Reaction of a peptide-αthioester with an N-terminal Asp(β-HA)-peptide yields a peptide-αO-acyl hydroxamic acid isopeptide under mild aqueous conditions. Reaction of a peptide-αthioester with an N-terminal Asp(β-HA)-peptide yields a peptide-αO-acyl hydroxamic acid isopeptide under mild aqueous conditions. Reaction scheme and LC-MS data showing conversion of thioester 2 to O-acyl hydroxamic acid isopeptide 1, by treatment with N-terminal Asp(β-HA)-peptide 3, at pH 9. Daniel L. Dunkelmann et al. PNAS 2018;115:15:3752-3757 ©2018 by National Academy of Sciences