A joint experimental and theoretical investigation on the oxidative coupling of resveratrol induced by copper and iron ions

Currently, a mounting interest exists on the biological activity of polyphenolic compounds, which have been suggested to exert positive effects on the human health. In this paper we report the first electrospray ionization mass spectrometry (ESI-MS) study on the gas-phase production of isomeric δ-viniferin and ɛ-viniferin dimers in racemic form, starting from acetonitrile/water solutions containing resveratrol and CuSO4 or FeCl3, respectively. Interestingly, the formation of racemic δ-viniferin dehydrodimer is observed in ESI-MS experiments carried out on resveratrol-copper mixtures, while the analogous resveratrol-iron reaction affords the racemic ɛ-viniferin dehydrodimer. The use of gas-phase techniques and of ab initio calculations, at BHandHLYP/LACV3P + +** level of theory, allowed us to elucidate some important aspects of these reaction mechanisms. In particular, a different stability for the resveratrol radicals involved in the oxidative coupling has been obtained in the presence of copper ion, favoring the formation of δ-viniferin, as proposed for the in vivo mechanism where copper is able to switch the resveratrol from an antioxidant to a prooxidant agent. Finally, the structure-reactivity relationship has been investigated for synthetic analogues of resveratrol, showing the crucial role of the OH group in para position.