Synthesis, characterization and electrochemical behavior of new acyclic mono and binuclear copper (II) complexes: DNA binding and cleavage studies

A new series of acyclic mono and binuclear copper (II) complexes of the general formula [Cu(L1–5)](ClO4)(H2O)2 (1–5) and [Cu2(L1–5)(H2O)2](ClO4)2 (6–10), {L1–5 refers to [N-(salicylaldimine)-N′-[(2-formyl-4-methyl-6-(4-methylpiperazine-1-yl)methyl)phenol] diamines, where the diamines are L1 = 1,2-diamino ethane, L2 = 1,3-diamino propane, L3 = 1,2-diamino benzene, L4 = 2-aminobenzylamine and L5 = 1,8-diamino naphthalene} have been synthesized and characterized. From the single crystal X-ray diffraction method, the mononuclear complex [Cu(L1)](ClO4)(H2O)2 (1), in which the copper atom exhibits a distorted square planar geometry, coordinates with two phenolic oxygen atoms and two ethylenediamine nitrogen atoms. The coordination bond angles have values between 83.7(2)° [N1–Cu1–N2] and 94.08(19)° [O2–Cu1–N2]. Electrochemical studies were performed using cyclic voltammetry, in which the appearance of one quasi-reversible peak within the potential range −0.91 to −1.21 V (Epc) for mononuclear and two quasi-reversible peaks within the potential ranges −0.70 to −0.88 V (E1pc) and −0.96 to −1.15 V (E2pc) for binuclear complexes were observed respectively. The CT-DNA binding ability of the complexes was investigated using absorption and fluorescence spectroscopic studies, viscosity measurements and circular dichroic techniques. In the absorption spectra, the binding constant (Kb) values for all the complexes were found to be of the order of 0.4 × 104–5.5 × 104 and from fluorescence spectra the apparent binding constant (Kapp) values were of the order of 1.7 × 106–6.6 × 106. From the above Kb and Kapp values, the binding propensity of the complexes to CT-DNA is revealed to be through the intercalative mode. The complexes cleave supercoiled pBR322 DNA in the presence of mercaptoethanol as a reducing agent and with a mechanistic pathway involving the formation of singlet oxygen as a reactive oxygen species. Overall, the more aromatic containing binuclear Cu(II) complex (10) exhibits better DNA binding and cleavage activity than the substituted aliphatic complexes.

Graphical abstractNew acyclic mono and binuclear copper (II) complexes have been synthesized and characterized. The cyclic voltammograms show one quasi-reversible peak for the mononuclear and two quasi-reversible peaks for the binuclear copper (II) complexes. The complexes bind to DNA through the intercalative mode. The mono and binuclear complexes cleave DNA through an oxidative mechanism and singlet oxygen plays a role in the cleavage.Figure optionsDownload full-size imageDownload as PowerPoint slide