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X-ray diffraction data demonstrate that the adduct formed upon the reaction of dirhodium(II,II) tetraacetate with RNase A reacts with imidazole, leading to the formation of an unexpected product with the imidazole that binds the dirhodium center at an equatorial site rather than an axial site. The origin of this result has been dissected using quantum-chemical calculations.

The catalytic and biological properties of dirhodium tetracarboxylates ([Rh2(μ-O2CR)4L2], L = axial ligand and R = CH3-, CH3CH2-, etc.) largely depend on the nature of bridging carboxylate equatorial μ-O2CR ligands, which can be easily exchanged by solvent molecules when R is CF3 (i.e. μ-O2CR is trifluoroacetate, tfa). Here, we prepared the [Rh2(OAc)(tfa)3] compound and investigated its interaction with bovine pancreatic ribonuclease and lysozyme under the same conditions used to study the reactivity of these proteins with [Rh2(OAc)4] and [cis-Rh2(OAc)2(tfa)2]. UV-vis absorption spectroscopy and 19F nuclear magnetic resonance studies indicate that [Rh2(OAc)(tfa)3] rapidly loses tfa ligands and interacts with the proteins. Crystallographic data demonstrate that the reaction of [Rh2(OAc)(tfa)3] with proteins can lead to products that are significantly different when compared to those obtained with [Rh2(OAc)4] and [cis-Rh2(OAc)2(tfa)2]: the dirhodium centre can bind the side chain of His residues at both axial and equatorial sites, at variance with what is found in the case of [Rh2(OAc)4] and [cis-Rh2(OAc)2(tfa)2]. These data indicate that the hydrolysis of dirhodium tetracarboxylates plays a significant role in defining their reaction with proteins allowing the formation of unexpected reaction products. These results suggest that [cis-Rh2(OAc)2(tfa)2] and [Rh2(OAc)(tfa)3] can be used to obtain different dirhodium/peptide and dirhodium/protein adducts with distinct catalytic properties and can explain the different cytotoxicity exhibited by tfa-containing dirhodium tetracarboxylates.

Dirhodium complexes of general formula [Rh2(O2CR)4]L2 are a well-known class of bimetallic compounds that are used as efficient catalysts for a variety of reactions and have been shown to be potent antibacterial and anticancer agents. The catalytic and biological properties of these complexes largely depend on the nature of the bridging carboxylate ligands. Trifluoroacetate (tfa)-containing dirhodium compounds have been used to build artificial metalloenzymes upon reaction with peptides and have been shown to be more cytotoxic than dirhodium tetraacetate. However, there is no structural information on the interaction between these compounds and proteins. Here, cis-Rh2(μ-O2CCH3)2(μ-O2CCF3)2 ([cis-Rh2(OAc)2(tfa)2]) has been synthesized and its reaction with bovine pancreatic ribonuclease (RNase A) and hen egg white lysozyme (HEWL) was analyzed using a combination of different techniques, including Fluorine-19 nuclear magnetic resonance spectroscopy and macromolecular X-ray crystallography, with the aim to unveil the differences in the reactivity of tfa-containing dihrodium complexes with proteins when compared to [Rh2(OAc)4]. [cis-Rh2(OAc)2(tfa)2] and [Rh2(OAc)4] bind the N atoms of His side chains of RNase A at the axial position; however the fluorine-containing compound rapidly loses its tfa ligands, while [Rh2(OAc)4] can retain the acetate ligands upon protein binding. The reactivity of [cis-Rh2(OAc)2(tfa)2] with HEWL is slightly distinct when compared to that of [Rh2(OAc)4] under the same experimental conditions; however, both [cis-Rh2(OAc)2(tfa)2] and [Rh2(OAc)4] degrade when soaked within HEWL crystals. These results provide a structural-based guide for the design of new heterogenous chiral dirhodium/peptide and dirhodium/protein adducts with application in the fields of organic synthesis and asymmetric catalysis.