Exchange of equatorial ligands in protein-bound paddlewheel Ru25+ complexes: new insights from X-ray crystallography and quantum chemistry

Abstract

Here we report the binding of the diruthenium complex [Ru₂Cl(D-p-CNPhF)(O₂CCH₃)₃]_n (D-p-CNPhF⁻ = N,N′-bis(4-cyanophenyl)formamidinate) to the model protein bovine pancreatic ribonuclease (RNase A), investigated for the first time by means of X-ray crystallography and Quantum Chemistry. The crystal structure reveals that the compound binds a histidine side chain with the diruthenium core anchored to RNase A at the axial site, without significantly altering the overall protein structure. The protein binding to the diruthenium core is associated with the replacement of an equatorial acetate ligand by two water molecules. This species is expected to be highly reactive in the absence of the protein. Thus, the Ru₂/RNase A structure here reported can be associated with the entatic state of the artificial metalloenzyme produced upon reaction of RNase A with [Ru₂Cl(D-p-CNPhF)(O₂CCH₃)₃]_n. Quantum chemical investigations unveil the possible reaction mechanisms and help in dissecting the role of the imidazole group axial ligands on the convenient replacement of equatorial acetate ligands by water molecules.