Influence of 6‐Trifluoromethyluracil Versus Chloride Counterions in the Properties of Bis‐(1,10‐Phenanthroline)chloridecopper(II) Motif: Spectroscopic and In Vitro Biological Activity Assays

Copper(II) metallonucleases have emerged as a promising class of compounds to serve as alternatives to the platinum drugs. Here, we evaluated how different counterions, 6‐trifluoromethyluracil (6tfmu) and chloride, impact the biological activity of copper(II) complexes containing the bis‐(1,10‐phenanthroline)chloridecopper(II) motif. Compounds were characterized by chemical and spectroscopic measurements. Crystallographic studies were conducted for [Cu(phen) 2 Cl]6tfmu (1), while data from complex [Cu(phen) 2 Cl]Cl (2) were previously described. In vitro antiproliferative assays demonstrated that both complexes exhibited lower IC 50 values than cisplatin, with melanoma A375 cells as the most sensitive, being 1.3 and 0.9 µM 1 and 2, respectively. Both compounds bind DNA to an indistinguishable extent, through a hybrid mode combining minor‐groove recognition with partial intercalation of a single phenanthroline ligand. Molecular dynamics performed on the common [Cu(phen) 2 Cl] + cation and microplate fluorophore assays converge on this mode. The main reactive oxygen species generated by complexes 1 and 2 were ·OH and H 2 O 2 , respectively. Substrate depletion assays in a human Topoisomerase‐I‐mediated DNA relaxation experiment indicated the preference of the [Cu(phen) 2 ] 2+ motif for degrading the DNA substrate over directly inhibiting the enzyme itself. The counterion modulates antibacterial potency and identity of the dominant reactive species, while leaving DNA recognition by the shared cation essentially unchanged.