[Development of Synthetic Methodology for Mid-size Peptide Based on Disulfide Bond Formation]

Disulfide bonds in peptides contribute to the immobilization and rigidity of their structures, leading to the expression of biological activity and resistance to metabolic enzymes. In addition, disulfide bonds are important in the construction of conjugates comprising two bioactive molecules such as peptides, sugars and drugs. Therefore, new methods of disulfide bond formation contribute to a more efficient construction of disulfide products. This article reviews studies on development of synthetic methodology for disulfide bond formation by using 3-nitro-2-pyridinesulfenyl (Npys) compounds. We have developed a one-pot solid-phase disulfide ligation (SPDSL) method by using an Npys resin, which can easily afford an asymmetric disulfide bond that is generated using two types of thiol-containing components such as peptides and small molecules. The disulfide-linked conjugation between a hydrophobic molecule and a hydrophilic peptide can be easily prepared. Based on the SPDSL strategy, we also developed a disulfide-driven cyclic peptide synthesis, which represents a new strategy to prepare cyclic peptides from two different fragments. By generating a disulfide bond between two fragments, the entropically favorable intramolecular amide bond formation can be achieved, resulting in the reduction of racemization at the coupling site. We found that methyl 3-nitro-2-pyridinesulfenate (Npys-OMe) functions as a disulfide bond-forming reagent possessing mildly oxidative activity. This reagent enhances intramolecular disulfide bond formation between two thiols for the synthesis of cyclic peptides under mildly acidic conditions. As the applications of Npys-OMe, we demonstrated the disulfide bond formation on thiols-containing peptidyl resin.

Chemoselective Disulfide Formation by Thiol-Disulfide Interchange in SIT-Protected Cysteinyl Peptides

Chemoselective disulfide formation is accomplished through a thiol-disulfide interchange approach using sec-isoamyl mercaptan (SIT) as an alkyl sulfenyl-protecting group of one of the Cys residues involved in the pairing. SIT has a dual and unique characteristic, acting as a masking group during the synthesis and directing disulfide formation in the presence of a free thiol. This novel approach is illustrated by the synthesis of several peptides of biological interest.

Cysteine protecting groups: applications in peptide and protein science

Protecting group chemistry for the cysteine thiol group has enabled a vast array of peptide and protein chemistry over the last several decades. Increasingly sophisticated strategies for the protection, and subsequent deprotection, of cysteine have been developed, facilitating synthesis of complex disulfide-rich peptides, semisynthesis of proteins, and peptide/protein labelling in vitro and in vivo. In this review, we analyse and discuss the 60+ individual protecting groups reported for cysteine, highlighting their applications in peptide synthesis and protein science.

Deprotection of S-acetamidomethyl cysteine with copper(ii) and 1,2-aminothiols under aerobic conditions

Ring-opening by CuSO₄ of a 1,3-thiazolidine carbonyl structure (Thz) as an N-terminal cysteine (Cys) residue revealed that an intramolecular S-acetamidomethyl cysteine (Cys(Acm)) can also be deprotected with concomitant formation of a disulphide bond connecting the two Cys residues. A mechanistic study on the disulphide formation led to a general protocol for deprotection of the S-Acm group by CuSO₄ and a 1,2-aminothiol under aerobic conditions. Application of this new deprotection reaction allowed for the synthesis of Apamin, a peptide with two-disulphides in a one-pot/stepwise disulphide-bridging procedure.

The Chemical Methods of Disulfide Bond Formation and Their Applications to Drug Conjugates

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The disulfide bond possesses unique chemical and biophysical properties which distinguish it as one of the key structural elements of bioactive proteins and peptides, important drugs and other materials. The chemo-selective synthesis of these structures and the exploration of their function have been of longstanding interest to the chemistry community. The past decades have witnessed significant progress in both areas. This review will summarize the historically established and recently developed chemical methods in disulfide bond formation. The discussion will also be extended to the use of the disulfide linkers in small molecules, and peptide- and protein-drug conjugates. It is hoped that the combined overview of the fundamental chemistries and applications to drug discovery will inspire creative thinking and stimulate future novel uses of these versatile chemistries.

Use of solid-supported 4-fluorophenyl 3-nitro-2-pyridinesulfenate in the construction of disulfide-linked hybrid molecules

To construct disulfide-linked hybrid molecules systematically and efficiently, we established a more practical solid-phase disulfide ligation (SPDSL) system with enhanced utility. The group Npys-OPh(pF) shows reactivity similar to that of Npys-Cl, but it is more stable. An efficient synthesis of the cyclic peptide oxytocin and a peptide-sugar conjugate was accomplished as models. These results indicate that the Npys-OPh(pF) resin functions as a common synthetic platform in SPDSL.

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.

Design and synthesis of potent myostatin inhibitory cyclic peptides

Myostatin is a negative regulator of skeletal muscle growth and myostatin inhibitors are promising lead compounds against muscle atrophic disorders such as muscular dystrophy. Previously, we published the first report of synthetic myostatin inhibitory 23-mer peptide 1, which was identified from a myostatin precursor-derived prodomain protein. Our structure-activity relationship study afforded the potent inhibitory peptide 3. In this paper, we report an investigation of the synthesis of conformationally-constrained cyclic peptide based on the linear peptide 3. To examine the potency of side chain-to-side chain cyclized peptides, a series of disulfide-, lactam- and diester-bridged derivatives were designed and synthesized, and their myostatin inhibitory activities were evaluated. The diester-bridged peptide (11) displayed potent inhibitory activity with an in vitro IC₅₀ value of 0.26 µM, suggesting that it could serve as a new platform for development of cyclic peptide inhibitors.

Novel Hybrid Compound of a Plinabulin Prodrug with an IgG Binding Peptide for Generating a Tumor Selective Noncovalent-Type Antibody-Drug Conjugate

Although several approaches for making antibody-drug conjugates (ADC) have been developed, it has yet to be reported that an antibody binding peptide such as Z33 from protein A is utilized as the pivotal unit to generate the noncovalent-type ADC (NC-ADC). Herein we aim to establish a novel probe for NC-ADC by synthesizing the Z33-conjugated antitumor agent, plinabulin. Due to the different solubility of two components, including hydrophobic plinabulin and hydrophilic Z33, an innovative method with a solid-supported disulfide coupling reagent is required for the synthesis of the target compounds with prominent efficiency (29% isolated yield). We demonstrate that the synthesized hybrid exhibits a binding affinity against the anti-HER2 antibody (Herceptin) and the anti-CD71 antibody (6E1) (Kd = 46.6 ± 0.5 nM and 4.5 ± 0.56 μM, respectively) in the surface plasmon resonance (SPR) assay. In the cell-based assays, the hybrid provides a significant cytotoxicity in the presence of Herceptin against HER2 overexpressing SKBR-3 cells, but not against HER2 low-expressing MCF-7 cells. Further, it is noteworthy that the hybrid in combination with Herceptin induces cytotoxicity against Herceptin-resistant SKBR-3 (SKBR-3HR) cells. Similar results are obtained with the 6E1 antibody, suggesting that the synthesized hybrid can be widely applicable for NC-ADC using the antibody of interest. In summary, a series of evidence presented here strongly indicate that NC-ADCs have high potential for the next generation of antitumor agents.

H-β-zeolite catalyzed transamidation of carboxamides, phthalimide, formamides and thioamides with amines under neat conditions

Efficient transamidation of unactivated carboxamides, phthalimides, formamides and thioamides with amines under solvent-free conditions using H-β-zeolite as a green and recyclable heterogeneous catalyst is described.