3-Nitro-2-pyridinesulfenates as Efficient Solution- and Solid-Phase Disulfide Bond Forming Agents

Development of Water-Soluble 3-Nitro-2-Pyridinesulfenate for Disulfide Bond Formation of Peptide Under Aqueous Conditions

Establishing an efficient method for the synthesis of disulfide bonds in peptides is an important challenge for the further development of several research fields, including peptide-based medicinal chemistry and biochemistry. Herein, we report the development of a novel highly water-soluble 3-nitro-2-pyridine (Npy) sulfenate that efficiently forms intramolecular disulfide bonds in reduced peptides. Moreover, Npy-sulfenate formed a disulfide bond in a thiol-containing peptide prepared by thiol-additive-free native chemical ligation (NCL), as demonstrated by the one-pot synthesis of adrenomedullin, which contains 52 amino acid residues. This study provides a new one-pot synthetic methodology for preparing mid-sized cyclic disulfide peptides via sulfenate-mediated oxidation.

Advances in the synthesis and engineering of conotoxins

Conotoxins, isolated from the venom of carnivorous marine snails of the Conus genus, are disulfide-rich peptides and proteins with well-defined three-dimensional structures. Conotoxins' ability to target a wide range of ion channels and receptors, including voltage- and ligand-gated ion channels, G protein-coupled receptors, monoamine transporters, and enzyme, at exquisite potency and selectivity make them valuable research and therapeutic tools. Despite their potentials, Conus venom peptides are present in limited quantities in nature and possess structural complexity that raises significant synthetic challenges for both chemical synthesis and recombinant expression. Here, we document recent advances in the expression and synthesis of conotoxins, particularly focusing on directed formation of disulfide bonds, chemical ligation techniques, and the integration of non-native functional groups. These advances can provide access to even the most complex conotoxins, accelerating conotoxin-based drug discovery and functional analysis, as well as opening new avenues for the development of drug candidates.

Structure Derivatization of IgG-Binding Peptides and Analysis of Their Secondary Structure by Circular Dichroism Spectroscopy

Mid-sized cyclic peptides are a promising modality for modern drug discovery. Their larger interaction area coupled with an appropriate secondary structure is more suitable than small molecules for binding to the target protein. In this study, we conducted a structure derivatization of an immunoglobulin G (IgG)-binding peptide (15-IgBP), a β-hairpin-like cyclic peptide with a twisted β-strand and assessed the effect of the secondary structure on IgG-binding activity using circular dichroism (CD) spectra analysis. As a result, derivatization at the Ala5 and Gly9 positions affected the secondary structure of 15-IgBP, in particular the appearance of a small positive peak in the 220-240 nm region characteristic of 15-IgBP in the CD spectrum. Maintaining this peak at a moderate level may be important for the expression of IgG binding activity. We found the small methyl group at Ala5 to be crucial for retaining the preferred secondary structure; we also found Gly9 could be replaced by D-amino acids. By integrating these findings with previous results of the structure-activity relationship, we obtained four potent affinity peptides for IgG binding (Kd = 4.24-5.85 nM). Furthermore, we found the Gly9 position can be substituted for D-Lys. This is a new potential site for attaching functional units for conjugation with IgG for the preparation of homogeneous antibody-drug conjugates.

Rapid and Highly Productive Assembly of a Disulfide Bond in Solid-Phase Peptide Macrocyclization

Here we report a highly efficient disulfide-driven peptide macrocyclization in 15 min on a solid support using persulfate as a crucial additive in iodine-mediated oxidative cyclization. The method eliminates the side products of classical iodine-mediated peptide cyclization. It is operationally simple and convenient for cyclizing small to lengthier peptides embodying popular cysteine building blocks in a single step.

[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.

Stearylated Macropinocytosis-Inducing Peptides Facilitating the Cellular Uptake of Small Extracellular Vesicles

Macropinocytosis is a form of endocytosis that allows massive uptake of extracellular materials and is a promising route for intracellular delivery of biofunctional macromolecules and nanoparticles. Our laboratory developed a potent macropinocytosis-inducing peptide named P4A. However, the ability of this peptide is not apparent in the presence of serum. This study aims to endow P4A and related peptides with the ability to induce macropinocytosis in the presence of serum by N-terminal acylation with long-chain fatty acids (i.e., decanoic, myristic, and stearic acids). Stearylated P4A (stearyl-P4A) had the highest effect on stimulating macropinocytotic uptake. Moreover, the intramolecularly disulfide-bridged analogue, stearyl-oxP4A, showed an even higher ability. The effect of stearyl-oxP4A to facilitate the intracellular delivery of small extracellular vesicles (sEVs) was evaluated in terms of (i) cellular uptake using sEVs labeled with an enhanced green fluorescent protein (EGFP) and (ii) cytosolic liberation and expression of sEV-encapsulated luciferase mRNA in recipient cells. The two- to threefold uptake of both sEVs in the presence of stearyl-oxP4A suggests the potential of the peptide for sEV delivery in the presence of serum.

Development of a High-Affinity Antibody-Binding Peptide for Site-Specific Modification

Immunoglobulin G (IgG)-binding peptides such as 15-IgBP are convenient tools for the site-specific modification of antibodies and the preparation of homogeneous antibody-drug conjugates. A peptide such as 15-IgBP can be selectively crosslinked to the fragment crystallizable region of human IgG in an affinity-dependent manner via the ϵ-amino group of Lys8. Previously, we found that the peptide 15-Lys8Leu has a high affinity (K_d =8.19 nM) due to the presence of the γ-dimethyl group in Leu8. The primary amino group required for the crosslinking to the antibodies has, however, been lost. Here, we report the design and synthesis of a novel unnatural amino acid, 4-(2-aminoethylcarbamoyl)leucine (Aecl), which possesses both the γ-dimethyl fragment and a primary amino group. A peptide containing Aecl8 (15-Lys8Aecl) was synthesized and showed a binding affinity ten times higher (K_d =24.3 nM) than that of 15-IgBP (K_d =267 nM). Fluorescein isothiocyanate (FITC)-labeled 15-Lys8Aecl with an N-hydroxy succinimide ester at the side chain of Aecl8 (FITC-15-Lys8Aecl(OSu)) successfully labeled an antibody (trastuzumab, Herceptin^® ) with the fluorophore. This peptide scaffold has both strong binding affinity and crosslinking capability, and could be a useful tool for the selective chemical modification of antibodies with molecules of interest such as drugs.

Expanding the Toolbox for Peptide Disulfide Bond Formation via l-Methionine Selenoxide Oxidation

In this study, l-methionine selenoxide (MetSeO) was used as an oxidant for the construction of peptide disulfide bonds. Excellent yields for various disulfide-containing peptides were achieved via the MetSeO oxidation method in different solvents and on a resin. Most importantly, the construction of disulfide bonds can be performed in the trifluoroacetic acid cocktail used for the cleavage of peptides from the resin, which obviates the steps of peptide purification and lyophilization. This facilitates and simplifies the synthesis of disulfide-containing peptides. Kinetic and mechanistic studies of the reaction between MetSeO and dithiothreitol (DTT, a model compound of dicysteine-containing peptide) show that the reaction is first order in both [MetSeO] and [DTT], and a reaction mechanism is proposed that can help us gain insights into the reaction of the oxidative synthesis of disulfide bonds via MetSeO oxidation.

Detection of Thiol Functionality and Disulfide Bond Formation by Polyoxometalate

The detection of thiol functionality and intramolecular disulfide bond formation of peptides using the α-Keggin type polyoxometalate molybdenum-oxygen cluster (H₃PMo₁₂O₄₀·nH₂O) is described. Our method entails the addition of this polyoxometalate to solutions of thiol, whereupon the color of the solution changes from colorless to deep blue. Reduction of the polyoxometalate from Mo(VI) to Mo(V) occurs with concomitant oxidation of the thiol functionality, to form disulfide bonds. To exemplify the utility this phenomenon, we accomplished the oxidation of glutathione, reduced linear oxytocin, bactenecin, and α-conotoxin SI; all of which proceeded smoothly and in good conversion in 24 h to less and were accomplished by a change in the color of the reaction solutions.

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.

Stepwise Construction of Disulfides in Peptides

The disulfide bond plays an important role in biological systems. It defines global conformation, and ultimately the biological activity and stability of the peptide or protein. It is frequently present, singly or multiply, in biologically important peptide hormones and toxins. Numerous disulfide-containing peptides have been approved by the regulatory agencies as marketed drugs. Chemical synthesis is one of the prerequisite tools needed to gain deep insights into the structure-function relationships of these biomolecules. Along with the development of solid-phase peptide synthesis, a number of methods of disulfide construction have been established. This minireview will focus on the regiospecific, stepwise construction of multiple disulfides used in the chemical synthesis of peptides. We intend for this article to serve a reference for peptide chemists conducting complex peptide syntheses and also hope to stimulate the future development of disulfide methodologies.

A study to develop platinum(iv) complex chemistry for peptide disulfide bond formation

Platinum(iv) complexes with a heterocyclic ligand and an ancillary ligand have been investigated and applied for treating various tumour cell lines. Another application of the Pt(iv) complexes in forming peptide disulfide bonds was investigated in this work. For development of Pt(iv) complex chemistry for disulfide bond formation in peptides, two Pt(iv) complexes, [PtCl₂(phen)(en)]Cl₂ and [PtCl₂(bpy)(en)]Cl₂, were synthesized and characterized using elemental analysis, ESI-MS and NMR. Subsequently, they were investigated as oxidants for the formation of disulfide bonds in various peptides. Excellent purities and yields of disulfide-containing peptides were achieved when the reactions were carried out in aqueous solution. The reactions were completed rapidly in a wide range of pH values even in acidic medium at room temperature. An intramolecular disulfide bond was formed in each of the peptides in a solution containing two dithiol-containing peptides, making the Pt(iv) complexes useful for generating disulfide-containing peptide libraries. In addition, the two Pt(iv) complexes can be used as oxidants for the synthesis of disulfide bonds on a resin, which is a more convenient method to synthesize disulfide-containing peptides through automation.

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.

Synthesis and structure-activity relationship studies of IgG-binding peptides focused on the C-terminal histidine residue

Currently, IgG-binding peptides are widely utilized as a research tool, as molecules that guide substrates to the Fc site for site-selective antibody modification, leading to preparation of a homogeneous antibody-drug conjugate. In this study, a structure-activity relationship study of an IgG-binding peptide, 15-IgBP, that is focused on its C-terminal His residue was performed in an attempt to create more potent peptides. A peptide with a substitution of His17 by 2-pyridylalanine (2-Pya) showed a good binding affinity (15-His17(2-Pya), K d = 75.7 nM). In combination with a previous result, we obtained 15-Lys8Leu/His17(2-Pya)-OH that showed a potent binding affinity (K d = 2.48 nM) and avoided three synthetic problems concerning the p-hydroxybenzyl amidation at the C-terminus, the difficulty associated with coupling at the His7 position and the racemization of 2-Pya.

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.

4-Fluorophenyl 3-nitro-2-pyridinesulfenate as a practical protecting agent for amino acids

We report a new protecting agent (1, Npys-OPh(pF)) for 3-nitro-2-pyridine (Npy) sulfenylation of amino, hydroxy, and thiol functional groups. Several Npys phenoxides were synthesized from Npys chloride (Npys-Cl) and phenols in the presence of base in 1-step reaction, and their ability for Npy-sulfenylation was evaluated. As a result, 1 was selected as a new Npy-sulfenylation agent with advantages including improved physicochemical stability, more controllable reactivity, and easier handling than the conventional protecting agent Npys-Cl.