Effective Antioxidants as Plausible Ligands in Chromium(III) Supplementation: How Complexation Modulates Catechol-Based Polyphenols

This study examines the impact of metal coordination on the antioxidant and pro-oxidant properties of 3,4-dihydroxybenzoic acid (3,4-DHBA) and caffeic acid (CA). Their Na(I), K(I) salts and Cr(III) complexes were evaluated in vitro using radical scavenging assays (ABTS, DPPH, hydroxyl, and superoxide), ferric- and cupric-reducing power, and inhibition of linoleic acid peroxidation. Alkali metal coordination generally decreased radical scavenging activity, though K complexes and Cr–3,4-DHBA improved lipid peroxidation inhibition. Cr(III) chelation produced ligand-dependent effects: it markedly increased the reducing power of CA while reducing that of 3,4-DHBA and uniquely promoted pro-oxidant behavior in CA under superoxide conditions. These outcomes reflect how chromium chelation alters electronic distribution and charge transfer, enhancing reducing power in single-electron transfer assays while enabling redox cycling in radical scavenging systems, underscoring its dual and ligand-dependent biological significance.