Anticancer drug design of potassium ion (K+) assisted self-assembled Cu(II) Schiff base complex and its Ni(II) and Zn(II) analogues (3d-4s): A comparative study of DNA/protein (BSA) interactions and cytotoxicity

Abstract

In the research for the therapeutic agents, three novel dinuclear 3d-4s complexes with a glycine-derived Schiff base ligand and a side-on bridging azido (µ-1,3-N₃) ligand were synthesized and characterized, with formulas [K(CuL)(µ-1,3-N₃)] (KCuL), [K(NiL)(µ-1,3-N₃)] (KNiL) and [K(ZnL)(µ-1,3-N₃)] (KZnL), where L indicates the Schiff base containing stoichiometric ratio of glycine and salicylaldehyde. Different structural, spectral and analytical methods, such as single-crystal X-ray crystallography, CHN analysis, FT-IR, UV–Vis and mass spectroscopy, were used for the characterization. The single-crystal X-ray diffraction of the KCuL complex confirmed the presence of a dinuclear complex containing one Cu(II) and K⁺centre, one Schiff base ligand and one side-on exogenous bridging azido ligand (μ-1,3-N₃) in the asymmetric unit. A considerable anticancer activity of the complexes was explored on different cancerous cell lines such as breast, lung carcinoma, gastric and enocarcinoma and normal fibroblasts. DNA cleavage experiment showed that all the complexes cleave pGFPC1 supercoiled DNA in the presence of an activator like H₂O₂. The interaction between the complexes and Fish Sperm DNA (FS-DNA) was inspected by UV–Vis and fluorescence titration spectrometry, viscosity measurement, cyclic voltammetry and docking studies, with intercalation found as the main binding mode. The order of intercalation propensity of the complexes was deduced from K_b values as KCuL > KNiL > KZnL, and this order was confirmed by the competitive EtBr/complex binding experiment. Absorption and fluorescence spectroscopy revealed a static quenching mechanism as the interaction mechanism for binding between the complexes and bovine serum albumin protein (BSA). Synchronous fluorescence and site-marker competitive experiments completed the investigations. Also, Forster resonance energy transfer theory (FRET) was employed to calculate the distance, r, between the acceptor (complex) and the donor (BSA). The quantitative tendency of the complexes to interact with BSA was calculated as K_SV, which varied in the order KCuL > KNiL > KZnL. Besides the results from the experimental methods, the theoretical molecular docking study and the Hirshfeld surface study alongside finger print plots were applied to confirm the mode and site of binding of the KCuL complex with DNA and BSA.