Reduction of Copper(II) to Copper(I) in the Copper-Curcumin Complex Induces Decomposition of Curcumin

We report the decomposition of curcumin due to reduction of Cu(II) to Cu(I). Cu(II) binds tightly with curcumin to form a complex which exhibits a high stability in methanol, but it decomposes readily in acetonitrile and in SDS micelles in the presence of ascorbic acid, coincident with reduction of Cu(II) to Cu(I). In this study, the UV-Vis absorption of the Cu-curcumin complex shows a monotonic decrease as a function of time, consistent with the decomposition of curcumin. At a high copper : curcumin molar ratio of 10 : 1, the UV-Vis absorption spectrum of the Cu(II)-curcumin complex in acetonitrile exhibits a substantial blue shift of the absorption maximum from 420 nm to 350 nm, which is indicative of a significant decrease in conjugation length of curcumin in the presence of Cu(II). Time-dependent mass spectrometry and high performance liquid chromatography (HPLC) data are also consistent with the decomposition of curcumin as a consequence of reduction of Cu(II) to Cu(I).