Development of tCAP(N3): Affinity Peptide‐Aided, pH‐Triggered Strategy for Site‐Specific Native IgG Modification

ABSTRACT Direct modification of native antibodies with affinity peptides remains a significant challenge, primarily because the affinity peptide often stays bound to the antibody and blocks its interaction with key receptors. In this study, we developed tCAP(N3), a novel traceless chemical conjugation method that employs an affinity peptide. tCAP(N3) can be stored for over a year at refrigerated temperature, as it contains an active ester precursor that can be spontaneously activated under neutral conditions. When tCAP(N3) was mixed with a target native IgG, Ac‐Lys(N 3 )‐Gly‐Gly was site‐selectively transferred onto Lys248, yielding divalently azidated IgG that can be further modified with DBCO compounds. The resulting antibody conjugates retained Fc receptor binding and antigen recognition comparable to the parent IgG.