A Cu(ii) pseudopeptide complex: a surrogate to a formal Cu(iii) species in water and an inhibitor of cancer cell proliferation

Metalloenzymes play a crucial role in life, performing myriad functions in aqueous environments in nature. Understanding these systems’ structural and functional mimics in water at ambient temperature is essential for developing new catalysts. Inspired by nature, we report here the synthesis of a remarkably stable formal [(L)Cu(III)]+ (2) species supported by a tetradentate pseudopeptide ligand, which we then characterized by various spectroscopic techniques in water at ambient temperature and using DFT methods. Our experimental and DFT studies indicate that the observed stability is caused by hydrogen bonding of water molecules to the amide moiety of the ligand bound to the formal Cu(III) center. The Natural Bonding Orbital (NBO) analysis indicated the best description of the oxidized species to be formal Cu(III). The formal Cu(III) species (2) could also be prepared by electrochemical oxidation methods in water. The generated species 2 was found to be reactive towards various phenols in H2O : CH3CN (1 : 1). The ability to stabilize formal Cu(III) in water encouraged us to investigate the biological activity of [(L)Cu(II)] (1) in cancer cell proliferation. Studies of these metalloenzyme mimics in cells indicated that intracellular and extracellular reactive oxygen species (ROS) generation, in turn, selectively prevented the growth of liver cancer cells over other cancer cell lines and normal kidney cells.