Flavonoids are an important class of polyphenolic compounds widely distributed in nature. They have been acknowledged to possess pharmacological effects (i.e. anti-cancer, anti-viral and anti-inflammatory actions) which are related to their antioxidant properties [1]. Flavonoids as antioxidant may act in different ways including scavenging of reactive oxidising species, chelation of transition metal ions, 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 [2]. 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. Moreover, large efforts are being made to establish sensitive analytical techniques to detect and discern flavonoid molecule in a mixture where these compounds could be found at very low concentrations, such as artworks. This study was aimed to obtain a wider insight into the molecular mechanisms of the beneficial action of two flavonoids, quercetin (I) and catechin (II), having the same number and arrangement of hydroxyl functions but a different conjugation structure between rings A and B (Fig. 1). The investigations, performed by using several techniques, were addressed on both the capability of these compounds in binding metal ions and scavenging oxidising radicals. The different steps of complexation were followed by Raman and Surface-Enhanced Raman Scattering (SERS) spectroscopy and the structures of the main complexes were proposed. Concomitantly, the reactivity of both compounds towards oxidising radicals and the nature of their intermediate species, in absence and the presence of copper ions, was studied by pulse radiolysis, which allows the production of different radicals in aqueous solution and the characterisation of the derived transient products. Depending on the state of protonation and the metal/ligand molar ratio, different sites of quercetin involved in the Cu(II) chelation were identified. On the other hand, the pulse radiolysis results showed as the scavenging of oxidizing radicals by quercetin and catechin gives mainly rise to the formation of phenoxyl-type radicals, quite stable and scarcely reactive because of the extensive electron delocalization through the whole system.
Interactions between Metals and Flavonoids: Spectroscopic and Pulse Radiolysis Studies
A TORREGGIANI;
2007
Abstract
Flavonoids are an important class of polyphenolic compounds widely distributed in nature. They have been acknowledged to possess pharmacological effects (i.e. anti-cancer, anti-viral and anti-inflammatory actions) which are related to their antioxidant properties [1]. Flavonoids as antioxidant may act in different ways including scavenging of reactive oxidising species, chelation of transition metal ions, 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 [2]. 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. Moreover, large efforts are being made to establish sensitive analytical techniques to detect and discern flavonoid molecule in a mixture where these compounds could be found at very low concentrations, such as artworks. This study was aimed to obtain a wider insight into the molecular mechanisms of the beneficial action of two flavonoids, quercetin (I) and catechin (II), having the same number and arrangement of hydroxyl functions but a different conjugation structure between rings A and B (Fig. 1). The investigations, performed by using several techniques, were addressed on both the capability of these compounds in binding metal ions and scavenging oxidising radicals. The different steps of complexation were followed by Raman and Surface-Enhanced Raman Scattering (SERS) spectroscopy and the structures of the main complexes were proposed. Concomitantly, the reactivity of both compounds towards oxidising radicals and the nature of their intermediate species, in absence and the presence of copper ions, was studied by pulse radiolysis, which allows the production of different radicals in aqueous solution and the characterisation of the derived transient products. Depending on the state of protonation and the metal/ligand molar ratio, different sites of quercetin involved in the Cu(II) chelation were identified. On the other hand, the pulse radiolysis results showed as the scavenging of oxidizing radicals by quercetin and catechin gives mainly rise to the formation of phenoxyl-type radicals, quite stable and scarcely reactive because of the extensive electron delocalization through the whole system.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
