Labeling and Natural Post-Translational Modification of Peptides and Proteins via Chemoselective Pd-Catalyzed Prenylation of Cysteine

Iridium-catalyzed diacylmethylation of tyrosine and its peptides with sulfoxonium ylides

Pyridyloxy-directed Ir(III)-catalyzed diacylmethylation of protected tyrosines was achieved with alkyl and (hetero)aryl sulfoxonium ylides, furnishing tyrosine-based unnatural amino acids in good yields. Furthermore, the late stage exemplification of the strategy was successfully accomplished in tyrosine-containing dipeptides, tripeptides and tetrapeptides in moderate yields. This methodology is distinguished by its site-selectivity, tolerance of sensitive functional groups, scalability, and retention of the chiral configuration for tyrosine motifs.

Ultrafast Au(III)-Mediated Arylation of Cysteine

Through mechanistic work and rational design, we have developed the fastest organometallic abiotic Cys bioconjugation. As a result, the developed organometallic Au(III) bioconjugation reagents enable selective labeling of Cys moieties down to picomolar concentrations and allow for the rapid construction of complex heterostructures from peptides, proteins, and oligonucleotides. This work showcases how organometallic chemistry can be interfaced with biomolecules and lead to a range of reactivities that are largely unmatched by classical organic chemistry tools.

Semisynthesis of segmentally isotope-labeled and site-specifically palmitoylated CD44 cytoplasmic tail

CD44, a ubiquitously expressed transmembrane receptor, plays a crucial role in cell growth, migration, and tumor progression. Dimerization of CD44 is a key event in signal transduction and has emerged as a potential target for anti-tumor therapies. Palmitoylation, a posttranslational modification, disrupts CD44 dimerization and promotes CD44 accumulation in ordered membrane domains. However, the effects of palmitoylation on the structure and dynamics of CD44 at atomic resolution remain poorly understood. Here, we present a semisynthetic approach combining solid-phase peptide synthesis, recombinant expression, and native chemical ligation to investigate the impact of palmitoylation on the cytoplasmic domain (residues 669-742) of CD44 (CD44ct) by NMR spectroscopy. A segmentally isotope-labeled and site-specifically palmitoylated CD44 variant enabled NMR studies, which revealed chemical shift perturbations and indicated local and long-range conformational changes induced by palmitoylation. The long-range effects suggest altered intramolecular interactions and potential modulation of membrane association patterns. Semisynthetic, palmitoylated CD44ct serves as the basis for studying CD44 clustering, conformational changes, and localization within lipid rafts, and could be used to investigate its role as a tumor suppressor and to explore its therapeutic potential.

Enhancing Tsuji-Trost deallylation in living cells with an internal-nucleophile coumarin-based probe

In recent years, bioorthogonal uncaging reactions have been developed to proceed efficiently under physiological conditions. However, limited progress has been made in the development of protecting groups combining stability under physiological settings with the ability to be quickly removed via bioorthogonal catalysis. Herein, we present a new water-soluble coumarin-derived probe bearing an internal nucleophilic group capable of promoting Tsuji-Trost deallylation under palladium catalysis. This probe can be cleaved by a bioorthogonal palladium complex at a faster rate than the traditional probe, namely N-Alloc-7-amino-4-methylcoumarin. As the deallylation process proved to be efficient in mammalian cells, we envision that this probe may find applications in chemical biology, bioengineering, and medicine.

Highly efficient site-specific protein modification using tyrosinase from Streptomyces avermitilis: Structural insight

Tyrosinase-mediated protein conjugation has recently drawn attention as a site-specific protein modification tool under mild conditions. However, the tyrosinases reported to date act only on extremely exposed tyrosine residues, which limits where the target tyrosine can be located. Herein, we report a tyrosinase from Streptomyces avermitilis (SaTYR), that exhibits a much higher activity against tyrosine residues on the protein surface than other tyrosinases. We determined the crystal structure of SaTYR and revealed that the enzyme has a relatively flat and shallow substrate-binding pocket to accommodate a protein substrate. We demonstrated SaTYR-mediated fluorescence dye tagging and PEGylation of a surface tyrosine residue that was unreacted by other tyrosinases with an approximately 95.2 % conjugation yield in 1 h. We also present a structural rationale that considers the steric hindrance from adjacent residues and surrounding structures along with the extent of solvent exposure of residues, as necessary when determining the optimal positions for introducing target tyrosine residues in SaTYR-mediated protein modification. The study demonstrated that the novel tyrosinase, SaTYR, extends the scope of tyrosinase-mediated protein modification, and we propose that site-specific tyrosine conjugation using SaTYR is a promising strategy for protein bioconjugation in various applications.

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.

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

Constraining and Modifying Peptides Using Pd-Mediated Cysteine Allylation

Over the past decades, several strategies for inducing and stabilizing secondary structure formation in peptides have been developed to increase their proteolytic stability and their binding affinity to specific interaction partners. Here, we report how our recently introduced chemoselective Pd-catalyzed cysteine allylation reaction can be extended to stapling and how the resulting alkene-containing staples themselves can be further modified to introduce additional probes into such stabilized peptides. The latter is demonstrated by introducing a fluorophore as well as a PEG moiety into different stapled peptides using bioorthogonal thiol-ene and Diels-Alder reactions. Furthermore, we investigated structural implications of our allyl staples when used to replace conformationally relevant disulfide bridges. To this end, we chose a selective binder of integrin α3 β1 (LXY3), which is only active in its cyclic disulfide form. We replaced the disulfide bridge by different stapling reagents in order to increase stability and binding affinity towards integrin α3 β1 .

Chemical modification of proteins - challenges and trends at the start of the 2020s

Ribosomally expressed proteins perform multiple, versatile, and specialized tasks throughout Nature. In modern times, chemically modified proteins, including improved hormones, enzymes, and antibody-drug-conjugates have become available and have found advanced industrial and pharmaceutical applications. Chemical modification of proteins is used to introduce new functionalities, improve stability or drugability. Undertaking chemical reactions with proteins without compromising their native function is still a core challenge as proteins are large conformation dependent multifunctional molecules. Methods for functionalization ideally should be chemo-selective, site-selective, and undertaken under biocompatible conditions in aqueous buffer to prevent denaturation of the protein. Here the present challenges in the field are discussed and methods for modification of the 20 encoded amino acids as well as the N-/C-termini and protein backbone are presented. For each amino acid, common and traditional modification methods are presented first, followed by more recent ones.

Palladium-peptide oxidative addition complexes for bioconjugation

The synthesis of palladium oxidative addition complexes derived from unprotected peptides is described. Incorporation of 4-halophenylalanine into a peptide during solid phase peptide synthesis allows for subsequent oxidative addition at this position upon treatment with a palladium precursor and suitable ligand. The resulting palladium-peptide complexes are solid, storable, water-soluble, and easily purified via high-performance liquid chromatography. These complexes react with thiols in aqueous buffer, offering an efficient method for bioconjugation. Using this strategy, peptides can be functionalized with small molecules to prepare modified aryl thioether side-chains at low micromolar concentrations. Additionally, peptide-peptide and peptide-protein ligations are demonstrated under dilute aqueous conditions.

Chemische Synthese und Semisynthese von lipidierten Proteinen

Lipidierung ist eine ubiquitäre Modifikation von Peptiden und Proteinen, die entweder co‐ oder posttranslational auftreten kann. Für die Vielzahl von Lipidklassen wurde gezeigt, dass diese viele entscheidende biologische Aktivitäten, z. B. die Regulierung der Signalweiterleitung, Zell‐Zell‐Adhäsion sowie die Anlagerung von Proteinen an Lipid‐Rafts und Phospholipidmembranen, beeinflussen. Während die Natur Enzyme nutzt, um Lipidmodifikationen in Proteine einzubringen, ist ihre Nutzung für die chemoenzymatische Herstellung von lipidierten Proteinen häufig ineffizient. Eine Alternative ist die Kombination moderner synthetischer und semisynthetischer Techniken, um lipidierte Proteine in reiner und homogen modifizierter Form zu erhalten. Dieser Aufsatz erörtert Fortschritte in der Entwicklung der Lipidierungs‐ und Ligationschemie und deren Anwendung in der Synthese und Semisynthese homogen lipidierter Proteine, die es ermöglichen, den Einfluss dieser Modifikationen auf die Proteinstruktur und ‐funktion zu untersuchen.

Palladium-catalyzed [3 + 3] annulations of 1-alkyl-indolin-2-imines and dialkyl (2-methylenepropane-1,3-diyl) dicarbonates

Palladium-catalyzed [3 + 3] annulations of 1-alkyl-indolin-2-imines and dialkyl (2-methylenepropane-1,3-diyl) dicarbonates leading to pyrido[2,3-b]indole derivatives have been developed for the first time.

Gold-catalyzed thioetherification of allyl, benzyl, and propargyl phosphates

Supported gold catalysts showed high activity for thioetherification of various phosphates. The surface of gold nanoparticles supported on ZrO2 served as a source for active cationic Au species.

α-Vinyl azide–cysteine click coupling reaction enabled bioorthogonal peptide/protein modification

α-Alkyl and α-aryl vinyl azides were found to be able to couple with cysteine-derived alkyl thiols chemoselectively under mild conditions, providing the corresponding β-ketosulfides with simultaneous extrusion of N2 and ammonia.

Reactions of proteins with a few organopalladium compounds of medicinal interest

Pd compounds form a promising class of experimental anticancer drug candidates whose mechanism of action is still largely unknown; in particular, a few organopalladium compounds seem very attractive. To gain mechanistic insight into medicinal palladium compounds, we have explored here – through ESI MS analysis – the interactions of four organopalladium agents (1–4) – showing remarkable in vitro antiproliferative properties – with a few representative model proteins, i.e., lysozyme (HEWL), ribonuclease A (RNase), and carbonic anhydrase (hCAI). The tested panel included three Pd allyl compounds with one or two carbene ligands and a palladacyclopentadienyl complex. Notably, the Pd allyl compounds turned out to manifest, on the whole, a modest tendency to react with the above proteins. Only complex 3 produced small amounts of characteristic adducts with hCAI bearing either one or two Pd allyl groups. In contrast, the palladacyclopentadienyl complex 4 manifested a greater and peculiar reactivity with all the above proteins generating invariably protein adducts with a mass increase of +256 Da where a butadienyl group – with no associated Pd – is attached to the proteins. Afterwards, we extended our investigations to the C-terminal dodecapeptide of thioredoxin reductase bearing the –Cys–Sec– reactive motif. In this latter case adducts were formed with all tested Pd compounds; however, complex 4 manifested towards this dodecapeptide a type of reactivity deeply different from that observed with HEWL, RNase A and hCAI. The mechanistic implications of these findings are discussed.

Pd compounds form a promising class of experimental anticancer drug candidates whose mechanism of action is still largely unknown; in particular, a few organopalladium compounds seem very attractive.

A Tale of Two Bioconjugations: pH Controlled Divergent Reactivity of Protein α-oxo-Aldehydes in Competing α-oxo-Mannich and Catalyst-Free Aldol Ligations

Site-selective chemical methods for protein bioconjugation have revolutionized the fields of cell and chemical biology through the development of novel protein/enzyme probes bearing fluorescent, spectroscopic, or even toxic cargos. Herein, we report two new methods for the bioconjugation of α-oxo aldehyde handles within proteins using small molecule aniline and/or phenol probes. The "α-oxo-Mannich" and "catalyst-free aldol" ligations both compete for the electrophilic α-oxo aldehyde, which displays pH divergent reactivity proceeding through the "Mannich" pathway at acidic pH to afford bifunctionalized bioconjugates, and the "catalyst-free aldol" pathway at neutral pH to afford monofunctionalized bioconjugates. We explore the substrate scope and utility of both of these bioconjugations in the construction of neoglycoproteins, in the process formulating a mechanistic rationale for how both pathways intersect with each other at different reaction pH's.

Chemical Synthesis and Semisynthesis of Lipidated Proteins

Lipidation is a ubiquitous modification of peptides and proteins that can occur either co‐ or post‐translationally. An array of different lipid classes can adorn proteins and has been shown to influence a number of crucial biological activities, including the regulation of signaling, cell–cell adhesion events, and the anchoring of proteins to lipid rafts and phospholipid membranes. Whereas nature employs a range of enzymes to install lipid modifications onto proteins, the use of these for the chemoenzymatic generation of lipidated proteins is often inefficient or impractical. An alternative is to harness the power of modern synthetic and semisynthetic technologies to access lipid‐modified proteins in a pure and homogeneously modified form. This Review aims to highlight significant advances in the development of lipidation and ligation chemistry and their implementation in the synthesis and semisynthesis of homogeneous lipidated proteins that have enabled the influence of these modifications on protein structure and function to be uncovered.

Nonenzymatic Stereoselective S-Glycosylation of Polypeptides and Proteins

Here we report a nonenzymatic glycosylation reaction that builds axial S-glycosidic bonds under biorelevant conditions. This strategy is enabled by the design and use of allyl glycosyl sulfones as precursors to glycosyl radicals and exploits the exceptional functional group tolerance of radical processes. Our method introduces a variety of unprotected glycosyl units to the cysteine residues of peptides in a highly selective fashion. Through developing the second-generation protocol, we applied our method in the direct glycosylation of complex polypeptides and proteins. Computational studies were performed to elucidate the reaction mechanism.

Amphiphilic Iodine(III) Reagents for the Lipophilization of Peptides in Water

We report the functionalization of cysteine residues with lipophilic alkynes bearing a silyl group or an alkyl chain using amphiphilic ethynylbenziodoxolone reagents (EBXs). The reactions were carried out in buffer (pH 6 to 9), without organic co-solvent or removal of oxygen, either at 37 °C or room temperature. The transformation led to a significant increase of peptide lipophilicity and worked for aromatic thiols, homocysteine, cysteine, and peptides containing 4 to 18 amino acids. His6 -Cys-Ubiquitin was also alkynylated under physiological conditions. Under acidic conditions, the thioalkynes were converted into thioesters, which could be cleaved in the presence of hydroxylamine.

Chemically modified and conjugated antimicrobial peptides against superbugs

Antimicrobial resistance (AMR) is one of the greatest threats to human health that, by 2050, will lead to more deaths from bacterial infections than cancer. New antimicrobial agents, both broad-spectrum and selective, that do not induce AMR are urgently required. Antimicrobial peptides (AMPs) are a novel class of alternatives that possess potent activity against a wide range of Gram-negative and positive bacteria with little or no capacity to induce AMR. This has stimulated substantial chemical development of novel peptide-based antibiotics possessing improved therapeutic index. This review summarises recent synthetic efforts and their impact on analogue design as well as their various applications in AMP development. It includes modifications that have been reported to enhance antimicrobial activity including lipidation, glycosylation and multimerization through to the broad application of novel bio-orthogonal chemistry, as well as perspectives on the direction of future research. The subject area is primarily the development of next-generation antimicrobial agents through selective, rational chemical modification of AMPs. The review further serves as a guide toward the most promising directions in this field to stimulate broad scientific attention, and will lead to new, effective and selective solutions for the several biomedical challenges to which antimicrobial peptidomimetics are being applied.

An Organometallic Strategy for Cysteine Borylation

Synthetic bioconjugation at cysteine (Cys) residues in peptides and proteins has emerged as a powerful tool in chemistry. Soft nucleophilicity of the sulfur in Cys renders an exquisite chemoselectivity with which various functional groups can be placed onto this residue under benign conditions. While a variety of reactions have been successful at producing Cys-based bioconjugates, the majority of these feature sulfur-carbon bonds. We report Cys-borylation, wherein a benchtop stable Pt(II)-based organometallic reagent can be used to transfer a boron-rich cluster onto a sulfur moiety in unprotected peptides forging a boron-sulfur bond. Cys-borylation proceeds at room temperature and tolerates a variety of functional groups present in complex polypeptides. Further, the bioconjugation strategy can be applied to a model protein modification of Cys-containing DARPin (designed ankyrin repeat protein). The resultant bioconjugates show no additional toxicity compared to their Cys alkyl-based congeners. Finally, we demonstrate how the developed Cys-borylation can enhance the proteolytic stability of the resultant peptide bioconjugates while maintaining the binding affinity to a protein target.

Conversion of Pd(I) off-cycle species into highly efficient cross-coupling catalysts

We report on the facile conversion of [Pd2(μ-Cl)(μ-η3-R-allyl)(NHC)2] complexes, which are commonly considered undesirable off-cycle species in cross-coupling reactions, into active [PdCl(μ-Cl)(NHC)]2 pre-catalysts. All reactions proceed under mild conditions (40 °C, 1-2 hours in acetone) using inexpensive HCl as both an oxidant and chloride source. DFT calculations were performed to explore the possible mechanism of this transformation, which appears to involve a combination of two different pathways. Moreover this study provides insights into factors favoring and hindering Pd(i) dimer formation undesirable in catalysis.

Oligonucleotide Bioconjugation with Bifunctional Palladium Reagents

Organometallic reagents enable practical strategies for bioconjugation. Innovations in the design of water-soluble ligands and the enhancement of reaction rates have allowed for chemoselective cross-coupling reactions of peptides and proteins to be carried out in water. There are currently no organometallic-based methods for oligonucleotide bioconjugation to other biomolecules. Here we report bifunctional palladium(II)-oxidative addition complexes (OACs) as reagents for high-yielding oligonucleotide bioconjugation reactions. These bifunctional OACs react chemoselectively with amine-modified oligonucleotides to generate the first isolable, bench stable oligonucleotide-palladium(II) OACs. These complexes undergo site-selective C-S arylation with a broad range of native thiol-containing biomolecules at low micromolar concentrations in under one hour. This approach provided oligonucleotide-peptide, oligonucleotide-protein, oligonucleotide-small molecule, and oligonucleotide-oligonucleotide conjugates in >80 % yield and afforded conjugation of multiple copies of oligonucleotides onto a monoclonal antibody.

Gold(III) Aryl Complexes as Reagents for Constructing Hybrid Peptide-Based Assemblies via Cysteine S-Arylation

Organometallic complexes have recently gained attention as competent bioconjugation reagents capable of introducing a diverse array of substrates to biomolecule substrates. Here, we detail the synthesis and characterization of an aminophosphine-supported Au(III) platform that provides rapid and convenient access to a wide array of peptide-based assemblies via cysteine S-arylation. This strategy results in the formation of robust C-S covalent linkages and is an attractive method for the modification of complex biomolecules due to the high functional group tolerance, chemoselectivity, and rapid reaction kinetics associated with these arylation reactions. This work expands upon existing metal-mediated cysteine arylation by introducing a class of air-stable organometallic complexes that serve as robust bioconjugation reagents enabling the synthesis of conjugates of higher structural complexity including macrocyclic stapled and bicyclic peptides as well as a peptide-functionalized multivalent hybrid nanocluster. This organometallic-based approach provides a convenient, one-step method of peptide functionalization and macrocyclization, and has the potential to contribute to efforts directed toward developing efficient synthetic strategies of building new and diverse hybrid peptide-based assemblies.

Late-Stage Hydrocarbon Conjugation and Cyclisation in Synthetic Peptides and Proteins

The conventional S-alkylation of cysteine relies upon using activated electrophiles. Here we demonstrate high-yielding and selective S-alkylation and S-lipidation of cysteines in unprotected synthetic peptides and proteins by using weak electrophiles and a Zn²⁺ promoter. Linear or branched iodoalkanes can S-alkylate cysteine in an unprotected 38-residue Myc peptide fragment and in a 91-residue miniprotein Omomyc, thus highlighting selective late-stage synthetic modifications. Metal-assisted cysteine alkylation is also effective for incorporating dehydroalanine into unprotected peptides and for peptide cyclisation via aliphatic thioether crosslinks, including customising macrocycles to stabilise helical peptides for enhanced uptake and delivery to proteins inside cells. Chemoselective and efficient late-stage Zn²⁺ -promoted cysteine alkylation in unprotected peptides and proteins promises many useful applications.

Synthesis of Hydrophilic Phosphorus Ligands and Their Application in Aqueous-Phase Metal-Catalyzed Reactions

Transition metal-catalyzed reactions in aqueous media are experiencing a constant increase in interest. In homogenous catalysis the use of water as a solvent offers advantages in cost, safety, the possibility of two-phase catalysis and simplified separation strategies. In the life sciences, transition metal catalysis in aqueous systems enables the ligation or modification of biopolymers in buffer systems or even in their cellular environment. In biocatalysis, aqueous systems allow the simultaneous use of enzymes and transition metal catalysts in cascade reactions. The use of water-soluble phosphine ligands still represents the most reliable and popular strategy for transferring metal catalysts into the aqueous phase. This review summarizes the recent advancements in this field since 2009 and describes current synthetic strategies for the preparation of hydrophilic phosphines and phosphites. In addition, recent applications of transition metal catalysis in aqueous solvents using these hydrophilic ligands are presented.

Deconvoluting the biology and druggability of protein lipidation using chemical proteomics

Lipids are indispensable cellular building blocks, and their post-translational attachment to proteins makes them important regulators of many biological processes. Dysfunction of protein lipidation is also implicated in many pathological states, yet its systematic analysis presents significant challenges. Thanks to innovations in chemical proteomics, lipidation can now be readily studied by metabolic tagging using functionalized lipid analogs, enabling global profiling of lipidated substrates using mass spectrometry. This has spearheaded the first deconvolution of their full scope in a range of contexts, from cells to pathogens and multicellular organisms. Protein N-myristoylation, S-acylation, and S-prenylation are the most well-studied lipid post-translational modifications because of their extensive contribution to the regulation of diverse cellular processes. In this review, we focus on recent advances in the study of these post-translational modifications, with an emphasis on how novel mass spectrometry methods have elucidated their roles in fundamental biological processes.

Organoruthenium-catalyzed chemical protein synthesis to elucidate the functions of epigenetic modifications on heterochromatin factors

The application of organometallic compounds for protein science has received attention. Recently, total chemical protein synthesis using transition metal complexes has been developed to produce various proteins bearing site-specific posttranslational modifications (PTMs). However, in general, significant amounts of metal complexes were required to achieve chemical reactions of proteins bearing a large number of nucleophilic functional groups. Moreover, syntheses of medium-size proteins (>20 kDa) were plagued by time-consuming procedures due to cumbersome purification and isolation steps, which prevented access to variously decorated proteins. Here, we report a one-pot multiple peptide ligation strategy assisted by an air-tolerant organoruthenium catalyst that showed more than 50-fold activity over previous palladium complexes, leading to rapid and quantitative deprotection on a protein with a catalytic amount (20 mol%) of the metal complex even in the presence of excess thiol moieties. Utilizing the organoruthenium catalyst, heterochromatin factors above 20 kDa, such as linker histone H1.2 and heterochromatin protein 1α (HP1α), bearing site-specific PTMs including phosphorylation, ubiquitination, citrullination, and acetylation have been synthesized. The biochemical assays using synthetic proteins revealed that the citrullination at R53 in H1.2 resulted in the reduced electrostatic interaction with DNA and the reduced binding affinity to nucleosomes. Furthermore, we identified a key phosphorylation region in HP1α to control its DNA-binding ability. The ruthenium chemistry developed here will facilitate the preparation of a variety of biologically and medically significant proteins containing PTMs and non-natural amino acids.

Chemical protein synthesis assisted by an organoruthenium catalyst streamlined the production of heterochromatin factors bearing various patterns of epigenetic modifications, and their biological significance was elucidated.

Palladium in biological media: Can the synthetic chemist's most versatile transition metal become a powerful biological tool?

Palladium catalysed reactions are ubiquitous in synthetic organic chemistry in both organic solvents and aqueous buffers. The broad reactivity of palladium catalysis has drawn interest as a means to conduct orthogonal transformations in biological settings. Successful examples have been shown for protein modification, in vivo drug decaging and as palladium-protein biohybrid catalysts for selective catalysis. Biological media represents a challenging environment for palladium chemistry due to the presence of a multitude of chelators, catalyst poisons and a requirement for milder reaction conditions e.g. lower temperatures. This review looks to identify successful examples of palladium-catalysed reactions in the presence of proteins or cells and analyse solutions to help to overcome the challenges of working in biological systems.

Synthesis of hydroxy-α-sanshool

Hydroxy-α-sanshool was synthesized in a 13% overall yield through eight steps, which included two Wittig reactions that were used to form the carbon skeleton with ethyl 2-oxoacetate and 2 E,4 E-hexadienal being reacted with the appropriate ylides. Impurities in the processes could easily be separated. Ethyl 6-hydroxy-2Z-hexenoate was converted to its E-isomer with catalysis by I2 and 2E,6Z,8E,10E-dodecatetraenoic acid was crystallized from a solution in 1% ethyl acetate in n-hexane.

Transition-Metal-Mediated Modification of Biomolecules

The site-selective modification of biomolecules has grown spectacularly in recent years. The presence of a large number of functional groups in a biomolecule makes its chemo- and regioselective modification a challenging goal. In this context, transition-metal-mediated reactions are emerging as a powerful tool owing to their unique reactivity and good functional group compatibility, allowing highly efficient and selective bioconjugation reactions that operate under mild conditions. This Minireview focuses on the current state of organometallic chemistry for bioconjugation, highlighting the potential of transition metals for the development of chemoselective and site-specific methods for functionalization of peptides, proteins and nucleic acids. The importance of the selection of ligands attached to the transition metal for conferring the desired chemoselectivity will be highlighted.

Transition metal catalyzed site-selective cysteine diversification of proteins

Site-specific protein conjugation is a critical step in the generation of unique protein analogs for a range of basic research and therapeutic developments. Protein transformations must target a precise residue in the presence of a plethora of functional groups to obtain a well-characterized homogeneous product. Competing reactive residues on natural proteins render rapid and selective conjugation a challenging task. Organometallic reagents have recently emerged as a powerful strategy to achieve site-specific labeling of a diverse set of biopolymers, due to advances in water-soluble ligand design, high reaction rate, and selectivity. The thiophilic nature of various transition metals, especially soft metals, makes cysteine an ideal target for these reagents. The distinctive reactivity and selectivity of organometallic-based reactions, along with the unique reactivity and abundancy of cysteine within the human proteome, provide a powerful platform to modify native proteins in aqueous media. These reactions often provide the modified proteins with a stable linkage made from irreversible cross-coupling steps. Additionally, transition metal reagents have recently been applied for the decaging of cysteine residues in the context of chemical protein synthesis. Orthogonal cysteine protecting groups and functional tags are often necessary for the synthesis of challenging proteins, and organometallic reagents are powerful tools for selective, rapid, and water-compatible removal of those moieties. This review examines transition metal-based reactions of cysteine residues for the synthesis and modification of natural peptides and proteins.

Post-Translational Modification Mimicry for Programmable Assembly of Elastin-Based Protein Polymers

Post-translational modification (PTM) of protein polymers is emerging as a powerful bioinspired strategy to create protein-based hybrid materials with molecularly encoded assembly and function for applications in nanobiotechnology and medicine. While these modifications can be accomplished by harnessing native biological machinery (i.e., enzymes), the evolutionarily programmed specificity of these enzymes (recognition of select substrates and the limited repertoire of ligation chemistries catalyzed by these enzymes) can limit the type and linkage of PTMs appended to proteins. One approach to overcome this limitation is to leverage advances in site-selective biomolecular modification to prepare synthetic mimics of naturally occurring PTMs that are absent in nature. As a proof of concept, we used scalable bio-orthogonal reactions to prepare synthetic mimics of lipidated proteins with tunable assembly and disassembly. Additionally, we demonstrated that our PTM mimicry regulates the stimuli-responsive phase behavior of intrinsically disordered biopolymers, modulates their self-assembly at the nanoscale, and can be used for programmable disassembly of these materials in acidic environments. Synthetic PTM mimicry opens a path to encode new assembly and disassembly capabilities into hybrid materials that cannot be produced via biosynthesis.

Pd/BIPHEPHOS is an Efficient Catalyst for the Pd-Catalyzed S-Allylation of Thiols with High n-Selectivity

The Pd-catalyzed S-allylation of thiols with stable allylcarbonate and allylacetate reagents offers several advantages over established reactions for the formation of thioethers. We could demonstrate that Pd/BIPHEPHOS is a catalyst system which allows the transition metal-catalyzed S-allylation of thiols with excellent n-regioselectivity. Mechanistic studies showed that this reaction is reversible under the applied reaction conditions. The excellent functional group tolerance of this transformation was demonstrated with a broad variety of thiol nucleophiles (18 examples) and allyl substrates (9 examples), and could even be applied for the late-stage diversification of cephalosporins, which might find application in the synthesis of new antibiotics.