Experimental and theoretical study on the coordination propertiesof quercetin towards aluminum(III), iron(III) and copper(II) inaqueous solution

In this work we have studied the complexation of quercetin (3,3',4',5,7-pentahydroxyflavone, H(5)Que) with aluminum(III),iron(III) and copper(II) at 37 degrees C and in aqueous solution (0.16 M NaCl). To evaluate the competitionof 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-Visdata 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 H-1 NMR and C-13 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-hydroxylsite of the A and C rings of the ligand) could be excluded. (C) 2020 Elsevier B.V. All rights reserved.