Electrochemical Monitoring and Evaluation of Metallo-Quercetin Complexes’ Antioxidant Activity Toward Electrochemically Generated Superoxide Anion Radical

Abstract

The superoxide anion radical, O2˙– (SAR) is a reactive oxygen species (ROS) that is naturally generated within the biological environment via a one electron reduction of molecular oxygen.1-4 Excess production of ROS in the body can lead to undesired reactions that induce cellular death and furthermore, the onset of some pathological diseases.1,3 Antioxidants, such as flavonoids, react with ROS to prevent possible cell damage and development of health issues caused as a result of excess ROS production.2,3 Additionally, flavonoids, such as quercetin (QCR), are known to play a role as metal ion chelators.3,4 Biologically relevant metal ions are also known to influence the activity of ROS via Fenton reactions.3 However, the coordination of metal ions with QCR results in the formation of a metallo-QCR complex that may influence the antioxidant activity and capacity of the flavonoid. The role of metal ions and metallo-QCR complexes as antioxidants toward SAR is not fully understood. Electrochemistry is a valuable analytical tool that can be used to study and assess the reactivity of metal ions and polyphenolic compounds with SAR.1-4 In this work, we monitored and evaluated the ability of QCR and biologically relevant metal ions, Fe(III) and Cu(II) to influence the activity of electrochemically generated SAR. Additionally, we tested and compared the reactivity of metallo-QCR complexes prepared either by sequential addition of metal ion, followed by QCR (in-situ) or by direct addition of a preformed mixture to the electrochemically generated SAR. Specifically, 1:1 and 1:2 molar ratios of metal ion:QCR were explored. The O2˙–/O2 redox couple was generated in-situ using a three-electrode electrochemical cell, with a glassy carbon electrode, a platinum wire counter electrode and a Ag/AgNO3 reference electrode immersed in DMF.1 Using cyclic voltammetry (CV), the current and potential associated with the O2˙–/O2 redox couple was monitored and measured to assess radical scavenging ability of the additives of interest. The data revealed a decrease in SAR associated peak currents for all additives, indicating their role as free radical scavengers. CV data also suggested that the reaction between QCR, metal ions and metallo-QCR complexes is driven by an initial electron transfer mechanism that is followed by a proton transfer.2 Overall, compared to all other variations of additives tested, in-situ prepared 1:2 Cu:QCR complexes exhibited the greatest ability as antioxidants toward SAR. References N. L. Zabik, S. Anwar, I. Ziu, and S. Martic-Milne, Electrochim. Acta, 296, 174-180 (2019).Ahmed, F. Shakeel, Czech J. Food Sci., 30, 153-163 (2012).E. Bodini, G. Copia, R. Tapia, F. Leighton, and L. Herrera, Polyhedron, 18, 2233-2239 (1999).M. Kasprzak, A. Erxleben, J. Ochocki, RSC Adv., 5, 45853-45877 (2015).