A side-chain photoactivatable auxiliary-mediated native chemical peptide ligation

Photoactivatable ligation auxiliaries present an attractive approach for expanding the applicability of native chemical ligation (NCL). In contrast to thiol-based mercaptobenzyl-type ligation scaffolds, photoactivatable auxiliaries are cleaved after ligation by photolysis, thereby simplifying protein assembly in non-cysteine NCL and causing minimal perturbation to the native polypeptide chains. This study reports an easy-to-attach 1-[5-(thiomethyl)-2-nitrophenyl]ethyl-(TmNpE)-auxiliary at the internal aspartic/glutamic acid side-chain of the N-terminal peptide fragments. This auxiliary enables a long-range NCL and allows for photolytic removal in an aqueous buffer without a reagent. The versatility of the method was highlighted by the synthesis of a human programmed cell death-ligand 1 (hPD-L1) peptide inhibitor, Ar5Y-3. The TmNpE group offers easy introduction, compatibility with peptide ligation reactions, and mild cleavage conditions, which may be beneficial for peptides or protein synthesis other than cysteine ligation sites.