Electrophilic Sulfonium-Promoted Peptide and Protein Amidation in Aqueous Media

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.

Traceless Peptide and Protein Modification via Rational Tuning of Pyridiniums

Herein, we report a versatile reaction platform for tracelessly cleavable cysteine-selective peptide/protein modification. This platform offers highly tunable and predictable conjugation and cleavage by rationally estimating the electron effect on the nucleophilic halopyridiniums. Cleavable peptide stapling, antibody conjugation, enzyme masking/de-masking, and proteome labeling were achieved based on this facile pyridinium-thiol-exchange protocol.

4-Iodine N-Methylpyridinium-Mediated Peptide Synthesis

Through systematic optimization of halopyridinium compounds, we established a peptide coupling protocol utilizing 4-iodine N-methylpyridinium (4IMP) for solid-phase peptide synthesis (SPPS). The 4IMP coupling reagent is easily prepared, bench stable, and cost-effective. Employing 4IMP in the SPPS process has showcased remarkable chemoselectivity and efficiency, effectively eliminating racemization and epimerization. This achievement has been substantiated through the successful synthesis of a range of peptides via the direct utilization of commercially available amino acid substrates for SPPS.

Pyridinium-Based Strategy for a Bioorthogonal Conjugation-Assisted Purification Method for Profiling Cell Surface Proteome

Cell surface proteins (CSPs) are valuable targets for therapeutic agents, but achieving highly selective CSP enrichment in cellular physiology remains a technical challenge. To address this challenge, we propose a newly developed sulfo-pyridinium ester (SPE) cross-linking probe, followed by two-step imaging and enrichment. The SPE probe showed higher efficiency in labeling proteins than similar NHS esters at the level of cell lysates and demonstrated specificity for Lys in competitive experiments. More importantly, this probe could selectively label the cell membranes in cell imaging with only negligible labeling of the intracellular compartment. Moreover, we successfully performed this strategy on MCF-7 live cells to label 425 unique CSPs from 1162 labeled proteins. Finally, we employed our probe to label the CSPs of insulin-cultured MCF-7, revealing several cell surface targets of key functional biomarkers and insulin-associated pathogenesis. The above results demonstrate that the SPE method provides a promising tool for the selective labeling of cell surface proteins and monitoring transient cell surface events.

AITF (4-acetamidophenyl triflimide) mediated synthesis of amides, peptides and esters

A simple, broadly applicable protocol for amidation and esterification reactions is described. Thereby, 4-acetamidophenyl triflimide (AITF), a crystalline stable reagent, is employed for the activation of carboxylic acids. The use of AITF as a coupling agent is demonstrated in the synthesis of peptides, amides and esters under mild conditions in good to excellent yields. Notably, peptide segment condensations were also accomplished. A diverse array of synthetic protocols showcasing a broad substrate scope and good functional group compatibility were accomplished. Herein, we systematically summarized the use of AITF in peptide synthesis strategies.

Triflylpyridinium as Coupling Reagent for Rapid Amide and Ester Synthesis

An effective method has been developed to facilitate the synthesis of amides and esters at ambient temperature within 5 min, in which a stable and easily accessible triflylpyridinium reagent is used. Remarkably, this method not only has a wide range of substrate compatibility but also could realize the scalable synthesis of peptide and ester via a continuous flow process. Moreover, excellent chirality retentions are presented during activation of carboxylic acid.

The thiol-sulfoxonium ylide photo-click reaction for bioconjugation

Visible-light-mediated methods were heavily studied as a useful tool for cysteine-selective bio-conjugation; however, many current methods suffer from bio-incompatible reaction conditions and slow kinetics. To address these challenges, herein, we report a transition metal-free thiol-sulfoxonium ylide photo-click reaction that enables bioconjugation under bio-compatible conditions. The reaction is highly cysteine-selective and generally finished within minutes with naturally occurring riboflavin derivatives as organic photocatalysts. The catalysts and substrates are readily accessible and bench stable and have satisfactory water solubility. As a proof-of-concept study, the reaction was smoothly applied in chemo-proteomic analysis, which provides efficient tools to explore the druggable content of the human proteome.

Low-Toxicity Sulfonium-Based Probes for Cysteine-Specific Profiling in Live Cells

Despite being a low-abundance amino acid, cysteine plays an essential role in regulating protein function and serves as a satisfactory target of post-translational modifications and drug developments. To comprehensively assess reactive-cysteine-containing proteins, the development of chemical proteomic probes to label cysteine residues in human cells is an important objective. Cysteine modification using sulfonium-based probes is a novel method to identify reactive cysteine residues in proteins. Herein, we reported a set of "cysteine-reactive sulfonium-based (C-Sul)" probes to label the reactive cysteine sites in cellular proteins. Notably, water-soluble C-Sul probes have a significantly enhanced stability and cellular uptakes, displaying a high specificity toward reactive cysteines and compatibility with quantitative proteomic profiling. In comparison to the conventional iodoacetamide-based probe, C-Sul particularly has no inhibitory effects on cell viability, enabling its application in proteomic profiling of reactive cysteine residues under biorelevant conditions. We propose C-Sul probes as optimal tools of cysteine profiling for further broadly basic research.