O-acyl isopeptide method: efficient synthesis of isopeptide segment and application to racemization-free segment condensation

Synthetic Collagen-like Polymer That Undergoes a Sol–Gel Transition Triggered by O–N Acyl Migration at Physiological pH

We previously reported an artificial collagen gel that can be used as a cell-culture substrate by end-to-end cross-linking of collagen-like triple-helical peptides via disulfide bonds. However, the gel had to be formed a priori by polymerizing the peptide in an acidic solution containing dimethyl sulfoxide for several days, which prevented its use as an injectable gel or three-dimensional (3D) scaffold for cell culture. In this study, we developed a collagen-like peptide polymer by incorporating an O–N acyl migration-triggered triple helix formation mechanism into a collagen-like peptide, which formed a gel within 10 min. We demonstrated that the collagen-like peptide polymer can be used as a 3D cell scaffold and that the 3D structure formation of cells can be controlled by collagen-derived bioactive sequences introduced into the peptide sequence.

A Global Review on Short Peptides: Frontiers and Perspectives

Peptides are fragments of proteins that carry out biological functions. They act as signaling entities via all domains of life and interfere with protein-protein interactions, which are indispensable in bio-processes. Short peptides include fundamental molecular information for a prelude to the symphony of life. They have aroused considerable interest due to their unique features and great promise in innovative bio-therapies. This work focusing on the current state-of-the-art short peptide-based therapeutical developments is the first global review written by researchers from all continents, as a celebration of 100 years of peptide therapeutics since the commencement of insulin therapy in the 1920s. Peptide "drugs" initially played only the role of hormone analogs to balance disorders. Nowadays, they achieve numerous biomedical tasks, can cross membranes, or reach intracellular targets. The role of peptides in bio-processes can hardly be mimicked by other chemical substances. The article is divided into independent sections, which are related to either the progress in short peptide-based theranostics or the problems posing challenge to bio-medicine. In particular, the SWOT analysis of short peptides, their relevance in therapies of diverse diseases, improvements in (bio)synthesis platforms, advanced nano-supramolecular technologies, aptamers, altered peptide ligands and in silico methodologies to overcome peptide limitations, modern smart bio-functional materials, vaccines, and drug/gene-targeted delivery systems are discussed.

Synthesis of O-Acyl Isopeptides: Stepwise and Convergent Solid-Phase Synthesis

The O-acyl isopeptide method was developed for the preparation of difficult sequence-containing peptides, whose hydrophobic nature hampers both peptide chain construction on resin and purification with HPLC after deprotection. In the O-acyl isopeptide method, the target peptide is synthesized in an O-acyl isopeptide form, which contains an O-acyl isopeptide bond instead of the native N-acyl peptide bond at a hydroxy group-containing amino acid residue, such as Ser or Thr. The hydrophilic O-acyl isopeptide can be isolated, e.g., as a lyophilized TFA salt. The target peptide can be quantitatively obtained by a final O-to-N intramolecular acyl migration reaction with exposure to neutral conditions. Additionally, the O-acyl isopeptide is important as a hydrophilic precursor peptide for biological peptide assays that are difficult to handle. This chapter describes the synthesis of such O-acyl isopeptides by stepwise and convergent Fmoc solid-phase peptide synthesis.

Amide-forming chemical ligation via O-acyl hydroxamic acids

The facile rearrangement of "S-acyl isopeptides" to native peptide bonds via S,N-acyl shift is central to the success of native chemical ligation, the widely used approach for protein total synthesis. Proximity-driven amide bond formation via acyl transfer reactions in other contexts has proven generally less effective. Here, we show that under neutral aqueous conditions, "O-acyl isopeptides" derived from hydroxy-asparagine [aspartic acid-β-hydroxamic acid; Asp(β-HA)] rearrange to form native peptide bonds via an O,N-acyl shift. This process constitutes a rare example of an O,N-acyl shift that proceeds rapidly across a medium-size ring (t_1/2 ∼ 15 min), and takes place in water with minimal interference from hydrolysis. In contrast to serine/threonine or tyrosine, which form O-acyl isopeptides only by the use of highly activated acyl donors and appropriate protecting groups in organic solvent, Asp(β-HA) is sufficiently reactive to form O-acyl isopeptides by treatment with an unprotected peptide-^αthioester, at low mM concentration, in water. These findings were applied to an acyl transfer-based chemical ligation strategy, in which an unprotected N-terminal Asp(β-HA)-peptide and peptide-^αthioester react under aqueous conditions to give a ligation product ultimately linked by a native peptide bond.

Medicinal Chemistry Focusing on Aggregation of Amyloid-β

The aggregation of peptides/proteins is intimately related to a number of human diseases. More than 20 have been identified which aggregate into fibrils containing extensive β-sheet structures, and species generated in the aggregation processes (i.e., oligomers and fibrils) contribute to disease development. Amyloid-β peptide (designated Aβ), related to Alzheimer's disease (AD), is the representative example. The intensive aggregation property of Aβ also leads to difficulty in its synthesis. To improve the synthetic problem, we developed an O-acyl isopeptide of Aβ1-42, in which the N-acyl linkage (amide bond) of Ser(26) was replaced with an O-acyl linkage (ester bond) at the side chain. The O-acyl isopeptide demonstrated markedly higher water-solubility than that of Aβ1-42, while it quickly converted to intact monomer Aβ1-42 via an O-to-N acyl rearrangement under physiological conditions. Inhibition of the pathogenic aggregation of Aβ1-42 might be a therapeutic strategy for curing AD. We succeeded in the rational design and identification of a small molecule aggregation inhibitor based on a pharmacophore motif obtained from cyclo[-Lys-Leu-Val-Phe-Phe-]. Moreover, the inhibition of Aβ aggregation was achieved via oxygenation (i.e., incorporation of oxygen atoms to Aβ) using an artificial catalyst. We identified a selective, cell-compatible photo-oxygenation catalyst of Aβ, a flavin catalyst attached to an Aβ-binding peptide, which markedly decreased the aggregation potency and neurotoxicity of Aβ.

Patchwork protein chemistry: a practitioner's treatise on the advances in synthetic peptide stitchery

With the study of peptides and proteins at the heart of many scientific endeavors, the omics era heralded a multitude of opportunities for chemists and biologists alike. Across the interface with life sciences, peptide chemistry plays an indispensable role, and progress made over the past decades now allows proteins to be treated as molecular patchworks stitched together through synthetic tailoring. The continuous elaboration of sophisticated strategies notwithstanding, Merrifield's solid-phase methodology remains a cornerstone of chemical protein design. Although the non-practitioner might misjudge peptide synthesis as trivial, routine, or dull given its long history, we comment here on its many advances, obstacles, and prospects from a practitioner's point of view. While sharing our perspectives through thematic highlights across the literature, this treatise provides an interpretive overview as a guide to novices, and a recap for specialists.

Synthesis of O-acyl isopeptides

O-Acyl isopeptides, in which the N-acyl linkage on the hydroxyamino acid residue (e.g., Ser and Thr) is replaced with an O-acyl linkage, generally possess superior water-solubility to their corresponding native peptides, as well as other distinct physicochemical properties. In addition, O-acyl isopeptides can be rapidly converted into their corresponding native peptide under neutral aqueous conditions through an O-to-N acyl migration. By exploiting these characteristics, researchers have applied the O-acyl isopeptide method to various peptide-synthesis fields, such as the synthesis of aggregative peptides and convergent peptide synthesis. This O-acyl-isopeptide approach also serves as a means to control the biological function of the peptide in question. Herein, we report the synthesis of O-acyl isopeptides and some of their applications.

Click Peptide concept: o-acyl isopeptide of islet amyloid polypeptide as a nonaggregative precursor molecule

The O-acyl isopeptide (1) of islet amyloid polypeptide (IAPP), which contains an ester moiety at both Ala8-Thr9 and Ser19-Ser20, was prepared by sequential segment condensation based on the O-acyl isopeptide method. Isopeptide 1 possessed nonaggregative properties, retaining its random coil structure under the acidic conditions; this suggests that the insertion of the O-acyl isopeptide structures in IAPP suppressed aggregation of the molecule. As a result of the rapid O-to-N acyl shift of 1 under neutral pH, in situ-formed IAPP adopted a random-coil structure at the start of the experiment, and then underwent conformational change to α-helix/β-sheet mixed structures as well as aggregation. The click peptide strategy with the nonaggregative precursor molecule 1 could be a useful experimental tool to identify the functions of IAPP, by overcoming the handling difficulties that arise from IAPP's intense and uncontrollable self-assembling nature.

Epimerization-free synthesis of cyclic peptide by use of the O-acyl isopeptide method

A head-to-tail cyclization of a protected linear hexapeptide with a C-terminal O-acyl isopeptide proceeded to give a cyclic O-acyl isopeptide without epimerization. The cyclic O-acyl isopeptide possessed different secondary structures compared with the native cyclic peptide. The isopeptide was then efficiently converted to the desired cyclic peptide via an O-to-N acyl migration reaction using a silica gel-anchored base.