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 evaluation of the electronic properties of some flavonoids belonging to different classes such as quercetin (flavonols), catechin (flavanols), and naringenin (flavanones) and their interactions with oxidants in 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 rate constants (s^-1, 20 °C) and the activation energies (kJ/mol) of DPPH reduction were 0.34 ± 0.06 and 23.0 ± 2.5 in the case of quercetin, 0.09 ± 0.02 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 had the highest value of HOMO energy, a planar structure and optimal electron orbital delocalization on all the phenolic rings due to the C2=C3 double bond in the C ring (absent in catechin and naringenin).

Keywords: Antioxidants; Catechin; Chlorination; Naringenin; Quercetin; Structure.

Fig. 1

Optimized molecular structures of quercetin, catechin, and naringenin (the optimization was performed by the semi-empirical AM1 method), calculated excess charges of atoms of flavonoid molecules (using the non-empirical ab initio method with 6-31G basis and UHF method with the Polak–Ribière gradient algorithm)

Fig. 2

Calculated HOMO delocalization maps of quercetin, catechin, and naringenin molecules (using the non-empirical ab initio method with 6-31G basis and UHF method with the Polak–Ribière gradient algorithm)

Fig. 3

Concentration dependence of DPPH (100 µM) reduction by quercetin (a) and catechin (b) at different temperatures (10–40 °C) in ethanol, represented as the logarithm form

Fig. 4

Arrhenius plots of DPPH (100 µM) reduction by quercetin (a) and catechin (b) (2.5–20 μM) in ethanol

Fig. 5

UV–VIS spectra of naringenin under autoxidation (a) and chlorination (b) (in the presence of 0, 25, 50, 100, and 150 μM HOCl) in ethanol/PBS mixture (30%/70%), pH 7.4, the flavonoid concentration was 50 μM, the temperature of autoxidation was 50 °C, 1-h interval, the temperature of chlorination was 25 °C, 1 min of incubation with HOCl

Fig. 6

UV–VIS spectra of quercetin under autoxidation (a) and chlorination (b) (in the presence of 0, 25, 50, 100, and 150 μM HOCl) in ethanol/PBS mixture (30%/70%), pH 7.4, the flavonoid concentration was 50 μM, the temperature of autoxidation was 50 °C, 1-h interval, the temperature of chlorination was 25 °C, 1 min of incubation with HOCl

Fig. 7

UV–VIS spectra of catechin under autoxidation (a) and chlorination (b) (in the presence of 0, 25, 50, 100, and 150 μM HOCl) in ethanol/PBS mixture (30%/70%), pH 7.4, the flavonoid concentration was 50 μM, the temperature of autoxidation was 50 °C, 1-h interval, the temperature of chlorination was 25 °C, 1 min of incubation with HOCl

Fig. 8

Time dependences of quercetin absorbance changes (D₃₈₀) in the absence of antioxidants, and in the presence of melatonin, reduced glutathione, ascorbic acid in ethanol/PBS mixture (30%/70%), pH 7.4, at 25 °C, the flavonoid concentration was 50 μM, the antioxidant concentrations were 100 μM; *p < 0.05 in comparison with quercetin alone

Fig. 9

Inhibition by flavonoids (quercetin, catechin, naringenin) of rat erythrocyte hemolysis induced by HOCl. Erythrocyte suspension (hematocrit 0.05%) was mixed with HOCl (75 μM) in the absence or the presence of varying concentrations of flavonoids in PBS, pH 7.4, at 25 °C; *p < 0.05 compared with hemolysis in the absence of flavonoids