and 3-methylene-2-oxindole (3). Indole-3-carbinol (13) can be further oxidized (2e, 1H(+)) to the cation of 3-hydroxymethylene indolenine, the precursor of indole-3-aldehyde (2), 3-hydroxy-2-oxidole (8) and 3,3-dihydroxy-2-oxindole (12). Direct decarboxylation of the initial acetoxy radical yields a carbon-centered radical that rapidly dimerizes to a compound that is further oxidized to 3,3'-(1,2-ethanediyl)bis-1 H-indol-2-ol (9) and thence 5,7,12,13-tetrahydro-oxepino[2,3-b:7,6-b']diindole (10). A parallel oxidation pathway involves oxidation of IAA to the cation of 3-methyleneindolenine carboxylic acid, the precursor of 2-oxidole-3-acetic acid (11). Particularly when relatively high concentrations of IAA are electrochemically oxidized for long periods of time, many additional oligomeric and polymeric products are formed. The peroxidase/O-2-mediated oxidation of IAA at pH 7.4 gives exactly the same products formed in the electrochemical reaction suggesting that, in a chemical sense, the enzymatic reaction follows the same pathways as the electrochemical reaction. (C) 1997 Elsevier Science S.A.