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. Editor's summary: Chemical synthesis of proteins, which typically involves stitching together fragments produced by solid-phase synthesis, is generally limited by solubility of larger sequences and highly hydrophobic fragments. Schilling et al. developed a peptide ligation method that functions in water at orders of magnitude lower concentrations than would be required for traditional peptide-coupling reactions. The key pieces are a potassium acyltrifluoroborate (KAT)–modified carboxyl terminus and a hydroxylamine–derivatized amino terminus. The authors developed a protecting group strategy that enables the incorporation of KAT-modified amino acids during standard peptide synthesis and demonstrated the utility of this approach by synthesizing challenging protein targets, including an aggregation-prone immunoglobulin domain. —Michael A. Funk [ABSTRACT FROM AUTHOR]