Exploiting soft and hard X-Ray absorption spectroscopy to characterize metallodrug/protein interactions: The binding of [trans-RuCl₄(Im) (dimethylsulfoxide)][ImH] (Im = imidazole) to bovine serum albumin

The reaction of bovine serum albumin (BSA) with [trans-RuCl ₄(Im)(dimethylsulfoxide)][ImH] (Im = imidazole) (NAMI-A), an experimental ruthenium(III) anticancer drug, and the formation of the respective NAMI-A/BSA adduct were investigated by X-ray absorption spectroscopy (XAS) at the sulfur and chlorine K-edges and at the ruthenium K- and L₃-edges. Ruthenium K and L₃-edge spectra proved unambiguously that the ruthenium center remains in the oxidation state +3 after protein binding. Comparative analysis of the chlorine K-edge XAS spectra of NAMI-A and NAMI-A/BSA, revealed that the chlorine environment is greatly perturbed upon protein binding. Only modest changes were observed in the sulfur K-edge spectra that are dominated by several protein sulfur groups. Overall, valuable information on the nature of this metallodrug/protein adduct and on the mechanism of its formation was gained; XAS spectroscopy turns out to be a very suitable method for the characterization of this kind of systems.