Interaction of quercitin with copper ions and free radicals: a mechanism of antioxidant defence.

Many epidemiological studies demonstrated that diets rich in fruit and vegetables, red wine and tea influence health by decreasing the risk of cancer, cardiovascular and Alzheimer's diseases. These observations were attributed to the large amount of flavonoid compounds in the above foods and beverages. One of the most important dietary constituents belonging to the flavonoid class is quercetin (Querc), a pentahydroxyl flavonol, that has multiple biological and pharmacol-ogical activities. Most of the pharmacological effects of Querc seems to be associated to ther its antioxidant action that can have two main possible mode of action, free radicai scav-enging and metal chelation. In fact, transition metal ions, such as copper, are known to play a vital role in thè initiation of toxic oxygen species processes and, of consequence, their chelation could be an important pathway of protection in biological systems. This study was addressed to the elucidation of the mechanisms of the benefic effect of Querc by examining both its capability in binding Cu(II) ions and in scavenging oxidising radicals. The characterisation of the Cu(II)-Querc complexes was obtained by different spectroscopic techniques coupled with thermogravimetry, whereas the reactivity of Querc towards free radicals was investigated by the pulse radiolysis technique which is able to generate specific radicals and follow their reactions with the compound under investigation. With respect to the coordination sites of Querc, there are three possible metal complexing domains that can interact with metal ions: the 3',4'-dihydroxy group located on the B ring and the 3- or 5- hydroxy and 4-carbonyl group in the C ring. Depending on the state of protonation and the metal/ligand molar ratio, different sites of Querc involved in the Cu(II) chelation were identified. On the other hand, the pulse radiolysis results showed as the scavenging of oxidizing radicals by Querc gives rise to the formation of phenoxyl-type radicals, quite stable and scarcely reactive because of the extensive electron delocalization through the whole System.