Unlocking the Silent Proteome: Chemoselective Asn/Gln Activation for Multidimensional Protein Diversification.

Amides are ubiquitous in pharmaceuticals, natural products, and biomolecules, owing to their exceptional stability and hydrogen-bonding capacity. Among the amino acids, asparagine (Asn) and glutamine (Gln) contain neutral primary amide side chains and constitute over 8% of the human proteome. Despite their abundance, these residues have remained largely inaccessible to selective chemical modification due to their low intrinsic reactivity and the propensity of proteinogenic side chains to poison transition-metal catalysts via chelation. Here, we report a general strategy that converts the primary amides of Asn and Gln into bioorthogonal nitrile handles, which can be further diversified through carbometalation with aryl boronic acids to yield aryl ketone products. This transformation proceeds with exceptional chemoselectivity, enabling the modification of native peptides and proteins. We demonstrate its broad utility in the synthesis of unnatural amino acids, late-stage diversification of peptides, fluorosequencing of Asn residues, and site-selective protein modification, culminating in the synthesis of a functional antibody-fluorophore conjugate. The versatility and selectivity of this approach expand the accessible chemical space of biomolecules and provide a powerful route for uncovering previously uncharacterized Asn/Gln sites within the chemically silent proteome.