Antioxidative activity of aromatic amino acids and indole compounds for the autoxidation of linoleic acid was found to correlate in some extent with the highest occupied molecular orbital energy which represents the electron donor property of respective molecule. 5-Hydroxytryptophan, one of the best electron donor among the compounds tested, was the most effective antioxidant. However, antioxidative activity of some indole compounds could not be interpreted simply by their highest molecular orbital energies.
Neither the chelating action for the possible metal traces nor the accelerated decomposition of hydroperoxide produced during the course of the reaction explained these actions of indoles. Tryptophan, while preventing the autoxidation of linoleic acid, underwent the ring cleavage at the position of between C₂ and C₃ or hydroxylation at C₅ to yield formylkynurenine, kynurenine, 3-hydroxykynurenine, 5-hydroxytryptophan, 5-hydroxyindoleacetic acid, etc. Following mechanisms which were compatible with the experimental results were proposed for the antioxidative action of indoles; indole donates an electron from its π-pool to linoleic acid radical or peroxy radical produced during the autoxidation of linoleic acid to form a loose charge transfer complex through a “local” interaction; an electron transfer occurs within the complex, which brings cleavage of indole rings and an inhibition of autoxidation.