Cu(ii) and Zn(ii) complexes show very strong binding with human serum albumin and display remarkable cytotoxicity against the HeLa cell line.

The reaction of Cu(OAc)2·H2O with o‐HOC6H4‐C(H)N‐NH‐C(S)‐NH‐NC(H)‐C6H4OH‐o in methanol resulted in the formation of a dinuclear Cu(II) complex [(o‐phen)Cu(HL)Cu(HL)] (1) in the solution where HL2− is a new O−NN− ligand ο−HOC6H4‐normalC()H=normalN‐normalN=truenormalC‐normalS‐normalCfalse(=normalN‐NH⎵false)‐C6H4OH−ο formed in situ from cyclization of the Schiff base and the isolated product was found to be a dimer of 1 forming a tetranuclear compound in solid due to axial coordination of bridging phenolate O− as revealed from X‐ray crystallography. Electronic spectroscopic study using calf thymus DNA as well as fluorescence quenching study of ethidium bromide bound DNA in presence of 1 showed intercalative DNA binding with Kb = 1.12 × 105 M−1 and Kapp = 8.80 × 105 M−1, respectively. This compound was found to be efficient in hydrolytic as well as oxidative DNA cleavage. The binding interaction between complex 1 and human serum albumin (HSA) was investigated using fluorescence emission, absorption, and molecular docking studies. 3D fluorescence studies revealed that HSA structure was altered at secondary and tertiary levels. The binding constant Ka value (~106 M−1) suggested a strong binding affinity of 1 to HSA, and the bimolecular quenching constant kq value (1013 M−1 s−1) suggested a static quenching mechanism operative in the quenching of the intrinsic fluorescence of the protein. Such high kq value may also be accounted by Förster resonance energy transfer (FRET) process. The distance r (3.22 nm) between donor HSA and acceptor complex 1 calculated from FRET theory suggested that they are close to each other. 3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay with human cervical cancer HeLa cells and human breast cancer MCF7 cells (IC50 = 2.2 and 1.2 μM, respectively) suggested remarkable in vitro anticancer activity of 1. Cell death in HeLa cells was induced via apoptosis as revealed from nuclear staining assays using Hoechst 33342 and acridine orange/propidium iodide dual staining, and apoptosis induction in HeLa cells happened through reactive oxygen species accumulation.

A mononuclear Cu­(II) complex [Cu­(HL)­(o-phen)]·H2O (1) [H3L =, o-phen = 1,10-phenanthroline] was isolated from methanol, and its X-ray single-crystal structure was determined. Frozen glass X-band EPR of 1 in dimethylformamide (DMF) at LNT showed a spectrum that is characteristic of a monomeric tetragonal character with g ∥ = 2.164, g ⊥ = 2.087, A ∥ = 19.08 mT, and A ⊥ ≤ 4 mT. Electronic spectroscopic studies using calf thymus DNA (CT-DNA) showed strong binding affinity of 1 as reflected from its intrinsic binding constant (K b) value of 2.85 × 105 M–1. Competitive behavior of 1 with ethidium bromide (EB) displayed intercalative binding of DNA (K app = 1.3 × 106 M–1). The compound displayed significant oxidative cleavage of pUC19 DNA. The interaction between HSA and complex 1 was examined by employing fluorescence and electronic absorption spectroscopic experiments. The secondary and tertiary structures of HSA were found to be altered as suggested by three-dimensional (3D) fluorescence experiments. The affinity of 1 to bind to HSA was found to be strong as indicated from its value of the binding constant (K a = 2.89 × 105 M–1). Intrinsic fluorescence of the protein was found to be reduced through a mechanism of static quenching as suggested from the k q (2.01 × 1013 M–1 s–1) value, the bimolecular quenching constant. The Förster resonance energy transfer (FRET) process may also be accounted for such a high k q value. The r value (2.85 nm) calculated from FRET theory suggested that the distance between complex 1 (acceptor) and HSA (donor) is quite close. Complex 1 primarily bound to HSA in subdomain IIA as suggested by molecular docking studies. IC50 values (0.80 and 0.43 μM, respectively) obtained from the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay with HeLa and MCF7 cells suggested remarkable in vitro anticancer activity of 1. Nuclear dual staining assays revealed that cell death occurred via apoptosis in HeLa cells and reactive oxygen species (ROS) accumulation caused apoptosis induction. On treatment with a 5 μM dose of 1 in HeLa cells, the cell population significantly increased in the G2/M phase, while it was decreased in G0/G1 and S phases as compared to the control, clearly indicating G2/M phase arrest.