Antioxidant and DNA binding studies of Cu(II) complexes of N,N 0 -(1,1 0 -dithio-bis(phenylene))-bis(salicylideneimine): synthesis and characterization S. B. Moosun 1 • S. Jhaumeer-Laulloo 1 • E. C. Hosten 2 • T. I. A. Gerber 2 • M. G. Bhowon 1 Received: 25 January 2015 / Accepted: 23 March 2015 / Published online: 2 April 2015 Ó Springer International Publishing Switzerland 2015 Abstract Four Cu(II) complexes [Cu(HL–LH)Cl 2 ].H 2 O (1), [Cu 2 (L–L)Cl 2 ] n (2), [Cu(LH) 2 ]0.5H 2 O(3) and [CuL] n (4) derived from a disulfide Schiff base ligand N,N 0 -(1,1 0 - 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 an- tioxidant 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 10 3 –10 5 M -1 suggesting moderate binding affinity of the complexes toward CT-DNA, with complex 1 showing the maximum binding affinity. In competitive binding ex- periments with ethidium bromide, complex 1 displayed the highest quenching constant, consistent with the UV binding results. Complexes 1–4 were found to cleave DNA effi- ciently in its nicked or linear forms. Introduction In recent decades, studies of the interactions of metal complexes with DNA have attracted much attention due to their applications in biotechnology and medicine [1]. Knowledge of the types of interactions between small molecules and DNA provides a basis for understanding the reactions of nucleic acids in biological systems and can provide fundamental data in the development of DNA structural probes. DNA provides a number of binding sites and is capable of various binding modes involving inter- calation, groove binding, electrostatic and hydrogen- bonding interactions [2]. These various interaction modes of DNA with metal complexes and the quest to exploit them in antitumor or anticancer agents have prompted many studies into the binding or cleavage of DNA with such molecules. It has been reported that free radicals can cause damage to proteins, lipids and DNA, which is one of the causes of cancer [3]. The use of antioxidants can pre- vent the formation of free radicals, thus reducing cell damage. Hence, the design of compounds with both an- tioxidant and DNA binding properties has considerable potential for effective cancer therapy. The role of Cu Schiff base complexes as effective DNA binders and cleaving agents has been well documented [4, 5]. Such complexes are known for their strong interaction with DNA via surface association or intercalation [6] and poten- tial DNA-cleavage activities via hydrolytic or oxidative mechanisms [7]. The highly specific oxidative cleavage of DNA by Cu aminoglycosides and the sequence-specific non- covalent DNA binding of Cu complexes have contributed to the wide success of Cu Schiff base complexes in drug-DNA studies. Recently, Reedjik et al. reported novel Schiff base Cu complexes such as [Cu(pyrimol)Cl] (pyrimol = 4-methyl-2-N-(2-pyridylmethyl)aminophenol) that could & M. G. Bhowon mbhowon@uom.ac.mu 1 Department of Chemistry, University of Mauritius, Reduit, Mauritius 2 Department of Chemistry, Summerstrand Campus South, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa 123 Transition Met Chem (2015) 40:445–458 DOI 10.1007/s11243-015-9934-1