Stimuli‐Responsive Hydrazone/Oxime Ligation for Selective Labeling of Cell Surface Glycoproteins

The precise spatial and temporal control of bioorthogonal reactions in living systems is highly desirable for targeted biomolecular labeling. Traditional cell‐selective labeling strategies mainly regulate the insertion of bioorthogonal tags, while we herein report an alternative approach by manipulating the second stage of the labeling process via a controllable hydrazone/oxime ligation. Hydrazone/oxime ligation between hydrazides/hydroxylamines and aldehydes/ketones is ideal for biomolecular labeling due to the small size of these reactant groups, which can be easily integrated into various biomacromolecules. We introduced stimuli‐responsive protecting groups (photoresponsive ortho‐nitrobenzyloxycarbonyl, reactive oxygen species (ROS)‐responsive benzyloxycarbonyl boronic acid, and leucine aminopeptidase (LAP)‐responsive group) to the amino groups of hydroxylamine and hydrazide, thereby inhibiting their reactivity. Upon exposure to the corresponding stimuli (light irradiation, ROS, or LAP), the protecting groups are cleaved, restoring the reactivity of hydrazides/hydroxylamines and enabling hydrazone/oxime ligation. Combined with ketone‐based metabolic glycan labeling, this strategy achieves selective labeling of cell surface glycoproteins only in stimuli‐rich environments. This stimuli‐controlled approach holds significant promise for chemical biology labeling and controllable tumor targeting.