Kinetic analysis and double quenching experiments as tools to elucidate the action mechanism of antioxidants

The dependence of the oxidation rate with the concentration of an additive that reduces the rate of the process can be employed to determine if its effect is due to trapping of active radicals (chain-breaking antioxidant) or to its ability to chelate active metals (metal-chelating antioxidant). Kinetic expressions are deduced for metal-chelating antioxidants of high or low efficiency and for chain-breaking antioxidants. In particular, it is proposed that chain-breaking antioxidants must provide lineal SternVolmer's like relationships, while efficient primary antioxidants will give plots with a strong upward curvature. A second procedure is proposed that could allow a determination of the main mechanism of action of the additive based on double quenching experiments. The effect of the simultaneous addition of two antioxidant will depend of their mechanism of action. Two strong metal-chelating antioxidants will present a synergic behavior, while two chain-breaking antioxidants acting on the same species will present antagonic behavior. Antioxidants acting by different mechanisms will present an indifferent behavior, its efficiency being independent of the presence of the second antioxidant. Several different situations are contrasted with data obtained in the autoxidation of brain homogenates employing propyl gallate, desferrioxamine and BHT as antioxidants.