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.

Application of 2,2'-dipyridyl disulfide-mediated thiazolidine ring-opening reaction to glycoprotein synthesis: Total chemical synthesis of evasin-3

Thiazolidine ring-opening reaction is one of the key steps in protein chemical synthesis via sequential native chemical ligation strategy. We recently developed a novel thiazolidine ring-opening reaction with 2,2'-dipyridyl disulfide (DPDS). In order to investigate the applicability of this reaction to glycoprotein synthesis, we synthesized evasin-3, a cysteine-rich glycoprotein with chemokine-binding ability originally found in tick saliva. The sequence of evasin-3 was divided into three segments, and these segments were separately synthesized with the ordinary solid-phase peptide synthesis method. After the first ligation of middle and C-terminal segments, thiazolidine used as a protecting group of Cys residue at the N-terminus of the middle segment was converted to Cys with DPDS. In this thiazolidine ring-opening reaction, DPDS treatment did not affect the N-linked glycan moiety. After the second ligation with the N-terminal segment and the refolding reaction, evasin-3 could be obtained in good yield. The synthetic evasin-3 showed the binding ability specifically to CXCL chemokines. These results clearly indicate that this DPDS method is useful for glycoprotein synthesis.

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.

On-resin synthesis of cyclic peptides via tandem N-to-S acyl migration and intramolecular thiol additive-free native chemical ligation

A novel methodology for the synthesis of cyclic peptides by on-resin intramolecular native chemical ligation (NCL) assisted by N-ethylcysteine using Fmoc/SPPS is described.

Disulfide-Driven Cyclic Peptide Synthesis of Human Endothelin-2 with a Solid-Supported Npys-Cl

We report here the synthesis of human endothelin-2, a peptide of 21 amino acid residues with two disulfide bonds, based on the novel idea of a disulfide-driven cyclic peptide synthesis (DdCPS). This synthesis has two steps: (1) a one-pot solid-phase disulfide ligation of two different sulfur-containing peptide fragments using an Npys-Cl resin and (2) intramolecular cyclization of the disulfide peptide via amide bond formation using a thioester ligation. Human endothelin-2 was obtained in a total yield of 2.2% with two such DdCPS procedures and subsequent deprotection and HPLC purification. This strategy is the basis of a new solid-phase assisted practical synthesis of cyclic disulfide peptides.

Synthesis of selenocysteine-containing cyclic peptides via tandem N-to-S acyl migration and intramolecular selenocysteine-mediated native chemical ligation

Cyclic selenocysteine-containing peptides were synthesized via one-pot tandem conversion of N-alkylcysteine-containing selenopeptides.

One-pot native chemical ligation by combination of two orthogonal thioester precursors

We developed a one-pot peptide ligation method using two orthogonal thioester precursors and a protecting group for the ligation reaction between Asp and Cys.

Amino acid fluorides: viable tools for synthesis of peptides, peptidomimetics and enantiopure heterocycles

This review provides a broad perspective of the uses of amino acid fluorides in the synthesis of peptides and a wide range of other molecules including peptidomimetics, heterocycles and biologically active molecules.

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.

Peptide Thioester Formation via an Intramolecular N to S Acyl Shift for Peptide Ligation

In chemical protein synthesis, peptide building blocks are prepared by solid-phase peptide synthesis (SPPS), and then connected by chemical ligation methods. The peptide thioester is one of key building blocks used in chemical protein synthesis, and improvements in the Fmoc SPPS procedure for preparing such thioesters would be highly desirable. In this review we focus on a method for peptide thioester synthesis based on the use of an intramolecular N to S acyl shift reaction as a key reaction. Amide and thioester forms at the thiol-containing residue are in equilibrium as a result of a reversible intramolecular acyl shift, which is detectable by 13C NMR. The amide form is favored under neutral conditions, while the thioester predominates under acidic conditions. Thiol auxiliaries can be employed to facilitate the formation of a thioester from an amide via an intramolecular N-S acyl shift, and the peptide thioester is formed after intermolecular transthioesterification in the presence of excess amounts of thiols. Even under neutral conditions, thiol auxiliary-containing peptides can be ligated with a cysteinyl peptide via an intramolecular N-S acyl shift, followed by native chemical ligation (NCL) in a one-pot reaction. These procedures can be applied to the chemical synthesis of proteins which are post-translationally modified.

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.