Discovery of a Serine‐Directed Chemical Reaction for Site‐Specific Protein Modification via Phage Display Screening

Abstract Conventional biocompatible chemistry typically depends on unnatural functional groups, such as alkynes and azides. Here, we present a natural amino acid‐based alternative by leveraging phage display to discover Cu II ‐assisted serine arylation ( CASA ), a serine‐selective strategy for chemical protein modification, achieved through a Cu II ‐mediated hydroxyl‐arylation reaction. CASA enables fast and precise modification of a single serine hydroxyl group within complex proteins while leaving the other amino acids, including serines at other sites, unmodified. CASA demonstrates robust performance in on‐demand modification of diverse recombinant proteins, including therapeutic antibodies, with single‐residue precision. The system's ability to selectively target individual proteins within crude cell lysates suggests its potential for applications in complex biological environments and positions it as a versatile tool for protein engineering.