A spectroscopic and in silico study on ruthenium(ii) complexes bearing pyridine-carboxamide ligands interacting with DNA as potential therapeutics

Two half-sandwich ruthenium(ii) complexes (C1 and C2) bearing N,N-chelating picolinamide ligands were synthesised and comprehensively characterised to elucidate the role of ligand electronics in governing stability and DNA interactions. The complexes were fully characterised by HR-ESI-MS, NMR spectroscopy, FT-IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (SCXRD), confirming a pseudo-octahedral piano-stool geometry with bidentate coordination through pyridyl and amide nitrogen donors. TD-DFT calculations reproduced the experimental UV-vis spectra and revealed dominant metal-to-ligand and mixed metal-metal-ligand charge-transfer transitions. Solution stability studies demonstrated marked substituent-dependent behaviour: the unsubstituted complex (C1) readily underwent chloride-water exchange to form aqua, C1-aqua, and hydroxo species under physiological conditions, whereas the aryl-fluorinated analogue exhibited enhanced resistance to ligand substitution across a wide pH range. ctDNA-binding investigations using ethidium bromide displacement assays revealed concentration-dependent fluorescence quenching for both complexes, with C1-aqua displaying higher quenching efficiency and apparent binding affinity, K _SV 3.9 × 10³ M^-1 and 1.1 × 10³ M^-1, and log K _a of 4.17 and 2.38 for C1-aqua and C2, respectively. UV-CD, UV-LD, and thermal denaturation studies suggested that DNA binding involves a significant intercalative component, accompanied by helix stiffening and partial unwinding, although contributions from minor groove binding cannot be excluded based on the Hoechst-33258 displacement studies. Molecular docking and 100 ns molecular dynamics simulations further supported favourable interactions with AT-rich regions of DNA and indicated dynamically stable binding modes associated with local DNA distortion. Overall, the findings suggest that subtle ligand electronic modulation through aryl fluorination influences the stability, solution speciation, and DNA-binding behaviour of Ru(ii) half-sandwich complexes. These observations provide useful structure-activity insights for the continued development of ruthenium-based metallodrugs.