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 37C and in 0.16 M NaCl. Theprotonation constant of curcumin was also determined to evaluate the competition of the ligand for themetal cations and Hþ. Speciation profiles obtained by potentiometric titrations show that in aqueoussolution a complexation occurs at 1:1 ligand-to-cations ratio. With the aim to define the coordinationsites of curcumin with these metal ions a mass spectrometry and UVeVis spectroscopy characterizationwas carried out. It was found that aluminum binds to the keto-enol moiety of curcumin, while iron bindsto the guaiacol (2-methoxyphenol) one. This unexpected experimental evidence was supported by a thorough computational study.