Reactivity of antitumor coinage metal-based N-heterocyclic carbene complexes with cysteine and selenocysteine protein sites

The reaction of the antitumor M(I)-bis-N-heterocyclic carbene (M(I)-NHC) complexes, M = Cu, Ag, and Au, with their potential protein binding sites, i.e. cysteine and selenocysteine, was investigated by means of density functional theory approaches. Capped cysteine and selenocysteine were employed to better model the corresponding residues environment within peptide structures. By assuming the neutral or deprotonated form of the side chains of these amino acids and by considering the possible assistance of an external proton donor such as an adjacent acidic residue or the acidic component of the surrounding buffer environment, we devised five possible routes leading to the binding of the investigated M(I)-NHC scaffolds to these protein sites, reflecting their different location in the protein structure and exposure to the bulk. The targeting of either cysteine or selenocysteine in their neutral forms is a kinetically unfavored process, expected to be quite slow if observable at all at physiological temperature. On the other hand, the reaction with the deprotonated forms is much more favored, even though an external proton source is required to assist the protonation of the leaving carbene. Our calculations also show that all coinage metals are characterized by a similar reactivity toward the binding of cysteine and selenocysteine sites, although the Au(I) complex has significantly higher reaction and activation free energies compared to Cu(I) and Ag(I).