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Singled out for special treatment: Naturally occurring glutathione S-transferase (GST) was used to catalyze an efficient “click” ligation between polypeptides with an N-terminal glutathione sequence and biomolecules or chemical probes containing perfluorinated aromatic groups (see scheme). The site-specific modification of one cysteine residue was possible in the presence of other unprotected cysteine residues and reactive functional groups., National Institutes of Health (U.S.) (GM101762), National Institutes of Health (U.S.) (Award GM46059), MIT Faculty Start-up Fund, Damon Runyon Cancer Research Foundation, Amgen Inc. (Summer Graduate Research Fellowship)

Here we describe a general synthetic platform for side-chain macrocyclization of an unprotected peptide library based on the S[subscript N]Ar reaction between cysteine thiolates and a new generation of highly reactive perfluoroaromatic small molecule linkers. This strategy enabled us to simultaneously “scan” two cysteine residues positioned from i, i + 1 to i, i + 14 sites in a polypeptide, producing 98 macrocyclic products from reactions of 14 peptides with 7 linkers. A complementary reverse strategy was developed; cysteine residues within the polypeptide were first modified with non-bridging perfluoroaryl moieties and then commercially available dithiol linkers were used for macrocyclization. The highly convergent, site-independent, and modular nature of these two strategies coupled with the unique chemoselectivity of a S[subscript N]Ar transformation allows for the rapid diversity-oriented synthesis of hybrid macrocyclic peptide libraries with varied chemical and structural complexities., National Institutes of Health (U.S.) (GM101762), National Institutes of Health (U.S.) (GM046059), MIT Faculty Start-up Fund, Sontag Foundation (Distinguished Scientist Award), Deshpande Center for Technological Innovation, Massachusetts Institute of Technology (Charles E. Reed Faculty Initiative Fund), Damon Runyon Cancer Research Foundation