Identification of the Factors Affecting the Rate of Deactivation of Hypochlorous Acid by Melatonin

It has been found that melatonin reacts rapidly with hypochlorous acid in phosphate-buffered, ethanol–water solutions to produce 2-hydroxymelatonin. The rate law, d[2 − HOMel]/dt − kHOCl[Mel][HOCl] − kOCl−[Mel][OCl−], was obtained. At 37°C and at a water concentration of 23.5 M, kOCl−= 6.0 × 102L · mol−1· s−1, and kHOClwas found to be a function of the water concentration, kHOCl= 11 ± 3 L3· mol−3· s−1· [H2O]2, indicating that the availability of water at the site of the reaction plays a significant role. The part that the structural components of melatonin play in determining the reaction pathway was examined by comparing the rate of deactivation of HOCl by melatonin to that of the model compounds indole, 5-methoxyindole, and 3-methylindole. The relative reactivity is explained in terms of steric and electronic effects, and it was found that the presence of the substituent at the 3-position influences the nature of the oxidation product. Melatonin and 3-methylindole yielded hydroxylated products, whereas indole and 5-methoxyindole produce chlorinated products.