Zn–polyphenol chelation: complexes with quercetin, (+)-catechin, and derivatives: II Electrochemical and EPR studies

In this work, we have studied the formation of complexes between flavonols, (quercetin, rutin, quercitrin, kaempferol, luteolin, tamarixetin) and flavanols ((+)-catechin, (−)-epicatechin), flavanonol, (+)-taxifolin, and Zn acetate in two hydro-organic media at neutral pH in the absence of oxygen. The complexation was first studied by cyclic voltammetry. Then preparative electrolysis have been attempted followed by coulometry, UV–Vis optical absorption and circular dicroism spectroscopies for characterizing the oxidized compounds. Spectroelectrochemistries monitored either in the UV–Vis or in the EPR spectrometers at room temperature have been also used and we have identified o-semi-quinone intermediates in some cases. Different behaviour vis-à-vis the complexation by Zn2+ according to the molecular structures of these different families of polyphenols have been found. Some of them are more easily oxidizible.