Eukaryotic Cell Toxicity and HSA Binding of [Ru(Me4phen)(bb7)]2+ and the Effect of Encapsulation in Cucurbit[10]uril

The toxicity (IC ₅₀ ) of a series of mononuclear ruthenium complexes containing bis[4(4'-methyl-2,2'-bipyridyl)]-1,n-alkane (bb _n ) as a tetradentate ligand against three eukaryotic cell lines-BHK (baby hamster kidney), Caco-2 (heterogeneous human epithelial colorectal adenocarcinoma) and Hep-G2 (liver carcinoma)-have been determined. The results demonstrate that cis-α-[Ru(Me ₄ phen)(bb ₇ )] ²⁺ (designated as α-Me _4p hen-bb ₇ , where Me ₄ phen = 3,4,7,8-tetramethyl-1,10-phenanthroline) showed little toxicity toward the three cell lines, and was considerably less toxic than cis-α-[Ru(phen)(bb ₁₂ )] ²⁺ (α-phen-bb ₁₂ ) and the dinuclear complex [Ru(phen) _{2 2} μ-bb ₁₂ ] ⁴⁺ . Fluorescence spectroscopy was used to study the binding of the ruthenium complexes with human serum albumin (HSA). The binding of α-Me ₄ phen-bb ₇ to the macrocyclic host molecule cucurbit[10]uril (Q[10]) was examined by NMR spectroscopy. Large upfield ¹ H NMR chemical shift changes observed for the methylene protons in the bb ₇ ligand upon addition of Q[10], coupled with the observation of several intermolecular ROEs in ROESY spectra, indicated that α-Me ₄ phen-bb ₇ bound Q[10] with the bb ₇ methylene carbons within the cavity and the metal center positioned outside one of the portals. Simple molecular modeling confirmed the feasibility of the binding model. An α-Me ₄ phen-bb ₇ -Q[10] binding constant of 9.9 ± 0.2 × 10 ⁶ M ^-1 was determined by luminescence spectroscopy. Q[10]-encapsulation decreased the toxicity of α-Me ₄ phen-bb ₇ against the three eukaryotic cell lines and increased the binding affinity of the ruthenium complex for HSA. Confocal microscopy experiments indicated that the level of accumulation of α-Me ₄ phen-7 in BHK cells is not significantly affected by Q[10]-encapsulation. Taken together, the combined results suggest that α-Me ₄ phen-7 could be a good candidate as a new antimicrobial agent, and Q[10]-encapsulation could be a method to improve the pharmacokinetics of the ruthenium complex.