Native Peptide Cyclization, Sequential Chemoselective Amidation in Water.

Chemical synthesis offers robust tactics for structural alterations of peptides and proteins. It remains a labor-intensive and complex process due to the challenges in selectively modifying diverse amino acid side chains and termini. Direct α-peptide ligation without premodification is a significant hurdle, especially when aiming to include all proteinogenic amino acids at the ligation site. We introduce Native Peptide Cyclization (NPC), a chemoselective method enabling intramolecular peptidyl ligation without the need for premodification. NPC cyclizes unprotected linear peptides through controlled, sequential C- and N-terminal activation via pH modulation. Water-based NPC simplifies peptide ligation, easing the labor-intensive nature of peptide synthesis, aiding efficient cyclic peptide preparation and enabling cost-effective macrocycle-based therapeutics.