Site‐Selective Modification of Peptides and Proteins via Interception of Free‐Radical‐Mediated Dechalcogenation

The development of site‐selective chemistry targeting the canonical amino acids enables the controlled installation of desired functionalities into native peptides and proteins. Such techniques facilitate the development of polypeptide conjugates to advance therapeutics, diagnostics, and fundamental science. We report a versatile and selective method to functionalize peptides and proteins through free‐radical‐mediated dechalcogenation. By exploiting phosphine‐induced homolysis of the C−Se and C−S bonds of selenocysteine and cysteine, respectively, we demonstrate the site‐selective installation of groups appended to a persistent radical trap. The reaction is rapid, operationally simple, and chemoselective. The resulting aminooxy linker is stable under a variety of conditions and selectively cleavable in the presence of a low‐oxidation‐state transition metal. We have explored the full scope of this reaction using complex peptide systems and a recombinantly expressed protein. Functionalization of peptides and proteins through the trapping of peptide radical species is described. This novel and versatile strategy exploits the process of desulfurization and deselenization of cysteine and selenocystine residues to enable the site‐selective installation of a broad range of groups appended to a persistent radical trap.