Study on fisetin-aluminium(III) interaction in aqueous buffered solutions by spectroscopy and molecular modeling

Spectroscopic (UV/visible and IR) and theoretical studies were used to assess relevant interaction of fisetin, a tetrahydroxylated flavone molecule, and trivalent aluminium in a wide range of buffered aqueous solutions. The chelation sites, stoichiometry, stability and the dependence of the complexes structures on pH and aluminium/fisetin mole ratios were defined. Obtained results implicated successive formation of two complexes with aluminium(III)-fisetin stoichiometries of 1:1 and 2:1. Considering the fisetin molecular structure, results of vibrational analysis and theoretical calculations, it is possible to implicate 3-hydroxyl-4-carbonyl and 3'4'-dihydroxyl groups as those with the possible chelating power. The equilibrium geometries were optimized in vacuum at the B3LYP/6-31G(d) level of theory, which predict structural modifications between the ligand molecule in free state and in the complex structure. The theoretical model has been validated by both vibrational and electronic spectroscopies.