Architectural Tuning of Ruthenium Centers to Drive Mitochondria-Mediated Apoptosis in Melanoma Cells

New Ru(II) arene complexes, namely, Bis(η6-p-cymene)-3,3'-((2,4,6-trimethyl-1,3-phenylene)bis(methylene))bis(1-(pyridin-2-ylmethyl)-1,3-dihydro-2H-benzo[d]imidazol-2-thione)(dichloro)ruthenium(II) dichloride (Ru-BIS) and 1-isopropyl-3-(pyridin-2-yl)-1,3-dihydro-2H-naphtho[2,3-d]imidazol-2-thione(chloro)ruthenium(II) chloride (Ru-NIS), were synthesized and characterized by various spectral techniques. The single-crystal X-ray diffraction structures substantiate a piano-stool octahedral geometry around the Ru(II) center of the complexes. The electronic spectra of Ru-BIS and Ru-NIS showed characteristic metal-to-ligand charge-transfer (MLCT) bands at 436 and 453 nm, respectively. The measured octanol/water distribution coefficient (log D) values for Ru-BIS and Ru-NIS demonstrated a marked enhancement of their aqueous solubility compared to their respective ligands (BIS and NIS). The stability studies of the complexes in D2O/DMSO-d6 (9:1) indicated a rapid aquation followed by the formation of a stable hydrolyzed species under physiological conditions. Ru-BIS and Ru-NIS bind strongly and statically to the bovine serum albumin (BSA) protein (Kb ∼ 105 M-1) with one ligand per binding site. Cell viability studies revealed that both Ru-BIS and Ru-NIS are biocompatible with non-malignant cells (C2C12 and L929), in defined concentration ranges. Furthermore, Ru-BIS (IC50 = 0.05 mg/mL, 40.31 μM) exhibited greater cytotoxicity against B16 melanoma cells compared to Ru-NIS (IC50 = 0.135 mg/mL, 215.78 μM). Microscopic evaluations of melanoma cells treated with Ru-BIS and Ru-NIS revealed that the former exhibited more mitochondrial dysfunction, apoptotic induction, and DNA double-strand breaks.