Mapping the Reactivity of Two Pharmacologically Active Organopalladium Compounds toward the Model Protein RNase A Using a Dual ESI MS and X-ray Diffraction Investigative Approach

Organopalladium compounds are a new class of metal-based drugs with significant yet largely unexplored therapeutic potential. Previous studies have suggested that these compounds act as prodrugs that can strongly interact with protein targets. However, the underlying protein metalation processes have scarcely been investigated to date. To gain a deeper insight into this issue, we analyzed the interactions of two representative organopalladium compounds with the model protein RNase A using a strategy that combined electrospray ionization mass spectrometry (ESI MS) and X-ray crystallography. Time-course and titration ESI-MS studies revealed that the two palladium compounds readily react with RNase A to form stable adducts with distinct structural characteristics. The crystal structures of two organopalladium/RNase A adducts were then determined. These structures revealed that the palladium-containing fragments bind to histidine and methionine side chains. Interestingly, one of the two complexes interacts with the catalytically important residues His12 and His119, causing significant inhibition of the protein's enzymatic activity. Overall, these results provide detailed insights into the main features of protein metalation reactions promoted by organopalladium compounds as well as the nature of protein-bound palladium fragments.