The effects of caffeic acid and its related catechols on hydroxyl radical formation by 3-hydroxyanthranilic acid, ferric chloride, and hydrogen peroxide.

The effect of caffeic acid on hydroxyl radical formation through a reaction, which contained 0.22 M carbonate buffer (pH 7.4), 0.22 mM 3-hydroxyanthranilic acid, 87 mM 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 2.9 mM hydrogen peroxide, and 14 microM FeCl3, was investigated. The addition of 30 microM caffeic acid resulted in the decrease of hydroxyl radical formation in the reaction mixture. Chlorogenic acid, 3,4-dihydroxy-phenylalanine noradrenaline, gallic acid, dopamine, epicatechin, and D-(+)-catechin also suppressed the hydroxyl radical formation. In regard to the positional isomers of benzenediol, o-benzenediol inhibited the hydroxyl radical formation, but m and p-benzenediol did not. The inhibitory effect of the hydroxyl radical formation seems to be due to the chelation of iron ions by the catechols. Supporting evidence includes the diminished effect of catechols in the presence of EDTA (a potent iron ion chelator) and the observation of a visible band at 450 nm caused by the interaction between caffeic acid and iron ions. Additionally, the visible band (506 nm) was observed in the solution of o-benzenediol and ferric chloride but not in the solution of m- or p-benzenediol and ferric chloride. Thus compounds with adjacent hydroxyl groups on aromatic rings might inhibit hydroxyl radical formation.