SUMMARY The primary antioxidant activity of quercetin and some of its derivatives was studied in the 36–70°C range in dry systems, using two fatty‐ester substrates, respectively with linoleate and linolenate, as the main constituent undergoing oxidation. Metal contamination was avoided as far as possible, and any residual traces of metals in the thoroughly purified esters were chelated with citric acid. Methylation of the 3, or 5, or 3 and 7, or 5 and 7 hydroxyls of the quercetin molecule led to considerable reduction of the antioxidant activity, while reduction due to methylation of the 7 hydroxyl was slight. Methylation of the 3′, or 4′, or 3′ and 4′ hydroxyls, or of any single hydroxyl of the B ring, and of an additional hydroxyl or hydroxyls of the A ring, led to a drastic reduction (to 11% or less); the 3,7,3′,4′ tetramethoxy derivative was found to be completely inactive. Hydrogenation of the 2,3 double bond resulted in an antioxidant (dihydroquercetin) with only about half the activity of qnercetin. The primary antioxidant activity of quercetin seems to be a function of the molecule as a whole and cannot be regarded as an additive property of active hydroxyls. The effect produced by methylation of a particular hydroxyl may, however, be related to the probability of formation of a stabilized free radical by the hydroxyl in question. The type of the substituted alkyl radical had little or no effect on the activity of the derivative, but replacement of a hydroxyl with hydrogen failed to produce the same effect as methylation of the same hydroxyl. No indication was found of pro‐oxidant activity of the meta‐hydroxyl grouping in the A ring.
Journal of Food Science – Wiley
Published: Jul 1, 1966
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