Kinetics and Antioxidative Sites of Capsaicin in Homogeneous Solution

Reliable quantitative kinetic data on the antioxidant activity of capsaicin (CAP) is lacking, and the antioxidative mechanism of CAP is still unclear. Therefore, an investigation aimed at elucidating the antioxidative sites of CAP that react with chain-propagating peroxyl radicals was undertaken. First, the reaction of CAP with 2,2-diphenyl-1-picrylhydrazyl (DPPH) was investigated, and it was found that the stoichiometric factor of CAP is 2. Then, the rate constant for the reaction of CAP with peroxyl radicals derived from cumene was measured. CAP reacted with peroxyl radicals at a constant rate of k _inh = 5.6 × 10³ M⁻¹ s⁻¹. Furthermore, the inhibitory effects of various related compounds against cumene oxidation were measured, showing that the phenolic OH group is the active portion of the molecule. In addition, the kinetic solvent effects of DPPH/CAP reactions were measured in methanol, acetonitrile, acetone and tetrahydrofuran. In particular, an enhancement in the reaction rate was observed in alkaline methanol, indicating that these results are due to the partial ionization of the phenol of CAP and very fast electron transfers from the phenolate anion to DPPH. We interpreted these results as indicating that the phenolic OH group of CAP is mainly associated with peroxyl radical scavenging.