Comparison of quercetin and a non-orthohydroxy flavonol as antioxidants by competing in vitro oxidation reactions.

Two structurally related flavonols, quercetin and morin, along with protocatechuic acid (PA), beta-resorcylic acid (DHBA), and phloroglucinol carboxylic acid (PCA), which represent quercetin and morin degradation products, were assessed with respect to their antioxidant potency by chemical comparisons in competing oxidation reactions. The measurement of the antioxidant capacity was performed with the beta-carotene bleaching method, and the compounds were also tested with respect to their abilities to prevent lipid, protein, and DNA oxidation. The effect of concentration was also considered. The results obtained strongly suggested that quercetin is a powerful antioxidant in every system used, whereas morin is a much weaker antioxidant and in some cases may also have pro-oxidant action. PA and PCA were always inferior antioxidants compared to the parent molecule quercetin; DHBA and PCA exhibited activities comparable to that of morin in reaction comparisons.