Labeling of native proteins invites interest from diverse segments of science. However, there remains the significant unmet challenge in precise labeling at a single site of a protein. Here, we report the site-specific labeling of natural or easy-to-engineer N-terminus Gly in proteins with remarkable efficiency and selectivity. The method generates a latent nucleophile from N-terminus imine that reacts with an aldehyde to deliver an aminoalcohol under physiological conditions. It differentiates N-Gly as a unique target amongst other proteinogenic amino acids. The method allows single-site labeling of proteins in isolated form and extends to lysed cells. It administers an orthogonal aldehyde group primed for late-stage tagging with an affinity tag, 19 F NMR probe, and a fluorophore. A user-friendly protocol delivers analytically pure tagged proteins. The mild reaction conditions do not alter the structure and function of the protein. The cellular uptake of fluorophore-tagged insulin and its ability to activate the insulin-receptor mediated signaling remains unperturbed.

The necessity for precision labeling of proteins emerged during the efforts to understand and regulate their structure and function. It demands selective attachment of tags such as affinity probes, fluorophores, and potent cytotoxins. Here, we report a method that enables single‐site labeling of a high‐frequency Lys residue in the native proteins. At first, the enabling reagent forms stabilized imines with multiple solvent‐accessible Lys residues chemoselectively. These linchpins create the opportunity to regulate the position of a second Lys‐selective electrophile connected by a spacer. Consequently, it enables the irreversible single‐site labeling of a Lys residue independent of its place in the reactivity order. The user‐friendly protocol involves a series of steps to deconvolute and address chemoselectivity, site‐selectivity, and modularity. Also, it delivers ordered immobilization and analytically pure probe‐tagged proteins. Besides, the methodology provides access to antibody‐drug conjugate (ADC), which exhibits highly selective anti‐proliferative activity towards HER‐2 expressing SKBR‐3 breast cancer cells.

Chemical functionalization of proteins is an indispensable tool. Yet, selective labeling of native proteins has been an arduous task. The limited success of chemical methods allows N-terminus protein labeling, but the examples with side-chain residues are rare. Herein, we surpass this challenge through a multicomponent transformation that operates under physiological conditions in the presence of a protein, aldehyde, acetylene, and Cu-ligand complex. The methodology results in the labeling of a single lysine residue in nine distinct proteins.

We report a chemoselective and site-selective formylation of ε-amine in native proteins. The aldehyde auto-oxidation re-routing, regulated generation of formate, and reversible N-terminus protection drive the transformation. It labels a single ε-amine in a pool of its copies, other nucleophilic residues, and α-amine. The extension of the methodology leads to site-selective acylation.

Self-assembled states of proteins render efficient promiscuous nucleophilic biocatalysis in MBH reaction in a green process.