Acid-Catalyzed Tandem Thiol Switch for Preparing Peptide Thioesters from Mercaptoethyl Esters

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.

The C-Terminal O-S Acyl Shift Pathway under Acidic Condition to Propose Peptide-Thioesters

Peptide-thioester is a pivotal intermediate for peptide ligation and N-, C-terminal cyclization. In this study, desired pathway and the side products of two C-terminal handles, hydroxyethylthiol (HET) and hydroxypropylthiol (HPT) are described in different conditions as well as kinetic studies. In addition, a new mechanism of C-terminal residue racemization is proposed on the basis of differentiation of products derived from the two C-terminal handles in preparing peptide thioesters through an acid-catalyzed tandem thiol switch, first by an intramolecular O-S acyl shift, and then by an intermolecular S-S exchange.

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

An N-sulfanylethylaminooxybutyramide (SEAoxy) has been developed as a novel thioester equivalent for native chemical ligation. SEAoxy peptide was straightforwardly synthesized by conventional Fmoc solid-phase peptide synthesis without a problem. Moreover, SEAoxy peptide could be directly applied to native chemical ligation owing to the intramolecular N-to-S acyl shift that releases the peptide-thioester in situ. This methodology was successfully applied to the synthesis of two bioactive peptides.

Development of a Chemical Methodology for the Preparation of Peptide Thioesters Applicable to Naturally Occurring Peptides Using a Sequential Quadruple Acyl Transfer System

Peptide thioesters are very useful in protein chemistry, and chemistry- and biochemistry-based protocols are used for the preparation of thioesters. Among such protocols, only a few biochemistry-based approaches have been use for naturally occurring peptide sequences. The development of chemistry-based protocols applicable to natural sequences remains a challenge, and the development of such methods would be a major contribution to protein science. Here, we describe the preparation of peptide thioesters using innovative methodology that features nickel(II)-mediated alcoholysis of a naturally occurring peptide sequence, followed by O−N and N−S acyl transfers. This protocol involves sequential quadruple acyl transfer, termed SQAT. Notably, the SQAT system consists of sequential chemical reactions that allow naturally occurring peptide sequences to be converted to thioesters without requiring an artificial chemical unit.

Chemical synthesis of proteins using N-sulfanylethylanilide peptides, based on N-S acyl transfer chemistry

Native chemical ligation (NCL), which features the use of peptide thioesters, is among the most reliable ligation protocols in chemical protein synthesis. Thioesters have conventionally been synthesized using tert-butyloxycarbonyl (Boc)-based solid-phase peptide synthesis (SPPS); however, the increasing use of 9-fluorenylmethyloxycarbonyl (Fmoc) SPPS requires an efficient preparative protocol for thioesters which is fully compatible with Fmoc chemistry. We have addressed this issue by mimicking the naturally occurring thioester-forming step seen in intein-mediated protein splicing of the intein-extein system, using an appropriate chemical device to induce N-S acyl transfer reaction, avoiding the problems associated with Fmoc strategies. We have developed N-sulfanylethylanilide (SEAlide) peptides, which can be synthesized by standard Fmoc SPPS and converted to the corresponding thioesters through treatment under acidic conditions. Extensive examination of SEAlide peptides showed that the amide-type SEAlide peptides can be directly and efficiently involved in NCL via thioester species in the presence of phosphate salts, even under neutral conditions. The presence or absence of phosphate salts provided kinetically controllable chemoselectivity in NCL for SEAlide peptides. This allowed SEAlide peptides to be used in both one-pot/N-to-C-directed sequential NCL under kinetically controlled conditions, and the convergent coupling of large peptide fragments, which facilitated the chemical synthesis of proteins over about 100 residues. The use of SEAlide peptides, enabling sequential NCL operated under kinetically controlled conditions, and the convergent coupling, were used for the total chemical synthesis of a 162-residue monoglycosylated GM2-activator protein (GM2AP) analog.

A thioethylalkylamido (TEA) thioester surrogate in the synthesis of a cyclic peptide via a tandem acyl shift

The cyclic cystine-knot peptide, kalata B1, was synthesized by employing a novel Fmoc-compatible thioethylalkylamido (TEA) thioester surrogate via an N-S acyl shift followed by a thiol-thioester exchange reaction. TEA thioester surrogate is cost-effective, conveniently prepared in one-step with starting materials, readily available from commercial sources, and highly efficient in preparing peptide thioesters.