Mononuclear cobalt(II) complexes with polypyridyl ligands: Synthesis, characterization, DNA interactions and in vitro cytotoxicity towards human cancer cells.

Mononuclear cobalt(II) complexes [CoII(L1)Cl2]; 1, [CoII(L1)(bpy)Cl]PF6; 2, [CoII(L1)(phen)Cl]PF6; 3 and [CoII(L2)Cl2]; 4 (where L1 = N,N-bis(pyridin-2-ylmethyl)aniline, L2 = (2,4,6-trimethyl-N,N-bis(pyridin-2-ylmethyl)aniline, bpy = 2,2/-bipyridine, phen = 1,10-phenanthroline) were synthesized and characterized by different analytical and spectroscopic methods. All the complexes were structurally identified by single-crystal X-ray crystallography. Penta-coordinated complex 1 adopted distorted trigonal bipyramidal and hexacoordinated complexes 2 and 3 having distorted octahedral geometry whereas tetra-coordinated complex 4 has distorted tetrahedral geometry. The interactions of salmon sperm DNA (ss-DNA) with complexes (1-4) were investigated by absorbance, fluorescence spectroscopy and molecular docking studies. All the complexes are very susceptible to DNA binding and the binding affinity (Kb) follows the order 3 (2.05 × 104 M -1) > 4 (1.40 × 104 M -1) > 2 (1.36 × 104 M -1) > 1 (1.34 × 104 M -1) indicating they have superior DNA binding ability. The Stern-Volmer constant (Ksv) ranges from 1.10 × 104 M -1 to 1.95 × 104 M -1 suggesting weak or moderate binding with DNA. DNA cleavage study in plasmid DNA reveals very efficient DNA cleavage factors even in the absence of any external agents. Using multiple biochemical assays, we have demonstrated that 1-4 induces apoptosis of human cancer cells with IC50 values of 26.48 ± 1.45 μM, 10.89 ± 0.55 μM, 7.63 ± 0.4 μM and 37.67 ± 2.06 μM, respectively in A549 lung adenocarcinoma cells and 14.45 ± 0.73 μM, 1.97 ± 0.1 μM, 0.98 ± 0.05 μM and 24.43 ± 1.22 μM, respectively in MDA-MB-231 breast adenocarcinoma cells.