Kinetics and modeling of L-cysteine effect on the Cu(II)-induced oxidation of quercetin

Abstract

This study aimed at investigating the effect of L-cysteine on the Cu2+-induced oxidation of quercetin, within a pH range varying from 6.7 to 8.3 and temperatures varying from 53 to 87 °C. Initial examinations showed that quercetin degradation obeyed apparent first-order kinetics and it was significantly temperature-dependent. Modelling of the effect of L-cysteine by implementing response surface methodology suggested that L-cysteine did not inhibit quercetin oxidation, but it acted as an oxidation stimulant. Liquid chromatography-diode array-mass spectrometry analyses revealed the presence of typical quercetin degradation and oxidation products, including protocatechuic acid, 2-(hydroxybenzoyl)-2-hydroxybenzofuran-3(2H)-one and a quinone methide. However, the presence of two tentative quercetin/L-cysteine adducts evidenced the implication of L-cysteinyl radicals in the relevant reactions. It was concluded that the formation of L-cysteinyl radicals may promote quercetin oxidation, giving rise to substances with unknown biological significance. This fact merits further attention to illuminate possible beneficial or adverse nutritional consequences of such reactions in foods.