Comparative Analysis of Molecular Properties and Reactions With Oxidants for Quercetin, Catechin and Naringenin

Abstract Flavonoids, a large group of secondary plant phenolic metabolites, are important natural antioxidants and regulators of cellular redox balance. The present study addressed the evaluation of the electronic properties of some flavonoids belonging to different classes: quercetin (flavonols), catechin (flavanols), and naringenin (flavanones) and their interactions with oxidants in the model systems of DPPH reduction, flavonoid autoxidation and chlorination. According to our ab initio calculations, the high net negative excess charges of the C rings and the small positive excess charges of the B rings of quercetin, catechin, and naringenin make these parts of flavonoid molecules attractive for electrophilic attack. The 3’-OH group of the B-ring of quercetin has the highest excess negative charge and the lowest energy of hydrogen atom abstraction for the flavonoids studied. The apparent reaction rates (M-1s-1, 20 ºC) and the activation energies (kJ/mol) of DPPH reduction are 4000±1000 and 23.0±2.5 in the case of quercetin, 1100±200 and 32.5±2.5 in the case of catechin, respectively. The stoichiometry of the DPPH – flavonoid reaction was 1:1. The activation energies (kJ/mol) of quercetin and catechin autoxidations were 50.8±6.1 and 58.1±7.2, respectively. Naringenin was not oxidized by the DPPH radical and air oxygen (autoxidation) and the flavonoids studied effectively prevented HOCl-induced hemolysis due to direct scavenging of hypochlorous acid (flavonoid chlorination). The best antioxidant quercetin has the highest value of HOMO energy, the planar structure and the optimal electron orbital delocalization on all phenolic rings due to C2=C3 double bond in the C ring (absent in catechin and naringenin).