Practical N-to-C peptide synthesis with minimal protecting groups

Sustainable Ultrasound-Assisted Solid-Phase peptide synthesis (SUS-SPPS): Less Waste, more efficiency

The integration of low-frequency ultrasound with Solid-Phase Peptide Synthesis (SPPS) was explored to establish a Sustainable Ultrasound-assisted Solid-Phase Peptide Synthesis (SUS-SPPS) method. This innovative approach significantly reduces solvent consumption, washing steps, time, and reagent usage compared to conventional manual SPPS protocols. The SUS-SPPS method exploits ultrasound at every stage of synthesis and work-up, reducing the process to just two steps. The first step sequentially combines Fmoc-amino acid coupling, capping of unreacted amino groups, and Fmoc deprotection into a single operation, while the second one consists of a single washing procedure. Moreover, we demonstrated that the method is compatible with various resin types, including Rink-amide, Wang, and Cl-Trt resins, and facilitates the efficient synthesis of peptides of varying lengths (up to 20-mers) and compositions, including those traditionally considered "difficult sequences", with excellent yields and purity. Notably, SUS-SPPS reduces solvent usage per coupling cycle by 83-88%, marking a significant breakthrough in sustainable peptide synthesis.

Synthesis of Bis-Thioacid Derivatives of Diarylethene and Their Photochromic Properties

Diarylethenes (DAEs) are an important class of photoswitchable compounds that typically undergo reversible photochemical conversions between the open and closed cyclized forms upon treatment with UV light or visible light. In this study, we introduced thioacid functional groups to several photochromic dithienylethene (DTE) derivatives and established a method that can be used to prepare these photoswitchable thioacids. Four thioacid-functionalized diarylethene derivatives were synthesized through the activation of carboxylic acids with N-hydroxysuccinimide, followed by reactions with sodium hydrosulfide with yields over 90%. These derivatives exhibited reversible photoswitching and photochromic properties upon treatment with ultraviolet (UV) and visible lights. The thioacid groups on these compounds can act as reaction sites for attaching other desirable functionalities. The photochromic properties of these new derivatives were characterized by using ultraviolet-visible (UV-vis) spectroscopy. The photocyclizations of one of the derivatives and its potassium salt were also characterized by using nuclear magnetic resonance (NMR) spectroscopy. The anions of the thioacid formed water-soluble photochromic systems, and their applications as colorimetric sensors in agarose hydrogels were demonstrated.

Twisted Amide-Mediated Peptide Synthesis

A robust, practical, and sustainable isomerization-suppressed peptide bond formation via acyl sulfonamide, a twisted amide, is disclosed. Tosyl isocyanate and pentafluorobenzyl bromide were applied in combination to activate the peptide C-terminus, which then reacted with an amine to yield an elongated peptide with high stereochemical purity. Careful analysis of NMR spectra of the active intermediate revealed the presence of an intramolecular hydrogen bond, suggesting that the hydrogen bond suppressed Cα-epimerization during amidation. The isomerization suppression by the intramolecular hydrogen bond is expected to be effective even under high dilution conditions, making the present method a powerful tool for the synthesis of complex macrocyclic peptides. In addition to peptide synthesis, the developed synthetic entry to twisted amides can be applied to the investigation of transition metal-catalyzed N-C bond activation. Moreover, the application to the N-C bond activation returned insight into peptide synthesis, leading to the use of sulfonamide as a protecting group of carboxylic acid that can be orthogonally removed in the presence of other conventional protecting groups.

Promising Proteolysis-Targeting Chimera for Mutant p53-R175H

The tumor suppressor protein p53 is among the most commonly mutated proteins across a variety of cancer types. Notably, the p53 R175H mutation ranks as one of the most prevalent hotspot mutations. Proteolysis-targeting chimeras (PROTACs) represent a class of bifunctional molecules capable of harnessing the cellular ubiquitin-proteasome pathway to facilitate targeted protein degradation. Despite the potential of PROTACs, limited research has been directed toward the degradation of the p53-R175H mutant protein. In this study, we developed a series of peptide-based PROTACs, leveraging known peptide ligands for both the p53-R175H mutation and the E3 ubiquitin ligase VHL. Our findings indicate that one of these peptide-based PROTACs is capable of directing the p53-R175H protein to the proteasome for degradation within a recombinant expression system. Moreover, by synthesizing a fusion peptide PROTAC molecule that incorporates a membrane-penetrating peptide, we have demonstrated its ability to traverse cellular membranes and subsequently reduce the levels of the p53-R175H mutant protein. Importantly, the degradation of p53-R175H was found to mitigate the cellular migration and invasion. In summary, our study introduces a novel class of protein degraders and establishes a foundational framework for the therapeutic management of cancers associated with p53 mutations.

Peptide Bond Formation Between Unprotected Amino Acids: Convergent Synthesis of Oligopeptides

In the history of peptide chemical synthesis, amino acid-protecting groups have become basic, essential, and common moieties. As the use of protecting groups for amino acids effectively suppresses unwanted side reactions and enables the desired reaction to proceed selectively, their use continues regardless of the complexities of the protection processes and the waste generated by the deprotection residues. We developed peptide bond formation between unprotected amino acids to form silacyclic dipeptides. This is the first report of the proceeding cross-condensation between an unprotected amino acid and another unprotected amino acid. The selectivity, reaction yields, and purity of the products were satisfactory. In addition, we demonstrated further elongation of these compounds and achieved convergent synthesis with peptide-peptide elongation without the use of coupling reagents. Thus, these methods showed the potential to unlock a new, more efficient synthetic path toward polypeptides.

Effect of Brønsted Acids on the Activation of Mixed Anhydride/Mixed Carbonic Anhydride and C-Terminal-Free N-Methylated Peptide Synthesis in a Micro-Flow Reactor

Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI⋅HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99 %) without severe racemization (<1 %). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI⋅HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.

Anion-Binding Properties of Short Linear Homopeptides

A comprehensive thermodynamic and structural study of the complexation affinities of tetra (L1), penta (L2), and hexaphenylalanine (L3) linear peptides towards several inorganic anions in acetonitrile (MeCN) and N,N-dimethylformamide (DMF) was carried out. The influence of the chain length on the complexation thermodynamics and structural changes upon anion binding are particularly addressed here. The complexation processes were characterized by means of spectrofluorimetric, 1H NMR, microcalorimetric, and circular dichroism spectroscopy titrations. The results indicate that all three peptides formed complexes of 1:1 stoichiometry with chloride, bromide, hydrogen sulfate, dihydrogen phosphate (DHP), and nitrate anions in acetonitrile and DMF. In the case of hydrogen sulfate and DHP, anion complexes of higher stoichiometries were observed as well, namely those with 1:2 and 2:1 (peptide:anion) complexes. Anion-induced peptide backbone structural changes were studied by molecular dynamic simulations. The anions interacted with backbone amide protons and one of the N-terminal amine protons through hydrogen bonding. Due to the anion binding, the main chain of the studied peptides changed its conformation from elongated to quasi-cyclic in all 1:1 complexes. The accomplishment of such a conformation is especially important for cyclopeptide synthesis in the head-to-tail macrocyclization step, since it is most suitable for ring closure. In addition, the studied peptides can act as versatile ionophores, facilitating transmembrane anion transport.

Ynamide Coupling Reagents: Origin and Advances

Since the pioneering work of Curtius and Fischer, chemical peptide synthesis has witnessed a century's development and evolved into a routine technology. However, it is far from perfect. In particular, it is challenged by sustainable development because the state-of-the-art of peptide synthesis heavily relies on legacy reagents and technologies developed before the establishment of green chemistry. Over the past three decades, a broad range of efforts have been made for greening peptide synthesis, among which peptide synthesis using unprotected amino acid represents an ideal and promising strategy because it does not require protection and deprotection steps. Unfortunately, C → N peptide synthesis employing unprotected amino acids has been plagued by undesired polymerization, while N → C inverse peptide synthesis with unprotected amino acids is retarded by severe racemization/epimerization owing to the iterative activation and aminolysis of high racemization/epimerization susceptible peptidyl acids. Consequently, there is an urgent need to develop innovative coupling reagents and strategies with novel mechanisms that can address the long-standing notorious racemization/epimerization issue of peptide synthesis.This Account will describe our efforts in discovery of ynamide coupling reagents and their application in greening peptide synthesis. Over an eight-year journey, ynamide coupling reagents have evolved into a class of general coupling reagents for both amide and ester bond formation. In particular, the superiority of ynamide coupling reagents in suppressing racemization/epimerization enabled them to be effective for peptide fragment condensation, and head-to-tail cyclization, as well as precise incorporation of thioamide substitutions into peptide backbones. The first practical inverse peptide synthesis using unprotected amino acids was successfully accomplished by harnessing such features and taking advantage of a transient protection strategy. Ynamide coupling reagent-mediated ester bond formation enabled efficient intermolecular esterification and macrolactonization with preservation of α-chirality and the configuration of the conjugated α,β-C-C double bond. To make ynamide coupling reagents readily available with reasonable cost and convenience, we have developed a scalable one-step synthetic method from cheap starting materials. Furthermore, a water-removable ynamide coupling reagent was developed, offering a column-free purification of the target coupling product. In addition, the recycle of ynamide coupling reagent was accomplished, thereby paving the way for their sustainable industrial application.As such, this Account presents the whole story of the origin, mechanistic insights, preparation, synthetic applications, and recycle of ynamide coupling reagents with a perspective that highlights their future impact on peptide synthesis.

Inverse Peptide Synthesis Using Transient Protected Amino Acids

Peptide therapeutics have experienced a rapid resurgence over the past three decades. While a few peptide drugs are biologically produced, most are manufactured via chemical synthesis. The cycle of prior protection of the amino group of an α-amino acid, activation of its carboxyl group, aminolysis with the free amino group of a growing peptide chain, and deprotection of the N-terminus constitutes the principle of conventional C → N peptide chemical synthesis. The mandatory use of the Nα-protecting group invokes two additional operations for incorporating each amino acid, resulting in poor step- and atom-economy. The burgeoning demand in the peptide therapeutic market necessitates cost-effective and environmentally friendly peptide manufacturing strategies. Inverse peptide chemical synthesis using unprotected amino acids has been proposed as an ideal and appealing strategy. However, it has remained unsuccessful for over 60 years due to severe racemization/epimerization during N → C peptide chain elongation. Herein, this challenge has been successfully addressed by ynamide coupling reagent employing a transient protection strategy. The activation, transient protection, aminolysis, and in situ deprotection were performed in one pot, thus offering a practical peptide chemical synthesis strategy formally using unprotected amino acids as the starting material. Its robustness was exemplified by syntheses of peptide active pharmaceutical ingredients. It is also amenable to fragment condensation and inverse solid-phase peptide synthesis. The compatibility to green solvents further enhances its application potential in large-scale peptide production. This study offered a cost-effective, operational convenient, and environmentally benign approach to peptides.

Hydroxy-directed peptide bond formation from α-amino acid-derived inert esters enabled by boronic acid catalysis

A boronic acid-catalyzed peptide bond formation from α-amino acid methyl esters is described. The catalysis showed high chemoselectivity for β-hydroxy-α-amino esters, affording the peptides in high to excellent yields with high functional group tolerance. This hydroxy-directed peptide bond formation could be applicable to oligopeptide syntheses. This is the first successful example of organoboron-catalyzed peptide bond formation from α-amino acid-derived inert esters.