Interactions of quercetin with iron and copper ions: Complexation and autoxidation

Abstract

Quercetin (3,3',4',5,7-pentahydroxyflavone), one of the most abundant dietary flavonoids, has been investigated for its ability to bind Fe(II), Fe(III), Cu(I) and Cu(II) in acidic to neutral solutions. In particular, analysis by UV-visible spectroscopy allows to determine the rate constants for the formation of the 1:1 complexes. In absence of added metal ion, quercetin undergoes a slow autoxidation in neutral solution with production of low hydrogen peroxide (H(2)O(2)) concentrations. Autoxidation is accelerated by addition of the metal ions according to: Cu(I) > Cu(II)>>Fe(II) Fe(III). In fact, the iron-quercetin complexes seem less prone to autoxidation than free quercetin in agreement with the observation that EDTA addition, while totally preventing iron-quercetin binding, slightly accelerates quercetin autoxidation. By contrast, the copper-quercetin complexes appear as reactive intermediates in the copper-initiated autoxidation of quercetin. In presence of the iron ions, only low concentrations of H(2)O(2) can be detected. By contrast, in the presence of the copper ions, H(2)O(2) is rapidly accumulated. Whereas Fe(II) is rapidly autoxidized to Fe(III) in the presence or absence of quercetin, Cu(I) bound to quercetin or its oxidation products does not undergo significant autoxidation. In addition, Cu(II) is rapidly reduced by quercetin. By HPLC-MS analysis, the main autoxidation products of quercetin are shown to be the solvent adducts on the p-quinonemethide intermediate formed upon two-electron oxidation of quercetin. Finally, in strongly acidic conditions (pH 1-2), neither autoxidation nor metal complexation is observed but Fe(III) appears to be reactive enough to quickly oxidize quercetin (without dioxygen consumption). Up to ca. 7 Fe(III) ions can be reduced per quercetin molecule, which points to an extensive oxidative degradation.

Review on iron availability in soil: interaction of Fe minerals, plants, and microbes

“…Among the organic molecules detected in the rhizosphere in addition to MS s , oxalate needs to be mentioned as for its abundance among rhizodepositions and for its direct involvement in the biological and efficient weathering of minerals (Jones and Wilson 1985;Adamo et al 1997). Flavonoids released by roots have a direct effect on soil minerals and on biological activity in the soil that play an important role in Fe solubilization processes (El Hajji et al 2006;Tomasi et al 2008;Cesco et al 2012). However, although the impact of flavonoids on soil microorganisms could also be of relevance in terms of microbe-mediated Fe oxidation-reduction in soil (Tomasi et al 2008;Cesco et al 2010Cesco et al , 2012, the relative contribution of these molecules to the Fe availability in the rhizosphere and to the use of this Fe source for plant nutrient acquisition has not yet been comprehensively studied.…”

Oxidation of Quercetin by Myeloperoxidase

“…At the same time simultaneous rise of the absorption band at 336-342 nm, depended on MPO concentration, was observed (Figure 1(c)), typical of oxidation product formation [16]. Moreover, two well-defined isosbestic points at 290 and 364 nm indicate that there was no significant accumulation of intermediates in the reaction of oxidation product formation (Figure 1(c)).…”

“…Moreover, two well-defined isosbestic points at 290 and 364 nm indicate that there was no significant accumulation of intermediates in the reaction of oxidation product formation (Figure 1(c)). These spectral changes are analogous to those occurring upon autoxidation of quercetin in water [16] and UV irradiation [17] as well as oxidation with some oxidants in organic solvents [16]. 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.…”

“…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 H 2 O 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).…”

Evaluation of Antimicrobial and Antioxidant Activities of Methanolic Extracts of Flavonoids Obtained from the Leaves of Solenostemma argel Plant Collected in the Region of Tamanrasset, Algeria

“…This shows the importance of certain high-reducing phenolic functions which, when involved in glycosidic linkages by engaging these same functions, give a lower activity due to the blocking of the active functions [14].…”