Facile synthesis of novel NNO-tethered copper(ii) complexes: characterization details, theoretical studies, promising enzyme-like activities, and biomolecular interactions

Abstract

Given the ubiquitous and multifaceted role of copper ions in various biological processes, we report herein the one-pot facile synthesis, X-ray structure, Hirshfeld surface analysis, enzyme-like activities, and biomolecular interactions of three mononuclear copper(II) complexes, [Cu(L)(X)] (1–3) with a tridentate quinoline-based salicylaldimine Schiff base (LH) having an N₂O donor set where X denotes NCS, N₃, and NO₃ for complexes 1, 2, and 3, respectively. Single-crystal X-ray study, spectroscopic techniques, DFT, and TD-DFT calculations were all used to fully characterize the complexes. The bio-inspired catalytic activities of the synthesized complexes were spectrophotometrically evaluated for the aerial oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) and 2-aminophenol (OAPH) in acetonitrile. The results of ESI mass spectrometry, EPR analysis of the reaction mixture, and DFT computations established that the aforementioned oxidation is metal-mediated and radical-driven, leading us to propose a viable mechanistic scheme. In complex 3, coordinated nitrate probably confers greater lability, allowing it to be the most effective enzyme for catecholase and phenoxazane-synthase activities. The biological activity of complexes 1–3 and the ligand LH towards calf thymus DNA and proteins (bovine serum albumin (BSA)) was explored using absorption and fluorescence spectral titrations, which affirmed that the compounds underwent avid binding with DNA, with high binding affinities (K_b) of approximately 10⁴–10⁵ M⁻¹. The observed DNA binding constants and viscosity measurement data suggested an intercalative mode of DNA binding with the copper(II) complexes. Spectral evidence also supports the high binding propensity (on the order of approximately 10⁵ M⁻¹) of the complexes with the protein. They actively suppressed the protein's intrinsic fluorescence in a static quenching mode, as further determined by fluorescence lifetime titration of protein with the complexes. Circular dichroism and synchronous spectroscopic experiments supported the protein's conformational alterations mediated by copper(II) complexes (1–3) in the microenvironment of the tryptophan residue of the protein. The typical binding distance between BSA and complexes was also computed using fluorescence resonance energy transfer. Of the three complexes (1–3), complex 3 stands out as the most efficacious.