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.

ABSTRACT:Introduction -Green tea, a popular drink with benefi cial health properties, is a rich source of specifi c fl avanols (polyphenols). There is a special interest in the water extraction of green tea polyphenols since the composition of the corresponding extracts is expected to refl ect the one of green tea infusions consumed worldwide. Objective -To develop a microwave-assisted water extraction (MWE) of green tea polyphenols. Methodology -MWE of green tea polyphenols has been investigated as an alternative to water extraction under conventional heating (CWE). The experimental conditions were selected after consideration of both temperature and extraction time. Conclusion -MWE appears more effi cient than CWE at both 80 and 100°C, particularly for the extraction of fl avanols and hydroxycinnamic acids. Although MWE at 100°C typically aff ords higher yields in total phenols, MWE at 80°C appears more convenient for the extraction of the green tea-specifi c and chemically sensitive fl avanols.

In this work, the affinity of common dietary phenols (gallic acid, caffeic acid, catechin, and rutin) for iron and copper ions was quantitatively investigated in neutral phosphate buffer as well as the reactivity of the complexes toward dioxygen. Contrasting behaviors were observed: because of the competing phosphate ions, Fe(III) binding is much slower than Fe(II) binding, which is rapidly followed by autoxidation of Fe(II) into Fe(III). With both ions, O2 consumption and H2O2 production are modest and the phenolic ligands are only slowly oxidized. By contrast, metal-phenol binding is fast with both Cu(I) and Cu(II). With Cu(I), O2 consumption and H2O2 production are very significant and the phenolic ligands are rapidly oxidized into a complex mixture of oligomers. The corresponding mechanism with Cu(II) is hampered by the preliminary rate-determining step of Cu(II) reduction by the phenols. The consequences of these findings for the stability and antioxidant activity of plant phenols are discussed.

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This work describes a straightforward synthesis of two 3'-(,fl-~-glycopyranosyloxy)flavylium ions thought to be good models of natural anthocyanins (pigments). For both pigments and for the non-glycosylated flavylium ion taken as a reference, H,O addition and proton-transfer reactions as well as formation of molecular complexes with chlorogenic acid and caffeine (copigmentation) are quantitatively investigated in mildly acidic aqueous solution. A remarkable affinity of caffeine for the trans-chalcone form of the pigments is demonstrated. Moreover, the differences in the flavylium pK, values are interpreted in terms of possible intramolecular H-bonding between the glycosyl residue and the chromophore. The discussion is then extended to a series of malonylated anthocyanins recently reported for their unusual pigmentation properties. A possible role for the malonyl group (frequently encountered in the structure of naturally occurring anthocyanins) in plant colour expression is outlined for the first time.