Computational and spectroscopic characterization of the molecular and electronic structure of the Pb(II)-quercetin complex.

The interactions of lead(II) ion with a polyhydroxylated flavonoid, the quercetin molecule, were investigated in methanol solution. The quercetin/metal stoichiometries and equilibrium stability constants for metal binding to quercetin have been determined by UV-vis spectroscopy combined with chemometrics methods. The 2:1, 1:2, and predominant 1:1 species are formed in solution. Among the three potential sites of chelation present in the quercetin structure, the catechol function presents the highest complexation power toward Pb(II), in opposition with previous results found for Al(III) complexation. This result has been confirmed by the good agreement of the experimental and theoretical features for both the electronic and vibrational spectra of the 1:1 complex. DT-DFT calculations show that the bathochromic shift of the long-wavelength band of the UV-vis spectra, that occurs upon complexation, is due to a ligand-to-metal charge transfer. The molecular structure of the ligand is not much modified by the coordination of lead at the level of the catecholate.