Copper(II)-quercetin complexes in aqueous solutions: spectroscopic and kinetic properties

Quercetin (Querc), one of the most common dietary flavonols, was investigated in the presence of Cu(II) ions under basic conditions by different techniques in order to obtain some elucidation on the mechanism of its beneficial action against free radical-mediated damage. The spectroscopic studies (UV/Vis, Raman and IR) were useful to assess the relevant interaction of Querc with Cu(II) ions, the chelation sites and the dependence of the complex structure from the metal/ligand ratio. In the presence of a slight excess of the ligand (0.5 M/L) Querc acts as a bidentate ligand trough the catechol moiety on B ring. At M/L > 1 the metal interaction also involves the C=O group and a close hydroxyl group. Thermogravimetric analysis confirmed the stoichiometry of the proposed complexes. Information on the reactivity of the Cu(II) chelates towards oxidizing radicals ((OH)-O-., N-3(.) and Br-2-(.)) were obtained by the pulse radiolysis data. The chelates appear to fast react with these oxidizing radicals, giving rise to intermediates transients, namely resonance-stabilised phenoxyl radicals. (c) 2004 Elsevier B.V. All rights reserved.