Mixed Ligand Mononuclear Copper(II) Complex as a Promising Anticancer Agent: Interaction Studies with DNA/HSA, Molecular Docking, and in Vitro Cytotoxicity Studies

Abstract

The isolated copper(II) complex [CuL(o-phen)]·H2O (1) [H2L = o-HO-C6H4C(H)═N-C6H4-SH-o, o-phen = 1,10-phenanthroline] was structurally characterized using single-crystal X-ray crystallography. 1 in CH3CN at liquid nitrogen temperature displayed a characteristic monomeric X-band electron paramagnetic resonance spectrum having a tetragonal character with g∥ = 2.1479 and g⊥ = 2.0691 and A∥ ≈ 18.0 mT and A⊥ ≤ 3.9 mT, respectively. 1 showed a strong binding affinity toward calf thymus DNA as reflected from its intrinsic binding constant (Kb = 7.88 × 105 M-1), and its competitive displacement of ethidium bromide suggested an intercalative DNA-binding mode (Kapp = 1.32 × 106 M-1). This was confirmed from the viscosity study that showed an increase in the viscosity of DNA with an increasing concentration of 1. Complex 1 is highly efficient in promoting oxidative and hydrolytic DNA cleavage (kobs = 1.987 h-1). 1 showed a strong binding affinity with the carrier protein human serum albumin (HSA) (Ka = 5.22 × 105 M-1). A high bimolecular quenching constant kq = 2.29 × 1013 M-1s-1 indicated a static quenching mechanism involved in the fluorescence quenching of HSA by 1. Fluorescence resonance energy transfer theory suggested that the distance (r = 3.52 nm) between 1 and HSA is very close. Molecular docking studies suggested that 1 primarily binds to HSA in subdomain IIA. A protein-ligand interaction profiler was used to visualize hydrophobic, hydrogen bonds, and π-cation interactions between HSA and 1. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay using HeLa and MDA-MB-231 cells showed a significant in vitro anticancer activity of 1 (IC50 2.63 and 2.68 μM, respectively). Nuclear staining assays suggested apoptotic cell death in HeLa cells treated with 1. The effect of 1 on the cytoskeletal actin filaments visualized using phalloidin staining showed extensive destruction of actin filaments. Flow cytometric analysis indicated that 1 inhibits the growth of HeLa cells through cell cycle arrest in the S phase. Western blot analysis showed upregulation in the expression of apoptotic marker proteins caspase 3, p53, and Bax. These results collectively indicate that 1 induces apoptosis by promoting DNA damage and has a high potential to act as an anticancer agent.