An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Chalcogen-Bonding-Enabled, Light-Driven Decarboxylative Oxygenation of Amino Acid Derivatives and Short Peptides Using O2

Here we report a photocatalytic method for the decarboxylative oxygenation of amino acid derivatives and short peptides using dioxygen as a green oxidant. A reverse catalytic strategy utilizing Lewis basic diphenyl diselenide as a Lewis acid catalyst to activate carboxylic acid via chalcogen bonding interaction is the key to this work. This synthetic method is tolerant of functionalities present in both natural and non-proteinogenic amino acids, enabling the efficient synthesis of C-terminal amides or imides. Mechanistic studies suggest there is a dual noncovalent interaction between diphenyl diselenide and carboxylic acid, which allows radical decarboxylation through photoinduced intermolecular electron transfer. This new activation mode of carboxylic acids will add a new dimension to classical decarboxylative reactions.

Formamidine as an Easy-On and Easy-Off Linker for Reversible Crosslinking of Two Alkyl Amines in Peptide Stapling and Conjugation

Amino groups are abundant in both natural and synthetic molecules, offering highly accessible sites for modifying native biorelevant molecules. Despite significant progress with more reactive thiol groups, methods for ligating two amino groups with reversible linkers for bioconjugation applications remain elusive. Herein, we report the use of oxidative decarboxylative condensation of glyoxylic acid to crosslink or ligate two alkyl amines via a compact formamidine linkage, applicable in both intra- and intermolecular contexts. This linking chemistry exhibits unique hetero-coupling selectivity between primary and secondary alkyl amines. Although the formamidine linkage is stable under pH-neutral buffers and acidic conditions, it can be readily cleaved with ethylenediamine or hydrazine under mild conditions in alcohol solvents or aqueous media, fully restoring the amino groups. This study introduces a rare 'easy-on and easy-off' strategy for connecting two native amines in peptide stapling and biomolecule conjugation.

Photochemically-enabled, post-translational production of C-terminal amides

C-terminal α-amidated peptides are attractive therapeutic targets, but preparative methods to access amidated pharmaceuticals are limited both on lab and manufacturing-scale. Here we report a straightforward and scalable approach to the C-terminal α-amidation of peptides and proteins from cysteine-extended polypeptide precursors. This amidation protocol consists of three highly efficient steps: 1) selective cysteine thiol substitution with a photolabel, 2) photoinduced decarboxylative elimination and 3) enamide cleavage by simple acidolysis or inverse electron demand Diels-Alder reaction. We provide a blueprint for applying this protocol to the semi-recombinant production of therapeutically relevant targets where gram scale C-terminal α-amidation is achieved in a photoflow reactor on a recombinantly prepared peptide YY analogue and a GLP-1/amylin co-agonist precursor peptide. Robust performance of this reaction cascade in flow highlights the potential of this chemistry to enable amidated drug leads to enter development that would not be viable on commercial scale using existing technology.

Recent Toolboxes for Chemoselective Dual Modifications of Proteins

Site-selective chemical modifications of proteins have emerged as a potent technology in chemical biology, materials science, and medicine, facilitating precise manipulation of proteins with tailored functionalities for basic biology research and developing innovative therapeutics. Compared to traditional recombinant expression methods, one of the prominent advantages of chemical protein modification lies in its capacity to decorate proteins with a wide range of functional moieties, including non-genetically encoded ones, enabling the generation of novel protein conjugates with enhanced or previously unexplored properties. Among these, approaches for dual or multiple modifications of proteins are increasingly garnering attention, as it has been found that single modification of proteins is inadequate to meet current demands. Therefore, in light of the rapid developments in this field, this review provides a timely and comprehensive overview of the latest advancements in chemical and biological approaches for dual functionalization of proteins. It further discusses their advantages, limitations, and potential future directions in this relatively nascent area.

Hypervalent Iodine Amino Acid Building Blocks for Bioorthogonal Peptide Macrocyclization

Ethynylbenziodoxol(on)es (EB(X)xs) reagents have emerged as useful reagents for peptide/protein modification due to their versatile reactivity and high selectivity. Herein, we report the successful introduction of ethynylbenziodoxoles (EBxs) on different amino acid building blocks (Lys/Orn/Dap), and show their compatibility with both solid phase peptide synthesis (SPPS) and solution phase peptide synthesis (SPS). The selective incorporation of the EBx core into peptide sequences enable efficient macrocyclizations under mild conditions for the synthesis of topologically unique cyclic and bicyclic peptides.

Synthesis of N-Glyoxylyl Peptides Enabled by a Lossen Rearrangement-Induced Intramolecular Redox Reaction of N-Terminal Glycyl Hydroxamic Acid

An oxidant-free approach to the synthesis of N-glyoxylyl peptides has been developed that utilizes the Lossen rearrangement of the N-terminal glycyl hydroxamic acid residue. The synthesis proceeds via an intramolecular redox mechanism to yield the glyoxylyl peptides, which are then subjected to various peptide cyclization procedures. The reaction scheme is suitable for oxidation-sensitive moieties including amino acids.

Electrochemical Nickel-Catalyzed Selective Inter- and Intramolecular Arylations of Cysteine-Containing Peptides

Here we report a simple electrochemical route towards the synthesis of S-arylated peptides by a site selective coupling of peptides with aryl halides under base free conditions. This approach demonstrates the power of electrochemistry to access both highly complex peptide conjugates and cyclic peptides.

Electrochemical Modification of Polypeptides at Selenocysteine

Mild strategies for the selective modification of peptides and proteins are in demand for applications in therapeutic peptide and protein discovery, and in the study of fundamental biomolecular processes. Herein, we describe the development of an electrochemical selenoetherification (e-SE) platform for the efficient site-selective functionalization of polypeptides. This methodology utilizes the unique reactivity of the 21st amino acid, selenocysteine, to effect formation of valuable bioconjugates through stable selenoether linkages under mild electrochemical conditions. The power of e-SE is highlighted through late-stage C-terminal modification of the FDA-approved cancer drug leuprolide and assembly of a library of anti-HER2 affibody conjugates bearing complex cargoes. Following assembly by e-SE, the utility of functionalized affibodies for in vitro imaging and targeting of HER2 positive breast and lung cancer cell lines is also demonstrated.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Fine-Tuning Electroauxiliary-Mediated Peptide Modifications Using Second-Generation Electroactive Amino Acids

Arylthioether functional groups serve as effective electroauxiliaries for tunable oxidations. Herein, we disclose the synthesis of second-generation glutamine building blocks bearing 2,4-dimethoxythiophenyl and 2,4-dichlorothiophenyl-derived electroauxiliaries. These building blocks improve SPPS efficiency and enable fine-tuning of the electrochemical window for selective anodic oxidation reactions in comparison to first-generation 4-methoxythiophenyl- and 4-nitrothiophenyl-substituted variants. Installation onto a segment of involucrin, a protein component of human skin, emphasizes the practical application of the new building blocks for iterative functionalizations.

Tunable Electrochemical Peptide Modifications: Unlocking New Levels of Orthogonality for Side-Chain Functionalization

Electrochemical transformations provide enticing opportunities for programmable, residue-specific peptide modifications. Herein, we harness the potential of amidic side-chains as underutilized handles for late-stage modification through the development of an electroauxiliary-assisted oxidation of glutamine residues within unprotected peptides. Glutamine building blocks bearing electroactive side-chain N,S-acetals are incorporated into peptides using standard Fmoc-SPPS. Anodic oxidation of the electroauxiliary in the presence of diverse alcohol nucleophiles enables the installation of high-value N,O-acetal functionalities. Proof-of-principle for an electrochemical peptide stapling protocol, as well as the functionalization of dynorphin B, an endogenous opioid peptide, demonstrates the applicability of the method to intricate peptide systems. Finally, the site-selective and tunable electrochemical modification of a peptide bearing two discretely oxidizable sites is achieved.

Electrochemistry for the Chemoselective Modification of Peptides and Proteins

Although electrochemical strategies for small-molecule synthesis are flourishing, this technology has yet to be fully exploited for the mild and chemoselective modification of peptides and proteins. With the growing number of diverse peptide natural products being identified and the emergence of modified proteins as therapeutic and diagnostic agents, methods for electrochemical modification stand as alluring prospects for harnessing the reactivity of polypeptides to build molecular complexity. As a mild and inherently tunable reaction platform, electrochemistry is arguably well-suited to overcome the chemo- and regioselectivity issues which limit existing bioconjugation strategies. This Perspective will showcase recently developed electrochemical approaches to peptide and protein modification. The article also highlights the wealth of untapped opportunities for the production of homogeneously modified biomolecules, with an eye toward realizing the enormous potential of electrochemistry for chemoselective bioconjugation chemistry.

Novel electrochemically-mediated peptide dethiylation in processes relevant to native chemical ligation

Here we explore electrochemical dethiylation in processes relevant to Native Chemical Ligation (NCL). NCL’s reliance on the redox active amino acid cysteine and β-mercaptoamine derivatives suggests a potential role for...

Electrochemical formal [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines

A catalyst-free electrochemical [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines without an external oxidant has been developed for constructing the 1,2,4-triazolidine skeleton.

Electrochemical Regioselective Cross-Dehydrogenative Coupling of Indoles with Xanthenes

An electrochemical cross-dehydrogenative coupling of indoles with xanthenes has been established at room temperature. This coupling reaction could proceed in the absence of any catalyst or external oxidant, and generate the indole derivatives in moderate yields. Mechanistic experiments support that a radical pathway maybe involved in this reaction system.