Effects of Indoleacetic Acid and Other Oxidation Regulators on in Vitro Peroxidation and Experimental Conversion of Eugenol to Lignin

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Abstract
Indoleacetic acid and other organic N compounds inhibit oxidations involving peroxidase (in vitro) and celery vascular tissue (in situ). The conversion of eugenol to lignin-like polymers in situ is especially sensitive to the antioxidants IAA, isonicotinylhydrazine, and mescaline. Oxidation in vitro of eugenol is more inhibited by some compounds than oxidation of pyrogallol. This difference may reside in the differential accessibility to inhibitors of intermediate radicals formed during the oxidation of these substrates. Preliminary kinetic evidence is consistent with the chemical picture of phenolic oxidations as radical-mediated reactions. Eugenol oxidation displays some features of a radical-initiated chain reaction, whereas pyrogallol does not. Free radical scavengers (chain breakers) are efficient inhibitors of eugenol oxidation in both systems. Using the H2O2/inhibitor ratio as an index, the inhibitory activity of the scavengers, 1AA and other compounds compares well with the activities of chain breakers and antioxidants in established oxidation and additional polymerization reactions. Quinones accelerate oxidation of eugenol in situ to lignin-like polymers, an effect which may account for the failure of autoxidizable hydroxyindoles as inhibitors in this system. The antioxidant activity of IAA may constrain the peroxidase which it induces from a premature attack upon metabolites essential to growth.