Experimental and theoretical study on the coordination properties of quercetin towards aluminum(III), iron(III) and copper(II) in aqueous solution

In this work we have studied the complexation of quercetin (3,3′,4′,5,7-pentahydroxyflavone, H5Que) with aluminum(III), iron(III) and copper(II) at 37 °C and in aqueous solution (0.16 M NaCl). To evaluate the competition of the ligand for the metal cations and H+, the protonation constant of quercetin was also determined under the same experimental conditions. Speciation profiles obtained by potentiometric titrations and supported by UV–Vis data show that in aqueous solution a complexation occurs at 1:1 ligand-to-cations ratio for Al(III) and Cu(II), and at 1:1 and 1:2 ligand-to-Fe(III) ratio. The coordination sites of quercetin to the different metal ions were determined with the aid of 1H NMR and 13C NMR spectroscopy as well as by a computational approach. Synergies between experiment and computation show that aluminum and iron ions show no clear preference towards any of the complexation sites of quercetin, while for copper the 4,5 site (i.e., the 4‑carbonyl-5-hydroxyl site of the A and C rings of the ligand) could be excluded.