Comparative study of copper(II)–curcumin complexes as superoxide dismutase mimics and free radical scavengers

Two stoichiometrically different copper(II) complexes of curcumin (stoichiometry, 1:1 and 1:2 for copper:curcumin), were examined for their superoxide dismutase (SOD) activity, free radical-scavenging ability and antioxidant potential. Both the complexes are soluble in lipids and DMSO. The formation constants of the complexes were determined by voltammetry. EPR spectra of the complexes in DMSO at 77 K showed that the 1:2 Cu(II)–curcumin complex is square planar and the 1:1 Cu(II)–curcumin complex is distorted orthorhombic. Cu(II)–curcumin complex (1:1) with larger distortion from square planar structure shows higher SOD activity. These complexes inhibit γ-radiation induced lipid peroxidation in liposomes and react with DPPH acting as free radical scavengers. One-electron oxidation of the two complexes by radiolytically generated azide radicals in Tx-100 micellar solutions produced phenoxyl radicals, indicating that the phenolic moiety of curcumin in the complexes participates in free radical reactions. Depending on the structure, these two complexes possess different SOD activities, free radical neutralizing abilities and antioxidant potentials. In addition, quantum chemical calculations with density functional theory have been performed to support the experimental observations.