Zwitterionic organoboron complexes for overcoming the concentration barrier in chemical protein synthesis.

Chemical protein synthesis enables the construction of specific protein architectures but is limited to millimolar reaction concentrations, restricting access to poorly soluble proteins. Potassium acyltrifluoroboronates (KATs) offer a promising alternative through fast and chemoselective amide bond formation, but their application to protein synthesis has been precluded by the lack of a masking strategy. We report chiral, zwitterionic organoboron complexes that mask amino acid-derived KATs. These molecules exhibit unexpected nitrogen-carbon-boron connectivity and are fully compatible with solid-phase peptide synthesis and stereoretentive deprotection. We synthesized C-terminal KAT peptides and demonstrated KAT ligation at micromolar concentrations for the convergent synthesis of the aggregation-prone programmed death ligand 2 (PD-L2) immunoglobulin V domain. This work establishes organoboron chemistry as an enabling strategy for chemical protein synthesis at low concentrations far more suitable for handling large, aggregation-prone biomolecules.