Methoxy Substitution and Alkyl Ligand Chain-Length Effects on DNA/BSA Binding and Anticancer Activity of Cobalt(III) Schiff Base Complexes

A series of cobalt(III) salophen-type Schiff base complexes bearing methoxy substituents at the 3-, 4-, and 5-positions of the ligand, along with linear-chain alkyl amines of varying lengths (heptylamine vs. dodecylamine), were synthesized and characterized. DNA binding constants (Kb) ranged from 2.290 × 104 M⁻¹ (complex 1) to 4.628 × 104 M⁻¹ (complex 6), as determined by viscosity and ethidium bromide displacement studies, supporting a mixed groove–intercalative binding mode. Bovine serum albumin (BSA) fluorescence quenching experiments yielded binding constants up to 5.102 × 106 M⁻¹ (complex 5), with positive entropy changes indicating a significant hydrophobic contribution. DFT calculations were performed to probe the electronic structures of these metal complexes. Molecular docking analyses revealed stabilizing interactions of the 5-methoxy substituent with nucleobases and preferential positioning near Trp-212 in BSA. In A549 lung carcinoma cells, the 5-methoxy dodecylamine complex (complex 6) exhibited the highest cytotoxicity (IC50 = 32 μM), surpassing the unsubstituted analogue (65 μM) and comparable to cisplatin (30 μM) under identical conditions. Importantly, cell-cycle analysis revealed a structure-dependent bifurcation: complexes 1, 3, and 5 predominantly induced Sub-G₀/G₁ populations, whereas complexes 2, 4, and 6 promoted G₂/M arrest. These findings establish a coherent structure–activity relationship (SAR) in which methoxy position and axial-ligand hydrophobicity govern biomolecular interactions and translate into distinct cellular outcomes, providing design principles for future cobalt(III) complex-based anticancer agents.