Metal binding ability of curcumin derivatives: a theoretical vs. experimental approach

Theoretical calculations employing DFT at the B3LYP/6-311G++** level are used to investigate the tautomeric equilibrium in curcumin derivatives. The solvent effect is evaluated using the CPCM continuum solvation method. The results are compared with experimental data obtained from the X-ray crystal structure of K2A23 and UV-vis data. The KE tautomer is more stable in a vacuum and in the solid state, while in water the DK tautomer reaches a population of 90%. In agreement with spectroscopic data, theoretical calculations predict a slight prevalence of the DK form in non-aqueous solvent systems. The ability to chelate metal ions [Fe3+, Ga3+ and Cu2+] is then explored by means of 1H, 13C NMR and UV-Vis spectroscopy. From the calculation of the overall stability constants of metal complexes and 1H NMR titrations with Ga3+, it is clear that the more stable species has a 1 : 2 M/L molar ratio. The curcuminoid coordinates the metal ion through the keto–enol function in the dissociated form; in addition 2D 1H 13C NMR experiments suggest the involvement of carboxylic oxygen in metal coordination it was found in the solid state for the complex [Ga(K2A33)2]PF6. The rate of the complexation reaction is strongly influenced by the type of substituent on the aromatic ring of the curcuminoid (K2A33 ≈ K2A23 ≫ K2A21). In addition DPPH assay evidences how antioxidant ability of curcumin derivatives is mainly due to the presence of a phenolic group and metal coordination by a keto–enolic moiety does not affect it, especially for K2A21.