Curcuminoids as potential new iron-chelating agents: spectroscopic, polarographic and potentiometric study on their Fe(III) complexing ability

The pK_a values of curcumin and diacetylcurcumin are, here doubtless, determined by means of spectroscopic and potentiometric measurements, and the enolic proton is the more acidic one. The interaction of Fe³⁺ with curcumin and diacetylcurcumin, in water/methanol 1:1 solution, leads to the formation of the complex species FeH₂CU(OH)₂] and FeDCU(OH)₂] (H₂CU and DCU=curcumin or diacetylcurcumin monoanion, respectively) which prevails near pH 7. At more basic condition the prevailing species are FeH₂CU(OH)₃]⁻ and FeDCU(OH)₃]⁻, which prevent metal hydroxide precipitation. ¹H NMR data state that the dissociated β-diketo moiety of the ligands is involved in metal chelation. The pK_a value of the deprotonation reaction is strongly anticipated by the metal ion, as shown by UV spectral data. The stability constants, evaluated from potentiometric data, are near to that of desferrioxamine, which is, by now, the only iron-chelating agent for clinical use.