Understanding Zinc(II) Chelation with Quercetin and Luteolin: A Combined NMR and Theoretical Study

The Zn(II) chelation with natural flavonoids, quercetin and luteolin, was investigated by the use of NMR spectroscopy and various levels of ab initio calculations. Very sharp phenolic OH 1H resonances in DMSO-d6 were observed for both free and complexed quercetin which allowed (i) the unequivocal assignment with the combined use of 1H–13C HSQC and HMBC experiments and (ii) the determination of complexation sites which were found to be the CO-4 carbonyl oxygen and the deprotonated C-5 OH group of quercetin and CO-4 carbonyl oxygen and the deprotonated C-5 OH group of luteolin. DOSY experiments allowed the determination of the effective molecular weight of the Zn–quercetin complex which was shown to be mainly 1:1. DFT calculations of the 1:1 complex in the gas phase demonstrated that the C-3 O– and CO-4 sites are favored for quercetin at both GGA and LDA approximations and the C-5 O– and CO-4 groups of luteolin at the LDA approximation. Quantum chemical calculations were also performed by means of the conductor polarizable model in DMSO by employing various functionals. The energetically favored Zn chelation sites of the 1:1 complex were found to be either the C-3 O– and CO-4 or C-5 O– and CO-4 sites, depending on the functional used, for quercetin and the C-5 O– and CO-4 sites for luteolin.