Peptide ligation by chemoselective aminonitrile coupling in water

Amide bond formation is one of the most important reactions in both chemistry and biology^1-4, but there is currently no chemical method of achieving α-peptide ligation in water that tolerates all of the 20 proteinogenic amino acids at the peptide ligation site. The universal genetic code establishes that the biological role of peptides predates life's last universal common ancestor and that peptides played an essential part in the origins of life^5-9. The essential role of sulfur in the citric acid cycle, non-ribosomal peptide synthesis and polyketide biosynthesis point towards thioester-dependent peptide ligations preceding RNA-dependent protein synthesis during the evolution of life^5,9-13. However, a robust mechanism for aminoacyl thioester formation has not been demonstrated¹³. Here we report a chemoselective, high-yielding α-aminonitrile ligation that exploits only prebiotically plausible molecules-hydrogen sulfide, thioacetate^12,14 and ferricyanide^12,14-17 or cyanoacetylene^8,14-to yield α-peptides in water. The ligation is extremely selective for α-aminonitrile coupling and tolerates all of the 20 proteinogenic amino acid residues. Two essential features enable peptide ligation in water: the reactivity and pK_aH of α-aminonitriles makes them compatible with ligation at neutral pH and N-acylation stabilizes the peptide product and activates the peptide precursor to (biomimetic) N-to-C peptide ligation. Our model unites prebiotic aminonitrile synthesis and biological α-peptides, suggesting that short N-acyl peptide nitriles were plausible substrates during early evolution.