Selective Activation of Peptide‐Thioester Precursors for Templated Native Chemical Ligations

Chemical protein synthesis enables access to proteins that would otherwise be difficult or impossible to obtain with traditional means such as recombinant expression. Chemoselective ligations provide the ability to join peptide segments prepared by solid-phase peptide synthesis. While native chemical ligation (NCL) is widely used, it is limited by the need for C-terminal thioesters with suitable reaction kinetics, properly placed native Cys or thiolated derivatives, and peptide segment solubility at low mM concentrations. Moreover, repetitive purifications to isolate ligated products are often yield-sapping, hampering efficiency and progress. In this work, we demonstrate the use of Controlled Activation of Peptides for Templated NCL (CAPTN). This traceless multi-segment templated NCL approach permits the one-pot synthesis of proteins by harnessing selective thioester activation and orthogonal conjugation chemistries to favor formation of the full-length ligated product while minimizing side reactions. Importantly, CAPTN provides kinetic enhancements allowing ligations at sterically hindered junctions and low peptide concentrations. Additionally, this one-pot approach removes the need for intermediate purification. We report the synthesis of two E. coli ribosomal subunits S16 and S17 enabled by the chemical tools described herein. We anticipate that CAPTN will expedite the synthesis of valuable proteins and expand on templated approaches for chemical protein synthesis.