Synthesis, Characterization, Crystal Structure, DNA and HSA Interactions, and Anticancer Activity of a Mononuclear Cu(II) Complex with a Schiff Base Ligand Containing a Thiadiazoline Moiety

A mononuclear Cu(II) complex [Cu(HL)(o-phen)].H2O (1) [H3L = (o-HOC5H4=C(H)=N-N=C-S-C(=N-NH)-C5H4OH-o), 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(parallel to) = 2.164, g(parallel to) = 2.087, A(perpendicular to) = 19.08 mT, and A(perpendicular to) = 4 mT. Electronic spectroscopic studies using calf thymus DNA (CT-DNA) showed strong binding affinity of 1 as reflected from its intrinsic binding constant (Kb) value of 2.85 x 10(5) M-1. Competitive behavior of 1 with ethidium bromide (EB) displayed intercalative binding of DNA (K-app= 1.3 x 10(6) 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 x 10(5) M-1). Intrinsic fluorescence of the protein was found to be reduced through a mechanism of static quenching as suggested from the kq (2.01 x 10(13) M-1 s(1)) value, the bimolecular quenching constant. The Forster resonance energy transfer (FRET) process may also be accounted for such a high kq 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 mu M, 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 mu M 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.