Interaction of flavonoids with transition metals can be partially responsible for their impact on humans. Stoichiometry of the iron/copper complex with a flavonoid glycoside isoquercitrin, a frequent component of food supplements, was assessed using competitive and non-competitive methods in four (patho)physiologically-relevant pH values (4.5. 5.5, 6.8, and 7.5). Isoquercitrin chelated all tested ions (Fe2+, Fe3+, Cu2+, and Cu⁺) but its affinity for Cu⁺ ions proved to be very low. In general, the chelation potency dropped with pH lowering. Metal complexes of 1:1 stoichiometry were mostly formed, however, they were not stable and the stoichiometry changed depending on conditions. Isoquercitrin was able to reduce both Cu2+ and Fe3+ ions at low ratios, but its reducing potential was diminished at higher ratios (isoquercitrin to metal) due to the metal chelation. In conclusion, this study emphasizes the need of using multiple different methods for the assessment of chelation potential in moderately-active metal chelators, like flavonoids.

Department of Pharmaceutical Botany Faculty of Pharmacy in Hradec Králové Charles University Heyrovského 1203 500 05 Hradec Králové Czech Republic

Department of Pharmacology and Toxicology Faculty of Pharmacy in Hradec Králové Charles University Heyrovského 1203 500 05 Hradec Králové Czech Republic

Laboratory of Biotransformation Institute of Microbiology Czech Academy of Sciences Vídeňská 1083 142 20 Prague Czech Republic

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