Intrinsic Antioxidant Potential of the Aminoindole Structure: A Computational Kinetics Study of Tryptamine

A computational kinetics study of the antioxidant activity of tryptamine toward HO^• and HOO^• radicals in water at 298 K has been carried out. Density functional methods have been employed for the quantum chemical calculations, and the conventional transition state theory was used for rate constant evaluation. Different mechanisms have been considered: radical adduct formation (RAF), single electron transfer (SET), and hydrogen atom transfer (HAT). For the reaction of tryptamine with the hydroxyl radical, nearly all channels are diffusion-controlled, and the overall rate constant is very high, 6.29 × 10¹⁰ M^-1 s^-1. The RAF mechanism has a branching ratio of 55%, followed by the HAT mechanism (31%), whereas the SET mechanism accounts just for 13% of the products. The less hindered carbon atom neighboring to the nitrogen of the indole ring seems to be the preferred site for the RAF mechanism, with a branching ratio of 16%. The overall rate constant for the reaction of tryptamine with the HOO^• radical is 3.71 × 10⁴ M^-1 s^-1, suggesting that it could be a competitive process with other reactions of hydroperoxyl radicals in biological environments. For this reaction only the HAT mechanism seems viable. Furthermore, only two centers may contribute to the HAT mechanism, the nitrogen atom of the indole ring and a carbon atom of the aminoethyl chain, the former accounting for more than 91% of the total products. Our results suggest that tryptamine could have a noticeable scavenging activity toward radicals, and that this activity is mainly related to the nitrogen atom of the indole ring, thus showing the relevance of their behavior in the study of aminoindoles.