Antioxidant and DNA binding studies of Cu(II) complexes of N,N′-(1,1′-dithio-bis(phenylene))-bis(salicylideneimine): synthesis and characterization

Four Cu(II) complexes [Cu(HL–LH)Cl2].H2O (1), [Cu2(L–L)Cl2]n (2), [Cu(LH)2]0.5H2O (3) and [CuL]n (4) derived from a disulfide Schiff base ligand N,N′-(1,1′-dithio-bis(phenylene))-bis(salicylideneimine) (HL–LH) were synthesized under different reaction conditions in the absence and presence of N-donor ligands and characterized by spectral, elemental, magnetic and thermal studies. A single-crystal X-ray analysis of 4 revealed that the Cu atom is in a square planar geometry, coordinated to phenolic oxygen, azomethine nitrogen and two sulfur atoms with cleavage of the S–S bond in HL–LH. Complexes 1–4 were found to be better antioxidants than HL–LH. Complexes 1 and 3, possessing free OH groups, showed higher antioxidant potentials. The interactions of the Schiff base Cu(II) complexes with DNA were investigated by UV–visible and fluorescence spectroscopies and agarose gel electrophoresis. The binding constants were in the order 103–105 M−1 suggesting moderate binding affinity of the complexes toward CT-DNA, with complex 1 showing the maximum binding affinity. In competitive binding experiments with ethidium bromide, complex 1 displayed the highest quenching constant, consistent with the UV binding results. Complexes 1–4 were found to cleave DNA efficiently in its nicked or linear forms.