Native Chemical Ligation at Serine Revisited

Polypeptide Preparation by β-Lactone-Mediated Chemical Ligation

Native chemical ligation (NCL) represents a cornerstone strategy in accessing synthetic peptides and proteins, remaining one of the most efficacious methodologies in this domain. The fundamental requisites for achieving a proficient NCL reaction involve chemoselective coupling between a C-terminal thioester peptide and a thiol-bearing N-terminal peptide. However, achieving coupling at sterically congested residues remains challenging. In addition, while most NCLs proceed without epimerization, β-branched (e.g., Ile, Thr, Val) and Pro-derived C-terminal thioesters react slowly and can be susceptible to significant epimerization and hydrolysis. Herein, we report an epimerization-free NCL reaction via β-lactone-mediated native chemical ligation which constructs sterically congested Thr residues. The constrained ring from the β-lactone allows rapid peptide ligation without detectable epimerization. The method has a broad side-chain tolerance and was applied to the preparation of cyclic peptides and polypeptidyl thioester, which could be difficult to obtained otherwise.

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.

Mirror-Image Single-Domain Antibody for a Novel Nonimmunogenic Drug Scaffold

Immunogenic responses by protein therapeutics often lead to reduced therapeutic effects and/or adverse effects via the generation of neutralizing antibodies and/or antidrug antibodies (ADA). Mirror-image proteins of the variable domain of the heavy chain of the heavy chain antibody (VHH) are potential novel protein therapeutics with high-affinity binding to target proteins and reduced immunogenicity because these mirror-image VHHs (d-VHHs) are less susceptible to proteolytic degradation in antigen-presenting cells (APCs). In this study, we investigated the preparation protocols of d-VHHs and their biological properties, including stereoselective target binding and immunogenicity. Initially, we established a facile synthetic process of two model VHHs [anti-GFP VHH and PMP12A2h1 (monomeric VHH of caplacizumab)] and their mirror-image proteins by three-step native chemical ligations (NCLs) from four peptide segments. The folded synthetic VHHs (l-anti-GFP VHH and l-PMP12A2h1) bound to the target proteins (EGFP and vWF-A1 domain, respectively), while their mirror-image proteins (d-anti-GFP VHH and d-PMP12A2h1) showed no binding to the native proteins. For biodistribution studies, l-VHH and d-VHH with single radioactive indium diethylenetriamine-pentaacid (111In-DTPA) labeling at the C-terminus were designed and synthesized by the established protocol. The distribution profiles were essentially similar between l-VHH and d-VHH, in which the probes accumulated in the kidney within 15 min after intravenous administration in mice, because of the small molecular size of VHHs. Comparative assessment of the immunogenicity responses revealed that d-VHH-induced levels of ADA generation were significantly lower than those of native VHH, regardless of the peptide sequences and administration routes. The resulting scaffold investigated should be applicable in the design of d-VHHs with various C-terminal CDR3 sequences, which can be identified by screening using display technologies.

Chemical Synthesis of a Potent Antimicrobial Peptide Murepavadin Using a Tandem Native Chemical Ligation/Desulfurization Reaction

Classical approaches for the backbone cyclization of polypeptides require conditions that may compromise the chirality of the C-terminal residue during the activation step of the cyclization reaction. Here, we describe an efficient epimerization-free approach for the Fmoc-based synthesis of murepavadin using intramolecular native chemical ligation in combination with a concomitant desulfurization reaction. Using this approach, bioactive murepavadin was produced in a good yield in two steps. The synthetic peptide antibiotic showed potent activity against different clinical isolates of P. aeruginosa. This approach can be easily adapted for the production of murepavadin analogues and other backbone-cyclized peptides.

Solid-Phase Synthesis of Head to Side-Chain Tyr-Cyclodepsipeptides Through a Cyclative Cleavage From Fmoc-MeDbz/MeNbz-resins

Cyclic depsipeptides constitute a fascinating class of natural products. Most of them are characterized by an ester formed between the β-hydroxy function of Ser/Thr -and related amino acids- and the carboxylic group of the C-terminal amino acid. Less frequent are those where the thiol of Cys is involved rendering a thioester (cyclo thiodepsipeptides) and even less common are the cyclo depsipeptides with a phenyl ester coming from the side-chain of Tyr. Herein, the preparation of the later through a cyclative cleavage using the Fmoc-MeDbz/MeNbz-resin is described. This resin has previously reported for the synthesis of cyclo thiodepsipeptides and homodetic peptides. The use of that resin for the preparation of all these peptides is also summarized.