Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-breaking reaction of quercetin and epicatechin in autoxidation of methyl linoleate

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 degreesC, k(inh) had values of 4.3 x 10(5) M-1 s(-1) for quercetin and 4.2 x 10(5) M-1 s(-1) for epicatechin, respectively. In the hydrogen-accepting "water-like" solvent tert-butyl alcohol, the values were 2.1. x 10(4) and 1.7 x 10(4) M-1 s(-1), respectively. The solvent effect (factor of 20) is more significant than for alpha -tocopherol (factor of 4), and the two flavonoids have efficiencies comparable to that of alpha -tocopherol in scavenging peroxyl radicals in the nonpolar solvent but not in the hydrogen-bonding solvent.