Palladium-triggered bioorthogonal phenylalanine decaging.

Phenylalanine (Phe) orchestrates protein interfacial hydrophobicity and π-driven interactions that are critical for diverse biological processes. Its nonpolar architecture, however, precludes traditional heteroatom-dependent reactions, leaving its functional manipulation in living systems unresolved. Here, by systematically evaluating exogenous heteroatom-based (X = O, N, B or I) caging groups under physiological conditions, we present a haloatom-assisted decaging strategy that uses iodine to cage Phe's side chain. This blockade can be tracelessly rescued through palladium-triggered bioorthogonal decaging. Through precise modulation, the haloatom-assisted decaging strategy enables: unmasking fluorophore activity, directing peptide disassembly, reactivating protein-protein interactions and rewiring cell-cell recognition. Using genetic code expansion, we site-specifically incorporated iodinated Phe into HER2-targeting affibodies for dynamic control of ligand-receptor interaction on the cell surface. Moreover, Phe decaging permits temporally controlled reshaping of antigenic peptide immunogenicity on tumour cells, thereby rewiring tumour-T cell engagement. We further optimized the decaging chemistry in cell lysates as well as inside living cells, which unlocks the possibility of chemically manipulating ubiquitous and functionally critical nonpolar groups in living systems.