Two copper(II) complexes of quercetin (Q) with metal:ligand stoichiometry ratios of 1:1 and 1:2 were synthesized. The synthesized complexes structures were determined by UV-Vis spectroscopy, infrared spectroscopy, thermogravimetry, elemental analyses as well as cyclic voltammetry. The spectroscopic data suggest that the chelation between Q and copper(II) can occur through a 3-hydroxy-4-carbonyl site. The antioxidant effectiveness and scavenging activity of quercetin and its complexes were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) method and luminol-hydrogen peroxide chemiluminescence system. The results showed that prepared compounds have strong quenching effects on the chemiluminescence intensity of luminol in the alkaline medium and are efficient in scavenging free radicals produced in this system. Reaction system variables such as luminol, quercetin and their complexes' concentration and various pH of luminol solution were investigated. Stren-Volmer equation was also used to study the kinetics of the luminol reaction system and linear relationship was obtained between chemiluminescence (CL) intensity and the concentration of these compounds in a range of 10-100 mg L-1. The Stern-Volmer quenching constant, Kq value in the presence of free quercetin is higher than the complexes, which indicates the fact that complexation with metal ions changes the chemical properties of quercetin and decreases the inhibitory ability and radical scavenging efficiency. The concentration-dependent inhibitory ability of quercetin and copper complexes is attributable to the presence of several hydroxyl groups in their structure that react with superoxide or peroxyl radicals available in luminol-H2O2 system. Our study strongly confirmed the potent radical scavenging property of polyphenolic compounds and their metal complexes.
