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

The isolated copper(II) complex [CuL( o -phen)]·H 2 O ( 1 ) [H 2 L = o -HO-C 6 H 4 C(H)=N-C 6 H 4 -SH- o , o -phen = 1,10-phenanthroline] was structurally characterized using single-crystal X-ray crystallography. 1 in CH 3 CN 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 ( K b = 7.88 × 10 5 M -1 ), and its competitive displacement of ethidium bromide suggested an intercalative DNA-binding mode ( K app = 1.32 × 10 6 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 ( k obs = 1.987 h -1 ). 1 showed a strong binding affinity with the carrier protein human serum albumin (HSA) ( K a = 5.22 × 10 5 M -1 ). A high bimolecular quenching constant k q = 2.29 × 10 13 M -1 s -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 (IC 50 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.