Evaluating the substitution effects of bis(β-iminoenolate)copper(ii) complexes on their photophysical, DNA binding/photocleavage, and cytotoxic activities

DNA minor groove binders (MGBs) are excellent carrier structures for chemotherapeutic applications with minimal side effects and dependable sequence specificity. The copper complex with the iminoenolate ligand exhibits excellent catalytic activity in azide-alkyne cycloaddition reactions. In this work, the structure-activity relationship (SAR) of copper(ii) complexes with methoxy, unsubstituted and fluoro β-iminoenolate ligands (1a-c) was investigated towards DNA interactions and cytotoxicity. Molar conductance of complexes 1a-c was studied in methanol, and it provided evidence for the non-electrolytic nature of all three complexes in the solution. The DNA interactions with complexes 1a-c were investigated by absorption titration, competitive fluorescence titration with 4′,6-diamidino-2-phenylindole (DAPI), thermal denaturation and viscosity measurements. These four techniques reveal that complexes 1a-c bind to ds-DNA in the minor groove region. The calculated binding constant (K_b) values from the interaction studies reveal a higher binding propensity for the fluoro-substituted Cu(ii) complex (1c) than the other complexes. Molecular docking studies reveal the DNA minor groove binding of complex 1c with a preference for the AT-rich region over the GC-rich region. Photocleavage studies revealed that 1a-c cleave PUC 19 DNA effectively in the presence of the reducing agent ascorbic acid. Mechanistic studies suggest that the hydroxyl radical (OH˙) plays a major role in DNA cleavage. The in vitro cytotoxicity study performed in human cancer cell lines viz., MDA-MB-231 (human breast cancer) and HEp-2 (human larynx epithelioma cancer) revealed that the fluoro-substituted complex (1c) was more cytotoxic than the other two complexes (1a-b) in vitro, with IC₅₀ values of 24.2 μg mL⁻¹ against the HeLa cell line and 32.7 μg mL⁻¹ against MDA-MB-231. The findings imply that tuning the substituent in a ligand can improve the bioactivity of a metal complex.