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 b e a growth hormone in plants. IAA is also found in the mammalian centr al nervous system although its biological roles, if any, are presently unknown. 19 this investigation the electrochemically driven and perox idase/O-2-mediated oxidation chemistry of IAA at physiological pH has been studied with the primary goal of identifying the major reaction p roducts. 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 o xidation/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 ca tion of 3-hydroxymethylene indolenine, the precursor of indole-3-aldeh yde (2), 3-hydroxy-2-oxidole (8) and 3,3-dihydroxy-2-oxindole (12). Di rect decarboxylation of the initial acetoxy radical yields a carbon-ce ntered radical that rapidly dimerizes to a compound that is further ox idized to 3,3'-(1,2-ethanediyl)bis-1 H-indol-2-ol (9) and thence 5,7,1 2,13-tetrahydro-oxepino[2,3-b:7,6-b']diindole (10). A parallel oxidati on pathway involves oxidation of IAA to the cation of 3-methyleneindol enine carboxylic acid, the precursor of 2-oxidole-3-acetic acid (11). Particularly when relatively high concentrations of IAA are electroche mically oxidized for long periods of time, many additional oligomeric and polymeric products are formed. The peroxidase/O-2-mediated oxidati on of IAA at pH 7.4 gives exactly the same products formed in the elec trochemical reaction suggesting that, in a chemical sense, the enzymat ic reaction follows the same pathways as the electrochemical reaction. (C) 1997 Elsevier Science S.A.