Application of a potentiometric system with data-analysis computer programs to the quantification of metal-chelating activity of two natural antioxidants: Caffeic acid and ferulic acid

The quantification of metal-chelating activity of caffeic and ferulic acids (1 and 2. resp.) was successfully performed by using a potentiometric system with data-analysis computer programs. The method was applied to two phenolic models, which have been systematically reported as antioxidants. Although a chain-breaking mechanism was proposed, several studies pointed out the possibility of complexation of transition metals that can participate in single-electron reactions and mediate the formation of oxygen-derived free radicals. In this work, the complexation properties towards Cu-11 were investigated by potentiometry with a glass electrode. Acidity constants of the ligands (phenolic acids) and the formation constants of the ligand-metal complexes were evaluated by potentiometry. The modeling of the titration curves and the data treatment were performed with the computer programs Superquad and Best. A detailed quantitative examination of the complexation species formed in the Cu-11/caffeic acid (1) and Cu-11/ferulic acid (2) systems is presented together with the formation constants (log P). Results have shown that the complexation properties of the two phenolic acids towards the transition metal are quite different: the activity of caffeic acid (1) was found higher than that of ferulic acid (2). ne data are important to get insight into the mechanism of action of antioxidants. and. in this case, could partially explain the efficacy of caffeic acid in the protection of LDL oxidative damage. In addition. the analytical method developed could be applied to quantify the chelating activity of important biological compounds, such as allopurinol, uric acid, cinnamic acids, flavonoids, and anthocianins, and, in that way, could be a valuable tool to understand the mechanisms underlying their protective effects.