Application of cysteinyl prolyl ester for the synthesis of cyclic peptides containing an RGD sequence and their biological activity measurement

Cysteinyl RGD-peptidyl cysteinyl prolyl esters, which have different configurations at the cysteine and proline residues, were synthesized by the solid-phase method and cyclized by the native chemical ligation reaction. Cyclization efficiently proceeded to give cyclic peptides, regardless of the difference in the configuration. The peptides were further derivatized to the corresponding desulfurized or methylated cyclic peptides at the Cys residues. The inhibition activity to αvβ6 integrin binding was then analyzed by ELISA. The results showed that the activity varied depending on the difference in the configuration and modification of the cysteinyl prolyl ester (CPC) moiety, demonstrating the usefulness of this method in the search for a good inhibitor of the protein-protein interaction.

Biocompatible strategies for peptide macrocyclisation

Peptides are increasingly important drug candidates, offering numerous advantages over conventional small molecules. However, they face significant challenges related to stability, cellular uptake and overall bioavailability. While individual modifications may not address all these challenges, macrocyclisation stands out as a single modification capable of enhancing affinity, selectivity, proteolytic stability and membrane permeability. The recent successes of in situ peptide modifications during screening in combination with genetically encoded peptide libraries have increased the demand for peptide macrocyclisation reactions that can occur under biocompatible conditions. In this perspective, we aim to distinguish biocompatible conditions from those well-known examples that are fully bioorthogonal. We introduce key strategies for biocompatible peptide macrocyclisation and contextualise them within contemporary screening methods, providing an overview of available transformations.

Dual Antagonism of α9α10 nAChR and GABAB Receptor-Coupled CaV2.2 Channels by an Analgesic αO-Conotoxin Analogue

Pain severely affects the physical and mental health of patients. The need to develop nonopioid analgesic drugs to meet medical demands is urgent. In this study, we designed a truncated analogue of αO-conotoxin, named GeX-2, based on disulfide-bond deletion and sequence truncation. GeX-2 retained the potency of its parent peptide at the human α9α10 nAChR and exhibited potent inhibitory activity at CaV2.2 channels via activation of the GABAB receptor (GABABR). Importantly, GeX-2 significantly alleviated pain in the rat model of chronic constriction injury. The dual inhibition of GeX-2 at both α9α10 nAChRs and CaV2.2 channels is speculated to synergistically mediate the potent analgesic effects. Results from site-directed mutagenesis assay and computational modeling suggest that GeX-2 preferentially interacts with the α10(+)α10(-) binding site of α9α10 nAChR and favorably binds to the top region of the GABABR2 subunit. The study offers vital insights into the molecular action mechanism of GeX-2, demonstrating its potential as a novel nonopioid analgesic.

Thioester deprotection using a biomimetic NCL approach

The reversibility of the thiol-thioester linkage has been broadly employed in many fields of biochemistry (lipid synthesis) and chemistry (dynamic combinatorial chemistry and material science). When the transthioesterification is followed by a S-to-N acyl transfer to give an amide bond, it is called Native Chemical Ligation (NCL), a high-yield chemoselective process used for peptide synthesis. Recently, we described thioglycolic acid (TGA) as a useful reagent for thioester deprotection both in solution and anchored to a solid-support under mild conditions. Inspired by NCL, in this work, we extended this approach and explored the use of 2-aminothiols for the deprotection of thiols bearing an acyl group. The best results were obtained using cysteamine or L-cysteine in an aqueous buffer pH 8 at room temperature for 30 min. The described approach was useful for S-acetyl, S-butyryl, and S-benzoyl heterocycles deprotection with yields up to 84%. Employing this methodology, we prepared six new analogs 2 of mercaptomethyl bisthiazolidine 1, a useful inhibitor of a wide-range of Metallo-β-Lactamases (MBLs). Compared with the previous methodologies (TGA polymer supported and TGA in solution), the biomimetic deprotection herein described presents better performance with higher yields, shorter reaction times, less time-consuming operations, easier setup, and lower costs.

Head-to-tail cyclization for the synthesis of naturally occurring cyclic peptides on organophosphorus small-molecular supports

4,4′-bis(diphenylphosphinyloxyl) diphenyl ketoxime and 4-diphenyl phospholoxy benzyl alcohol were designed and prepared as supports for peptide synthesis. The total synthesis of cyclic peptides in a resin-free manner was successfully demonstrated.

Three Methods for Peptide Cyclization Via Lactamization

Lactamization is the key step in the synthesis of many compounds with macrocyclic structure. As the interest for these types of molecules grows in various fields such as drug discovery and nanomaterials, different methodologies to access them are being developed. Three different strategies to obtain cyclic peptides via lactamization are described in this chapter: solution-phase macrocyclization following solid-phase peptide synthesis (SPPS) of the linear precursor, SPPS and on-resin cyclization on the 2-chlorotrityl chloride (2-CTC) resin, and SPPS and on-resin cyclization by native chemical ligation on the amino-PEGA resin.

One-Pot Cyclization and Cleavage of Peptides with N-Terminal Cysteine via the N,S-Acyl Shift of the N-2-[Thioethyl]glycine Residue

We developed a one-pot method for peptide cleavage from a solid support via the N,S-acyl shift of N-2-[thioethyl]glycine and transthioesterification using external thiols to produce cyclic peptides through native chemical self-ligation with the N-terminal cysteine. The feasibility of this methodology is validated by the syntheses of model short peptides, including a tetrapeptide, the bicyclic sunflower trypsin inhibitor SFTI-1, and rhesus Θ-defensin RTD-1. Synthesis of the whole peptide precursor can be fully automated and proceeds without epimerization or dimerization.

Enzyme-Primed Native Chemical Ligation Produces Autoinducing Cyclopeptides in Clostridia

Clostridia coordinate many important processes such as toxin production, infection, and survival by density‐dependent communication (quorum sensing) using autoinducing peptides (AIPs). Although clostridial AIPs have been proposed to be (thio)lactone‐containing peptides, their true structures remain elusive. Here, we report the genome‐guided discovery of an AIP that controls endospore formation in Ruminiclostridium cellulolyticum. Through a combination of chemical synthesis and chemical complementation assays with a mutant strain, we reveal that the genuine chemical mediator is a homodetic cyclopeptide (cAIP). Kinetic analyses indicate that the mature cAIP is produced via a cryptic thiolactone intermediate that undergoes a rapid S→N acyl shift, in a manner similar to intramolecular native chemical ligation (NCL). Finally, by implementing a chemical probe in a targeted screen, we show that this novel enzyme‐primed, intramolecular NCL is a widespread feature of clostridial AIP biosynthesis.

Recent advances in the synthesis of C-terminally modified peptides

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

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.