N-Sulfanylethylaminooxybutyramide (SEAoxy): A Crypto-Thioester Compatible with Fmoc Solid-Phase Peptide Synthesis

Peptide Alkyl Thioester Synthesis from Advanced Thiols and Peptide Hydrazides

Peptide alkyl thioesters are versatile reagents in various synthetic applications, commonly generated from peptide hydrazides and thiols. However, a notable limitation is the need for a substantial excess of the thiol reagent, restricting the usage to simple thiols. Here, we introduce an adapted procedure that significantly enhances thioester production with just a minimal thiol excess, facilitating the use of advanced thiol nucleophiles.

Insights into the Mechanism and Catalysis of Peptide Thioester Synthesis by Alkylselenols Provide a New Tool for Chemical Protein Synthesis

While thiol-based catalysts are widely employed for chemical protein synthesis relying on peptide thioester chemistry, this is less true for selenol-based catalysts whose development is in its infancy. In this study, we compared different selenols derived from the selenocysteamine scaffold for their capacity to promote thiol-thioester exchanges in water at mildly acidic pH and the production of peptide thioesters from bis(2-sulfanylethyl)amido (SEA) peptides. The usefulness of a selected selenol compound is illustrated by the total synthesis of a biologically active human chemotactic protein, which plays an important role in innate and adaptive immunity.

One Step Synthesis of Fmoc-Aminoacyl-N-alkylcysteine via the Ugi Four-Component Condensation Reaction

A prompt preparation method of the Fmoc-aminoacyl-N-alkylcysteine dipeptide by an Ugi four-component condensation reaction is described. Through a reaction with a commercially available Fmoc-amino acid, an amine, an isocyanide, and a mercaptoacetaldehyde derivative, one step synthesis of dipeptides containing 20 kinds of natural amino acid residues was achieved, which avoided the problematic N-alkylation of S-tritylcysteine and its coupling reaction. The dipeptide was applied to the Fmoc-solid-phase peptide synthesis, and peptide thioesters were successfully obtained in high efficiency via N-alkylcysteine (NAC)-assisted thioesterification.

Cysteinylprolyl imide (CPI) peptide: a highly reactive and easily accessible crypto-thioester for chemical protein synthesis

A new crypto-thioester, cysteinylprolyl imide (CPI) peptide, offers a practical synthetic pathway and reliable reaction rate to be successfully applied to chemical protein synthesis.

Native chemical ligation (NCL) between the C-terminal peptide thioester and the N-terminal cysteinyl-peptide revolutionized the field of chemical protein synthesis. The difficulty of direct synthesis of the peptide thioester in the Fmoc method has prompted the development of crypto-thioesters that can be efficiently converted into thioesters. Cysteinylprolyl ester (CPE), which is an N–S acyl shift-driven crypto-thioester that relies on an intramolecular O–N acyl shift to displace the amide-thioester equilibrium, enabled trans-thioesterification and subsequent NCL in one pot. However, the utility of CPE is limited because of the moderate thioesterification rates and the synthetic complexity introduced by the ester group. Herein, we develop a new crypto-thioester, cysteinylprolyl imide (CPI), which replaces the alcohol leaving group of CPE with other leaving groups such as benzimidazolidinone, oxazolidinone, and pyrrolidinone. CPI peptides were efficiently synthesized by using standard Fmoc solid-phase peptide synthesis (SPPS) and subsequent on-resin imide formation. Screening of the several imide structures indicated that methyloxazolidinone-t-leucine (MeOxd-Tle) showed faster conversion into thioester and higher stability against hydrolysis under NCL conditions. Finally, by using CPMeOxd-Tle peptides, we demonstrated the chemical synthesis of affibody via N-to-C sequential, three-segment ligation and histone H2A.Z via convergent four-segment ligation. This facile and straightforward method is expected to be broadly applicable to chemical protein synthesis.

Catalysis of Thiol-Thioester Exchange by Water-Soluble Alkyldiselenols Applied to the Synthesis of Peptide Thioesters and SEA-Mediated Ligation

N-Alkyl bis(2-selanylethyl)amines catalyze the synthesis of peptide thioesters or peptide ligation from bis(2-sulfanylethyl)amido (SEA) peptides. These catalysts are generated in situ by reduction of the corresponding cyclic diselenides by tris(2-carboxyethyl)phosphine. They are particularly efficient at pH 4.0 by accelerating the thiol-thioester exchange processes, which are otherwise rate-limiting at this pH. By promoting SEA-mediated reactions at mildly acidic pH, they facilitate the synthesis of complex peptides such as cyclic O-acyl isopeptides that are otherwise hardly accessible.

Acyl donors for native chemical ligation

Native chemical ligation (NCL) has become one of the most important methods in chemical syntheses of proteins. Recently, in order to expand its scope, considerable effort has been devoted to tuning the C-terminal acyl donor thioesters used in NCL. This article reviews the recent advances in the design of C-terminal acyl donors, their precursors and surrogates, and highlights some noteworthy progress that may lead the future direction of protein chemical synthesis.

Traceless β-mercaptan-assisted activation of valinyl benzimidazolinones in peptide ligations

Peptidyl thioesters or their surrogates with C-terminal β-branched hydrophobic amino acid residues usually exhibit poor reactivities in ligation reactions. Thus, activation using exogenous additives is required to ensure an acceptable reaction efficiency. Herein, we report a traceless ligation at Val-Xaa sites under mild thiol additive-free reaction conditions, whereby the introduction of β-mercaptan on the C-terminal valine residue effectively activates the otherwise unreactive N-acyl-benzimidazolinone (Nbz), and enables the use of a one-pot ligation-desulfurization strategy to generate the desired peptide products. The orthogonality between β-thiovaline-Nbz and a conventional alkyl thioester, as well as the convenient access to the former from readily available penicillamine, also allowed expedited assembly of the peptidic hormone β-LPH and hPTH analogues, based on a kinetically controlled one-pot three-segment ligation and desulfurization strategy.

Peptide Weinreb amide derivatives as thioester precursors for native chemical ligation

Peptide Weinreb amide derivatives with an N-substituted mercaptoethyl group are designed as thioester precursors for native chemical ligation. We show that these amides undergo rapid ligation with a cysteinyl peptide under normal NCL conditions to form various Xaa-Cys peptide bonds, including the difficult Val-Cys junction. Facile synthesis of the Weinreb amide linkers allows easy access to this new type of peptide thioester precursor by standard Fmoc solid phase synthesis.

1,2,4-Triazole-aided native chemical ligation between peptide-N-acyl-N′-methyl-benzimidazolinone and cysteinyl peptide

Novel thiol-additive-free NCL using easy-to-prepare peptide-MeNbz and 1,2,4-triazole can be readily combined with one-pot desulfurization and Cys-modification.