Raman and pulse radiolysis studies of the antioxidant properties of quercetin: Cu(II)-chelation and oxidizing radical scavenging.

Quercetin (Querc), a pentahydroxyflavonol, is suggested to give protection to living organisms by both direct scavenging of free radicals and metal chelation. The scavenging ability of Querc towards oxidizing radicals, such as OH, N3 and NO2 , was evaluated by pulse radiolysis studies in aqueous solutions at different pH. Phenoxyl radicals are the final transient products and are formed either by water elimination from OH-adducts or by one-electron transfer from the deprotonated OH groups. Their formation rate is strongly affected by pH and reaches the maximum values in alkaline medium. The Raman and IR spectra were useful to assess the relevant interaction of Querc with Cu(II) ions, which play an important role in the metal-catalysed generation of reactive oxygen species. Depending on pH and the metal-to-ligand ratio, the different chelating sites of Querc change their ability to complex copper ions. Under neutral conditions, the 5-OH group of ring A and CO-4 of ring C have a chelating power superior to that of the catechol group (ring B), whereas the complexation in alkaline medium occurs in the reverse order. In addition, experiments with Querc and Zn(II) ions, carried out at basic pH in order to verify the possible Cu(II)-catalysed oxidation of the ligand, indicated the absence of the above process. Copyright © 2005 John Wiley & Sons, Ltd.