Quercetin autoxidation

The HPLC analysis of quercetin solution treated with MPO for 30 minutes (Figure 1(d)) revealed that concentration of quercetin (retention time 10 minutes) was negligible. Also, formation of major oxidation product (retention time 5.26 minutes) which was more polar than quercetin was detected (Figure 1(d)). From literature data and the obtained spectrophotometric and HPLC results in this work, we assume that the oxidation product detectable at 336—342 nm results from H20 addition on the p-quinonemethide formed by H-atom abstraction at 3-OH and 4 -OH of quercetin and subsequent rearrangement of the central ring [16]. The complete sequence of quercetin autoxidation induced by oxidants and catalyzed by metal ions is described earlier [15, 16] (Scheme 1). 

PEDRIELLI p, PEDULLi G F and SKIBSTED L H (2001a) Antioxidant mechanism of flavonoids. Solvent effect on rate constant for chain-braining reaction of quercetin and epicatechin in autoxidation of methyl linoleate, JAgric Food Chem, 49, 3034-40. 

The effects of flavonoids on in vitro and in vivo lipid peroxidation have been thoroughly studied [123]. Torel et al. [124] found that the inhibitory effects of flavonoids on autoxidation of linoleic acid increased in the order fustin < catechin < quercetin < rutin = luteolin < kaempferol < morin. Robak and Gryglewski [109] determined /50 values for the inhibition of ascorbate-stimulated lipid peroxidation of boiled rat liver microsomes. All the flavonoids studied were very effective inhibitors of lipid peroxidation in model system, with I50 values changing from 1.4 pmol l-1 for myricetin to 71.9 pmol I 1 for rutin. However, as seen below, these /50 values differed significantly from those determined in other in vitro systems. Terao et al. [125] described the protective effect of epicatechin, epicatechin gallate, and quercetin on lipid peroxidation of phospholipid bilayers. 

A one-electron oxidation study of quercetin (see structure below) and quercetin derivatives (rutin) by DPBH, CAN, or dioxygen in protic and aprotic solvents has shown that quercetin radicals quickly disproportionate to generate quercetin and produce a quinone. This quinone adds water molecules and is then degraded. Oligomerization might be a minor route in media of low water content. Oxidation of quercetin-serum albumin complex retarded water to the quercetin quinone. The role of the quercetin 3-OH was established as follows (1) allows the formation of jo-quinonoid compounds, quickly converted into solvent adducts which still react with one-electron oxidants, and (2) in its deprotonated form stabilizes radicals, allowing autoxidation to proceed under mild conditions. 

A number of papers show that among flavonoids there are strong scavengers of lipidic radicals [33, 34], For example, quercetin and Gingko biloba extract show an important antilipoperoxidant action or a weak effect (rutin and kaempferol), moreover hesperidin increases the rate of autoxidation. The discrepancy between the activity of these flavonoids seems to be important in explain the different effects of, for example, quercetin (inhibitor) and rutin (stimulator) on the cell COX activity. It is, in fact, important to remember that COX activity includes lipoperoxidative steps and thus it is inhibited by antilipoperoxidant agents. 

Prooxidant activity of phenolic compounds Phenolic antioxidants can initiate an autoxidation process and act like prooxidants under conditions that favor their autoxidation. Instead of terminating a free-radical chain reaction by reacting with a second radical, the phenoxy radical may also interact with oxygen and produce quinones (P = 0) and superoxide anion (02 ). PO -I-O2—>P = 0- -02 . Small phenolic compounds which are easily oxidized, such as quercetin, gallic acid, possess prooxidant activity while high-molecular weight phenolic compounds, such as condensed and hydrolyzable tannins, have little or no prooxidant activity. 

Nordstrom CG. Autoxidation of quercetin in aqueous solution an elucidation of the autoxidation reaction. Suomen Kemistilehti 1%8 841 351-353. 

Flavonol quercetin (Q) (3,5,7,3 4 -pentahydroxyflavone) (Scheme 1) due to its phenolic structure is a strong antioxidant and free radical scavenger [1]. Oxidation of quercetin during its antioxidative functions is usually accompanied by the production of the quercetin radical anion, superoxide, and hydrogen peroxide [2]. Quercetin undergoes autoxidation—the nonenzymatic reaction with atmospheric... 

H. E. Hajji, E. Nkhili, V. Tomao, and O. Dangles, Interactions of quercetin with iron and copper ions complexation and autoxidation, Free Radical Research, vol. 40, no. 3, pp. 303-320, 2006. 

---