Mechanistic approach to the antioxidant activities of metal-associated superoxide with Trolox

Abstract

Trolox-C (TXC) is a water-soluble analogue of α-tocopherol that reduces the metal-bound superoxo complex (SOC), [(NH3)5Co(µ-O2)Co(NH3)5]5+ in water–ethanol media. In aqueous-acid media ([H+] = 0.02–0.70 mol/L), TXC quantitatively reduces SOC under the pseudo first-order reaction condition where the concentration of TXC, [TXC], was at least 10 times or more than that of [SOC]. Variations in media ionic strength suggest for the reaction between two cationic species, and a positive kinetic isotope effect ( kH/ kD < 1) indicates for a pre-equilibrium step of protonation of the pyran-oxygen of TXC. Both TXC and the protonated TXC, i.e., TXCH, are the potential reductants under the experimental range of media acidity. The reduction of SOC takes place through the single-electron transfer and the kinetic analysis yields the value for second-order rate constants for TXC and TXCH as 9.7 and 3.6 × 103 (mol/L)−1s−1 at 15 °C.