Chemical post-translational methods allow convergent side-chain editing of proteins without needing to resort to genetic intervention. Current approaches that allow the creation of constitutionally native side chains via C–C bond formation, using off-protein carbon-centered C· radicals added to unnatural amino acid radical acceptor (SOMOphile, singly occupied molecular orbital (SOMO)) “tags” such as dehydroalanine, are benign and wide-ranging. However, they also typically create epimeric mixtures of d /l-residues. Here, we describe a light-mediated desulfurative method that, through the creation and reaction of stereoretained on-protein l-alanyl Cβ· radicals, allows Cβ–Hγ, Cβ–Oγ, Cβ–Seγ, Cβ–Bγ, and Cβ–Cγ bond formation to flexibly generate site-selectively edited proteins with full retention of native stereochemistry under mild conditions from a natural amino acid precursor. This methodology shows great potential to explore protein side-chain diversity and function and in the construction of useful bioconjugates.

Chemical post-translational methods now allow convergent side-chain editing of proteins as a form of direct chemical mutagenesis without needing to resort to genetic intervention. Current approaches that allow the creation of constitutionally native side-chains via C-C formation using off-protein carbon-centred C· radicals added to unnatural amino acid radical acceptor SOMOphile 'tags' such as dehydroalanine are benign and wide-ranging. However, they also typically create epimeric mixtures of D-/L-residues. Here we describe a light-mediated desulfurative method that, through the creation and reaction of stereoretained on-protein L-alanyl Cβ· radicals, allows Cβ-Hγ, Cβ-Oγ, Cβ-Seγ, Cβ-Bγ and Cβ-Cγ bond formation to flexibly generate site-selectively edited proteins with full retention of native stereochemistry under mild conditions from a natural amino acid. This methodology shows great potential to explore protein side-chain diversity and construct useful bioconjugates.