Selective Synthesis of Lysine Peptides and the Prebiotically Plausible Synthesis of Catalytically Active Diaminopropionic Acid Peptide Nitriles in Water.

Why life encodes specific proteinogenic amino acids remains an unsolved problem, but a non-enzymatic synthesis that recapitulates biology's universal strategy of stepwise N-to-C terminal peptide growth may hold the key to this selection. Lysine is an important proteinogenic amino acid that, despite its essential structural, catalytic, and functional roles in biochemistry, has widely been assumed to be a late addition to the genetic code. Here, we demonstrate that lysine thioacids undergo coupling with aminonitriles in neutral water to afford peptides in near-quantitative yield, whereas non-proteinogenic lysine homologues, ornithine, and diaminobutyric acid cannot form peptides due to rapid and quantitative cyclization that irreversibly blocks peptide synthesis. We demonstrate for the first time that ornithine lactamization provides an absolute differentiation of lysine and ornithine during (non-enzymatic) N-to-C-terminal peptide ligation. We additionally demonstrate that the shortest lysine homologue, diaminopropionic acid, undergoes effective peptide ligation. This prompted us to discover a high-yielding prebiotically plausible synthesis of the diaminopropionic acid residue, by peptide nitrile modification, through the addition of ammonia to a dehydroalanine nitrile. With this synthesis in hand, we then discovered that the low basicity of diaminopropionyl residues promotes effective, biomimetic, imine catalysis in neutral water. Our results suggest diaminopropionic acid, synthesized by peptide nitrile modification, can replace or augment lysine residues during early evolution but that lysine's electronically isolated sidechain amine likely provides an evolutionary advantage for coupling and coding as a preformed monomer in monomer-by-monomer peptide translation.

Prebiotic Catalytic Peptide Ligation Yields Proteinogenic Peptides by Intramolecular Amide Catalyzed Hydrolysis Facilitating Regioselective Lysine Ligation in Neutral Water.

The prebiotic origin of catalyst-controlled peptide synthesis is fundamental to understanding the emergence of life. Building on our recent discovery that thiols catalyze the ligation of amino acids, amides, and peptides with amidonitriles in neutral water, we demonstrate the outcome of ligation depends on pH and that high pKa primary thiols are the ideal catalysts. While the most rapid thiol catalyzed peptide ligation occurs at pH 8.5-9, the most selective peptide ligation, that tolerates all proteinogenic side chains, occurs at pH 7. We have also identified the highly selective mechanism by which the intermediate peptidyl amidines undergo hydrolysis to α-peptides while demonstrating that the hydrolysis of amidines with nonproteinogenic structures, such as ß- and γ-peptides, displays poor selectivity. Notably, this discovery enables the highly α-selective protecting-group-free ligation of lysine peptides at neutral pH while leaving the functional ε-amine side chain intact.