Role of oxidizing radicals in damaging flavonoids and their metal complexes: some insights by spectroscopic and pulse radiolysis technique

Reactive oxygen species (ROS) and, more generally, free radicals are important mediators in biological systems. Free radicals are continuously produced in living cells during normal and pathological metabolic conditions. They are involved in the ageing degradation processes and are well known to be continuously produced under different environmental conditions (pollution, smoke, etc.). Flavonoids are an important class of low molecular weight polyphenols widely distributed in nature. They have been acknowledged to exert some beneficial actions (i.e. anti-inflammatory, anti-carcinogenic properties), most of which seem to be associated with antioxidant properties. Owing to the polyphenol structure of flavonoids and the extensive electronic delocalisation through the whole system, these compounds may act as antioxidants by different mechanisms including scavenging of reactive oxidizing species and chelation of transition metal ions, such as copper, able to promote free-radical reactions [1]. Another interesting aspect of flavonoids is their use as artistic colorants. They are the main components of a number of natural dyes used both as pigmenting agents in paintings and textile dyeing 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. Establishing the degradation mechanism and the identity of the intermediate compounds may allow, not only the determination of the used original compounds, but also the determination of the factors that influence this degradation. This allows the compilation of a set of rules for preventing their conservation. 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 to metal binding affects the behavior of these compounds towards oxidizing radicals. This study considers in particular the scavenging of damaging oxidizing radicals carried out by two flavonoids, quercetin and catechin, in the absence and presence of copper ions. Pulse radiolysis technique was useful to generate specific radicals and to follow their relevant reactions with the compounds under investigations.On the other hand, Raman and surface-enhanced Raman scattering (SERS) spectroscopy allowed to identify the metal coordination sites as well as the main complex species formed. It was seen that the interaction between the flavonoid molecules and metal ions depends on the protonation state of the ligand and the metal/ligand molar ratio. As a consequence of oxidizing radical scavenging by quercetin and catechin, the formation of phenoxyl-type radicals, quite stable and scarcely reactive, was evidenced.