Enhancement in intramolecular interactions and in vitro biological activity of a tripodal tetradentate system upon complexation

Abstract

Novel biomimetic mononuclear complexes, [Fe(Bz-NI)Cl₂]⁺ (1) and [Cu(Bz-NI)(H₂O)]²⁺ (2) based on naphthalimide appended tripodal tetradentate ligand (Bz-NI = 2,2′,2′′-(3,3′,3′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl))tris(propane-3,1-diyl))tris(1H-benzo-[de]isoquinoline-1,3(2H)-dione)) have been synthesized and characterized by various analytical and spectral techniques. In addition, the structures of the ligand (Bz-NI) and complex 2 were established unambiguously through X-ray crystal structure analysis. Uniquely, the coordination with a metal ion modified the ligand scaffold to interact efficiently with ct-DNA (groove binding) as well as protein (hydrophobic and/or electrostatic interactions). We have determined the affinity of these complexes for DNA/protein and the values are found to be in the range, K_DNA = 0.34–1.01 × 10⁴ M⁻¹ and K_BSA = 4.1–5.0 × 10⁵ M⁻¹. Furthermore, the fluorescence quenching of BSA with complexes 1 and 2 occurs through a static mechanism and affects the conformation of BSA around the tryptophan residues. The in vitro biological studies of these systems employing HeLa cell lines indicated that both these complexes exhibited enhanced cytotoxicity (IC₅₀ = 32 ± 0.19 and 10 ± 0.21 μM for complexes 1 and 2, respectively), when compared to the ligand (Bz-NI) (IC₅₀ = 150 μM). Interestingly, both the complexes (1 and 2) were found to be non-toxic to normal H9C2 cell lines. The mechanism of in vitro biological activity of these complexes has been evaluated through a variety of techniques: acridine orange/ethidium bromide, DAPI staining studies, annexin V-FITC/PI and poly(ADP-ribose)-polymerase (PARP) cleavage, which confirmed the apoptotic mediated cell death. Our results demonstrate the importance of complexation of the naphthalimide ligand (Bz-NI) as well as the potential of these biomimetic metal complexes as cytotoxic and anticancer agents.