Balancing Reactivity and Specificity: Fc-Binding O-Acyl Hydroxamate Reagents for Site-Selective Antibody Labeling at Lys248

We report the design and synthesis of O-acyl hydroxamates (OAHs) affinity labels for selective modification of human IgG1 at Lys248 via proximity-driven acylation. Optimized compound 7g exhibits high water stability and >99% site-selective conversion at Lys248 on Trastuzumab using only 3 equiv. Peptide mapping and fluorescence labeling confirm its precision, establishing 7g as a robust platform for efficient and site-selective antibody conjugation.