Neutral Borate Buffer Activates Diazo for Rapid Protein Labeling

Diazo, a carbene precursor, offers an effective chemistry for bioorthogonal labeling upon activation by acids, metals, and/or photo‐illumination. Here, we report neutral borate buffer can trigger diazo compounds into carbene for protein labeling without the need for acid, metal, or light. We show that such boron‐diazo‐mediated covalent modification rapidly occurs within seconds under physiological conditions and preferentially targets carboxylate‐containing amino acid residues. Towards applications of such boron‐diazo protein chemistry, we exemplify the design of an affinity‐based covalent inhibitor for dihydrofolate reductase by installing diazo on the non‐covalent trimethoprim drug. When extended to stressed cells, we also capture cellular aggresomes using a diazo‐Thioflavin T probe that selectively labels aggregated proteins. Finally, in Alzheimer's disease brain tissue, we employ this boron‐diazo chemistry to label, enrich, and profile amyloid plaques, identifying proteins related to pathological deposition. Together, the boron‐diazo chemistry reported herein provides a rapid and facile protein modification strategy under mild conditions.