Insight on the chelation of aluminum(III) and iron(III) by curcumin in aqueous solution

In this work we have studied the complexation of curcumin (HCur, 1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) with Al(III) and Fe(III) at 37 °C and in 0.16 M NaCl. The protonation constant of curcumin was also determined to evaluate the competition of the ligand for the metal cations and H+. Speciation profiles obtained by potentiometric titrations show that in aqueous solution a complexation occurs at 1:1 ligand-to-cations ratio. With the aim to define the coordination sites of curcumin with these metal ions a mass spectrometry and UV–Vis spectroscopy characterization was carried out. It was found that aluminum binds to the keto-enol moiety of curcumin, while iron binds to the guaiacol (2-methoxyphenol) one. This unexpected experimental evidence was supported by a thorough computational study. [Display omitted] •Experimental and theoretical results are employed to study the formation of Al(III)- and Fe(III)-curcumin complexes.•The potentiometric study of these systems allowed us to clearly identify the stoichiometry of the complexes.•The coordination sites of these complexes were defined by mass spectrometry and UV spectroscopy characterization.•Aluminum binds to the keto-enol moiety of curcumin.•Iron binds to the guaiacol (2-methoxyphenol) moiety of curcumin.