Exploring the structure of a ruthenium acetate cluster for biological purposes

Abstract

This work evaluates the cytotoxicity of [Ru3(μ3-O)(μ-OAc)5{μ-η1(C),η2(N,N)-phen}(py)2](PF6) (1), a triruthenium acetate cluster combined with an ortho-metallated 1,10-phenanthroline (phen) ligand, along with its interactions with biomolecules. In vitro cytotoxicity tests against the B16F10 (murine melanoma) and L929 (fibroblast) cell lines showed that 1 decreased cancer cell viability by 50% at 25 μM, while it requires 50 μM of free phen ligand to achieve the same effect. Importantly, 1 is not active against non-tumor cell model L929 up to 100 μM. Spectrophotometric titrations suggest that 1 and DNA interacted weakly through electrostatic attraction and semi-intercalation. Despite the presence of a planar and aromatic ligand, relative viscosity measurements are not consistent with the DNA intercalation of 1, presumably due to the small size of phenanthroline. The protein-binding ability of compound 1 was evaluated using human serum albumin (HSA) as a model and monitored by spectroscopic techniques, including steady-state emission and absorption, time-resolved luminescence, and circular dichroism, CD. Compound 1 quenches the HSA emission efficiently (Kap = 4.72 × 108 M−1, 298 K) mostly by static quenching, consistent with its relatively strong binding to HSA (Kb = 3.70 × 106 M−1, 298 K). Analysis of ΔH and ΔS values (604 kJ/mol and 2162 J/mol K, respectively) suggest the contribution of hydrophobic interactions. CD measurements demonstrated that the protein α-helical structure decreased in the presence of l, indicating a partial conformation change of HSA. Although the interaction of 1 with DNA is weak, its binding to the model protein HSA is strong, such that the cytotoxicity of the complex may be attributed to protein-complex interactions. This work provides a framework to design new triruthenium acetates to achieve stronger and more selective binding to biomolecules. Importantly, the selective activity of 1 towards a cancer cell line, while remaining inactive against healthy cells, represents a critical finding for the design of new cancer therapeutics based on triruthenium acetate clusters.