Zwitterionic chiral organoboron complexes for overcoming the concentration barrier in protein synthesis

Chemical protein synthesis enables access to protein variants and architectures that are difficult or impossible to produce recombinantly. The leading amide-forming ligation methods, such as native chemical ligation, require millimolar concentrations of peptide segments, limiting their applicability to aggregation-prone or poorly soluble proteins. Potassium acyltrifluoroborates (KATs) offer fast and chemoselective amide bond formation under acidic aqueous conditions, but their application to protein synthesis has been precluded by the lack of a masking strategy compatible with solid-phase peptide synthesis (SPPS). Here, we report a solution to this obstacle through the development of a new class of chiral, zwitterionic organoboron complexes that mask amino acid–derived KATs. The resulting hexacyclic protected acylboronates feature unprecedented chemical connectivity – including a carbon connected to one boron and two nitrogen atoms, and a pyridinium aminal that is stable to all steps of Fmoc-SPPS. It is cleanly deprotected with stereoretention. By subjecting these protected Fmoc-amino KATs to automated Fmoc-SPPS, we synthesized and purified long C-terminal KAT peptides and demonstrated that these undergo KAT ligation at micromolar concentrations for the convergent synthesis of the aggregation-prone IgV domain of PD-L2. This work establishes innovative organoboron chemistry as an enabling strategy for chemical protein synthesis at the low concentrations best suited for handling large, aggregation prone biomolecules.