Metal chelation and radical scavenging ability of catechin: spectroscopic and pulse radiolysis studies

Flavonoids are an important class of polyphenolic compounds widely distributed in nature. Beneficial effects of flavonoids on human health have gained increasing interest in the last years.1 Most of the benefits have been associated to the antioxidant action of these polyphenols.2 Flavonoids as antioxidant may act in different ways including scavenging of reactive oxidising species,3 chelation of transition metal ions,4 especially iron and copper able to promote free-radical reactions, or restoring other natural antioxidants. Another interesting aspect of flavonoids is their use as artistic colorants.5 They are the main components of a number of natural dyes used both as pigmenting agents in paintings and textile dyes for texile since antiquity. The analysis of organic molecules is crucial to advance on the conservation of the artworks, but such task is still extremely difficult due to their presence in complex mixtures or media in only trace concentrations, their high fluorescence and instability. Despite the importance of these natural compounds, there is still a lack of information on the ability of flavonoids to chelate metal ions and how the metal binding affects the behaviour of these compounds towards oxidising radicals. (+)-Catechin (Cat), belonging to this compound class, is a constituent of red wine and tea, widely present also in numerous fruits and vegetables. Its antioxidant activity has recently attracted attention because during food processing storage by limiting the fast chain reactions ending in the oxidation products of fatty acids, and in humans from risk of diseases.6 This study was aimed to obtain a wider insight into the molecular mechanisms of the beneficial action of catechin (Fig. 1). The investigations, performed by using several techniques, were addressed on both the capability of Cat in binding Zn(II) and Cu(II) ions, and scavenging oxidizing radicals, such as .OH, N3. and NO2.. The structural characterisation of the Zn-Cat and Cu-Cat complexes was performed by UV/Vis and vibrational spectroscopy, whereas the Cat reactivity towards radiation-induced radicals was investigated by pulse radiolysis. The UV /Vis, Raman and IR studies were useful to assess the chelation sites, the relevant interaction of Cat with Cu(II) ions, as well as the weak affinity with Zn(II). Cat acts as a bidentate ligand through the catechol moiety on B ring, but the metal complex structure depends on metal. The addition of Zn(II) ions to Cat solutions in different metal /ligand (M/L) ratios at pH 8 gave rise to small modifications in the absorption and Raman spectra of Cat, suggesting the existence of only a weak affinity between Cat and Zn(II).The analysis of the spectra suggested the existence of two competitive reactions: one involving Zn(II) ions and leading to the complex formation, and another one, slower, involving oxygen. This reaction is probably hampered, at the initially stage, by the presence of metal ions, which may bind to Cat by the same sites involved in the oxidation (B ring) (Scheme 1). The autoxidation is strongly pH-dependent and increases by going to basic pH. The pH-dependence of the process suggests that the initial step of the oxidation occurs on the B ring that is partially deprotonated at pH 8 (pKa1 about 8.5).