Antioxidant Mechanism of Flavonoids. Solvent Effect on Rate Constant for Chain-Breaking Reaction of Quercetin and Epicatechin in Autoxidation of Methyl Linoleate

Publisher Website
Google Scholar
Abstract
The rate of oxygen depletion, as measured by electron spin resonance spectroscopy (oximetry using a spin probe), in a homogeneous solution of peroxidating methyl linoleate (initiated by an azo initiator) in the presence or absence of antioxidants was converted to second-order rate constants for the inhibiting reaction of quercetin and epicatechin. In the non-hydrogen-bonding solvent chlorobenzene at 50 °C, kinh had values of 4.3 × 105 M-1 s-1 for quercetin and 4.2 × 105 M-1 s-1 for epicatechin, respectively. In the hydrogen-accepting “water-like” solvent tert-butyl alcohol, the values were 2.1 × 104 and 1.7 × 104 M-1 s-1, respectively. The solvent effect (factor of 20) is more significant than for α-tocopherol (factor of 4), and the two flavonoids have efficiencies comparable to that of α-tocopherol in scavenging peroxyl radicals in the nonpolar solvent but not in the hydrogen-bonding solvent. Keywords: Quercetin; epicatechin; antioxidant mechanism; hydrogen atom transfer; solvent effect