A treatise on benzimidazole-anchored Schiff base ligands and their metal (II) complexes: an exegesis on pharmacological potential through spectroscopic interrogation of DNA topology, genotoxic activity, and antimicrobial profiling

A series of Schiff base transition metal complexes was synthesized using the ligand (2E,4E)-N2,N4-bis(1H-benzo[ d ]imidazol-2-yl)pentane-2,4-diimine derived from 2-aminobenzimidazole and acetylacetone in a 2:1 M ratio and characterized by spectroscopic techniques and elemental analysis. Biophysical studies were conducted to evaluate their potential as DNA-interactive agents. Absorption studies determined the binding constant (kb) values and were in the order: Cu(II) (4.7 × 10 4 M −1 ) > Zn(II) (4.26 × 10 4 M −1 ) > Co(II) (3.8 × 10 4 M −1 ) > Fe(II) (2.3 × 10 4 M −1 ) > Mn(II) (1.3 × 10 4 M −1 ). Fluorescence quenching studies corroborated this trend, yielding Stern-Volmer constants (Kₛᵥ) of 4.1 × 10 4 M −1 , 3.21 × 10 4 M −1 , 2.8 × 10 4 M −1 , 2.2 × 10 4 M −1 , and 1.7 × 10 4 M −1 for the respective metal complexes. The results suggest a non-intercalative binding mode, such as groove binding, for all metal complexes. Based on their superior DNA-binding properties, the Cu(II), Co(II), and Zn(II) complexes were considered for further biological evaluation. Antioxidant assays (DPPH) showed moderate radical scavenging activity, though less potent than ascorbic acid, with IC₅₀ values of 23.2 μM, 65.5 μM, and 40.8 μM for the Cu(II), Co(II), and Zn(II) complexes, respectively. In antibacterial assays against Pseudomonas aeruginosa , these complexes exhibited inhibition zones of 14 mm, 10 mm, and 11 mm, respectively. Cytotoxicity studies against MCF-7 breast cancer cells revealed IC₅₀ values of 3.2 μM, 41.3 μM, and 23.0 μM for Cu(II), Co(II), and Zn(II) metal complexes, respectively, demonstrating cytotoxic effects, with the Cu(II) complex being the most potent. The Co(II) and Zn(II) complexes showed moderate activity. The consolidated results demonstrated a moderate DNA-binding affinity, suggestive of a non-intercalative mode, alongside measurable antioxidant, antimicrobial, and cytotoxic activities, particularly those of Cu(II), Co(II), and Zn(II), which make these compounds promising multifunctional candidates for therapeutic development. • Successful synthesis and characterization of a novel benzimidazole-anchored Schiff base ligand and its five transition metal complexes (Cu(II), Co(II), Zn(II), Fe(II), Mn(II)). • All metal complexes demonstrated a strong affinity for DNA, binding via the minor groove, with a binding affinity trend of Cu(II) > Zn(II) > Co(II) > Fe(II) > Mn(II). • Fluorescence quenching studies using ethidium bromide corroborated the DNA interaction trend, providing Stern-Volmer constants that aligned with the absorption data. • The Cu(II), Co(II), and Zn(II) complexes exhibited significant antioxidant activity in the DPPH assay, with the Cu(II) complex (IC₅₀ = 23.2 μM) being the most potent. • In antibacterial assays against Pseudomonas aeruginosa , the complexes showed measurable zones of inhibition, with the Cu(II) complex being the most effective (14 mm). • The complexes displayed potent, dose-dependent cytotoxicity against MCF-7 breast cancer cells, with the Cu(II) complex showing exceptional potency (IC₅₀ = 3.2 μM). • The Co(II) and Zn(II) complexes also showed considerable anticancer activity, with IC₅₀ values of 51.3 μM and 23 μM, respectively. • Mechanistic studies revealed that the complexes induce apoptosis (programmed cell death) in cancer cells, a clinically desirable pathway. • The apoptotic cell death was triggered by a dual mechanism: generation of Reactive Oxygen Species (ROS) and induction of mitochondrial dysfunction. • The Cu(II) complex consistently emerged as the most effective candidate across all biological evaluations, establishing it as a promising multifunctional therapeutic agent.