Formyl benzeno boronic acids rapidly and selectively react with N-terminal cysteines to yield a reversible boronated thiazolidine that may be used in the interactive orthogonal modification of peptides.

Widely used reagents in the peptide functionalization toolbox, Michael acceptors and N‐hydroxysuccinimide (NHS) activated esters, are combined in NHS‐activated acrylamides for efficient chemoselective amino‐sulfhydryl stapling on native peptides and proteins. NHS‐activated acrylamides allow for a fast functionalization of N‐terminal cysteines (k2=1.54±0.18×103 M−1 s−1) under dilute aqueous conditions, enabling selectivity over other nucleophilic amino acids. Additionally, the versatility of these new bioconjugation handles was demonstrated in the cross‐linking of in‐chain or C‐terminal cysteines with nearby lysine residues. NHS‐activated acrylamides are compatible with the use of other cysteine selective reagents, allowing for orthogonal dual‐modifications. This strategy was successfully applied to the late‐stage functionalization of peptides and proteins with a PEG unit, fluorescent probe, and cytotoxic agent. The level of molecular control offered by NHS‐activated acrylamides is expected to promote amino‐sulfhydryl stapling technology as a powerful strategy to design functional bioconjugates.