Direct 11CN-Labeling of Unprotected Peptides via Palladium-Mediated Sequential Cross-Coupling Reactions

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.

C-H radiocyanation of bioactive molecules via sequential iodination/copper-mediated cross-coupling

This report describes a net C-H radiocyanation reaction for the transformation of electron rich (hetero)aromatic substrates into 11CN-labeled products. Electrophilic C(sp2)-H iodination of the (hetero)arene with N-iodosuccinimide is followed by Cu-mediated radiocyanation with K11CN. This sequence is applied to a variety of substrates, including the nucleobases uracil and cytosine, the amino acids tyrosine and tryptophan, and the peptide LYRAGWRAFS, which undergoes selective C-H radiocyanation at the tryptophan (W) residue.

Deoxyfluorination of phenols for chemoselective 18F-labeling of peptides

The challenge of forming C-18F bonds is often a bottleneck in the development of new 18F-labeled tracer molecules for noninvasive functional imaging studies using positron emission tomography (PET). Nucleophilic aromatic substitution is the most widely employed reaction to functionalize aromatic substrates with the radioactive fluorine-18 but its scope is restricted to arenes containing electron-withdrawing substituents. Furthermore, many protic functional groups are incompatible with basic fluoride anions. Peptide substrates, which are highly desirable targets for PET molecular imaging, are particularly challenging to label with fluorine-18 because they are densely functionalized and sensitive to high temperatures and basic conditions. To expand the utility of nucleophilic aromatic substitution with fluorine-18, we describe two complementary procedures for the radiodeoxyfluorination of bench-stable and easy-to-access phenols that ensure rapid access to densely functionalized electron-rich and electron-poor 18F-aryl fluorides. The first procedure details the synthesis of an 18F-synthon and its subsequent ligation to the cysteine residue of Arg-Gly-Asp-Cys in 10.5 h from commercially available starting materials (189-min radiosynthesis). The second procedure describes the incorporation of commercially available CpRu(Fmoc-tyrosine)OTf into a fully protected peptide Lys-Met-Glu-(CpRu-Tyr)-Leu via solid-phase peptide synthesis and subsequent ruthenium-mediated uronium deoxyfluorination with fluorine-18 followed by deprotection, accomplished within 7 d (116-min radiosynthesis). Both radiolabeling methods are highly chemoselective and have conveniently been automated using commercially available radiosynthesis equipment so that the procedures described can be employed for the synthesis of peptide-based PET probes for in vivo imaging studies according to as low as reasonably achievable (ALARA) principles.

Mechanistic Study of the Copper(II)-Mediated Site-Selective O-Arylation of Glycosides with Arylboronic Acids

Glycosides having multiple free OH groups have been shown to undergo site-selective O-arylations in the presence of arylboronic acids and copper(II) acetate. Herein, a mechanistic analysis of these Chan-Evans-Lam-type couplings is presented based on reaction kinetics, mass spectrometric analysis of reaction mixtures, and substituent effect studies. The results establish that the formation of a substrate-derived boronic ester accelerates the rate-determining transmetalation step. Intramolecular transfer of the aryl group from the boronic ester is ruled out in favor of a pathway in which the key pre-transmetalation assembly is generated from a boronic ester, a copper complex, and a second equivalent of arylboronic acid.

11C, 12C and 13C-Cyanation of Electron-Rich Arenes via Organic Photoredox Catalysis

As a non-invasive imaging technology, positron emission tomography (PET) plays a crucial role in personalized medicine, including early diagnosis, patient screening, and treatment monitoring. The advancement of PET research depends on the discovery of new PET agents, which requires the development of simple and efficient radiolabeling methods in many cases. As bioisosteres for halogen and carbonyl moieties, nitriles are important functional groups in pharmaceutical and agrochemical compounds. Here, we disclose a mild organophotoredox-catalyzed method for efficient cyanation of a broad spectrum of electron-rich arenes, including abundant and readily available veratroles and pyrogallol trimethyl ethers. Notably, the transformations not only are compatible with various affordable 12C and 13C-cyanide sources, but also could be applied to carbon-11 synthons to incorporate [11C]nitriles into arenes. The aryl [11C]nitriles can be further derivatized to [11C]carboxylic acids, [11C]amides, and [11C]alkyl amines. The newly developed reaction can serve as a powerful tool for generating new PET agents.

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.

An Organometallic Gold(III) Reagent for 18F Labeling of Unprotected Peptides and Sugars in Aqueous Media

The ¹⁸F labeling of unprotected peptides and sugars with a Au(III)-[¹⁸F]fluoroaryl complex is reported. The chemoselective method generates ¹⁸F-labeled S-aryl bioconjugates in an aqueous environment in 15 min with high radiochemical yields and displays excellent functional group tolerance. This approach utilizes an air and moisture stable, robust organometallic Au(III) complex and highlights the versatility of designer organometallic reagents as efficient agents for rapid radiolabeling.

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [¹¹C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [¹¹C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

Synthesis of carbon-11 radiolabelled transition metal complexes using 11C-dithiocarbamates

A novel radiolabelling method exploiting ¹¹C-dithiocarbamate ligands has been used to generate ¹¹C-labelled Au(I), Au(III), Pd(II) and Pt(II) complexes in high radiochemical yields (71-99%). Labelled complexes were prepared in a rapid one-pot procedure via the substitution reaction of ¹¹C-dithiocarbamate ligands with appropriate transition metal chloride precursors.

Synthesis and Characterization of 5-(2-Fluoro-4-[11C]methoxyphenyl)-2,2-dimethyl-3,4-dihydro-2H-pyrano[2,3-b]pyridine-7-carboxamide as a PET Imaging Ligand for Metabotropic Glutamate Receptor 2

Metabotropic glutamate receptor 2 (mGluR2) is a therapeutic target for several neuropsychiatric disorders. An mGluR2 function in etiology could be unveiled by positron emission tomography (PET). In this regard, 5-(2-fluoro-4-[11C]methoxyphenyl)-2,2-dimethyl-3,4-dihydro-2H-pyrano[2,3-b]pyridine-7-carboxamide ([11C]13, [11C]mG2N001), a potent negative allosteric modulator (NAM), was developed to support this endeavor. [11C]13 was synthesized via the O-[11C]methylation of phenol 24 with a high molar activity of 212 ± 76 GBq/μmol (n = 5) and excellent radiochemical purity (>99%). PET imaging of [11C]13 in rats demonstrated its superior brain heterogeneity and reduced accumulation with pretreatment of mGluR2 NAMs, VU6001966 (9) and MNI-137 (26), the extent of which revealed a time-dependent drug effect of the blocking agents. In a nonhuman primate, [11C]13 selectively accumulated in mGluR2-rich regions and resulted in high-contrast brain images. Therefore, [11C]13 is a potential candidate for translational PET imaging of the mGluR2 function.

Palladium oxidative addition complex-enabled synthesis of amino-substituted indolyl-4(3H)-quinazolinones and their antitumor activity evaluation

The indolyl-4(3H)-quinazolinone core is an important structural motif in functional molecules. However, few methods exist for its direct modification, which limits its potential application. Reported herein is a palladium-mediated amination of halogen-containing indolyl-4(3H)-quinazolinones with a variety of primary and secondary amines via the corresponding palladium oxidative addition complexes. The protocol allows the facile synthesis of indolyl-4(3H)-quinazolinone derivatives with amino groups at all the positions of the benzene ring in moderate to good yields with mild reaction conditions and good functional group tolerance. Furthermore, the antitumor activity of these products was evaluated.

Radiolabeling with [11C]HCN for Positron emission tomography

Hydrogen cyanide (HCN) is a versatile synthon for generating carbon‑carbon and carbon-heteroatom bonds. Unlike other one-carbon synthons (i.e., CO, CO2), HCN can function as a nucleophile (as in potassium cyanide, KCN) and an electrophile (as in cyanogen bromide, (CN)Br). The incorporation of the CN motif into organic molecules generates nitriles, hydantoins and (thio)cyanates, which can be converted to carboxylic acids, aldehydes, amides and amines. Such versatile chemistry is particularly attractive in PET radiochemistry where diverse bioactive small molecules incorporating carbon-11 in different positions need to be produced. The first examples of making [11C]HCN for radiolabeling date back to the 1960s. During the ensuing decades, [11C]cyanide labeling was popular for producing biologically important molecules including 11C-labeled α-amino acids, sugars and neurotransmitters. [11C]cyanation is now reemerging in many PET centers due to its versatility for making novel tracers. Here, we summarize the chemistry of [11C]HCN, review the methods to make [11C]HCN past and present, describe methods for labeling different types of molecules with [11C]HCN, and provide an overview of the reactions available to convert nitriles into other functional groups. Finally, we discuss some of the challenges and opportunities in [11C]HCN labeling such as developing more robust methods to produce [11C]HCN and developing rapid and selective methods to convert nitriles into other functional groups in complex molecules.

A Ligand Exchange Process for the Diversification of Palladium Oxidative Addition Complexes

Palladium oxidative addition complexes (OACs) have recently emerged as useful tools to enable challenging bond connections. However, each OAC can only be formed with one dative ligand at a time. As no one ligand is optimal for every cross-coupling reaction, we herein disclose a ligand exchange protocol for the preparation of a series of OACs bearing a variety of ancillary ligands from one common complex. These complexes were further applied to cross-coupling transformations.

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.

Tyrosine-Reactive Cross-Linker for Probing Protein Three-Dimensional Structures

Cross-linking mass spectrometry (XL-MS) has made significant progress in understanding the structure of protein and elucidating architectures of larger protein complexes. Current XL-MS applications are limited to targeting lysine, glutamic acid, aspartic acid, and cysteine residues. There remains a need for the development of novel cross-linkers enabling selective targeting of other amino acid residues in proteins. Here, a novel simple cross-linker, namely, [4,4'-(disulfanediylbis(ethane-2,1-diyl)) bis(1,2,4-triazolidine-3,5-dione)] (DBB), has been designed, synthesized, and characterized. This cross-linker can react selectively with tyrosine residues in protein through the electrochemical click reaction. The DBB cross-links produced the characteristic peptides before and after electrochemical reduction, thus permitting the simplified data analysis and accurate identification for the cross-linked products. This is the first time a cross-linker is developed for targeting tyrosine residues on protein without using photoirradiation or a metal catalyst. This strategy might potentially be used as a complementary tool for XL-MS to probe protein 3D structures, protein complexes, and protein-protein interaction.

Amphiphilic Biaryl Monophosphine Ligands by Regioselective Sulfonation

Amphiphilic ligands are valued for their ability to facilitate organometallic reactions in the presence of water. The regioselective sulfonation of a series of commercially available biaryl monophosphines to generate amphiphilic ligands is presented. In this one-step protocol, the temperature and addition of fuming sulfuric acid were carefully controlled to arrive at sulfonated biaryl monophosphine ligands in high yields with >95% regioselectivity without the need for chromatographic purification.

Synthesis and [*C]CO-labelling of (C,N) gem-dimethylbenzylamine-palladium complexes for potential applications in positron emission tomography

Various aryl-palladium complexes were synthesised from gem-dimethylbenzylamine derivatives by C-H activation under extremely mild conditions. Interestingly, these highly stable structures reacted with [13C]carbon monoxide to produce the desired labelled lactams in 29% to 51% yields over the C-H activation/carbonylation steps. As representative examples, a non-natural amino acid and an estradiol-based conjugate were prepared and labelled in model experiments with [13C]CO in homogeneous or heterogeneous conditions. Especially, the latter was radiolabelled with [11C]CO using a convenient procedure from the resin-supported palladium complex precursor. Thus, these results strongly suggest that cyclometallated palladium complexes obtained from gem-dimethylbenzylamine moieties are promising precursors for the practical synthesis of new [11C]tracers for Positron Emission Tomography.

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.

Protein-Protein Cross-Coupling via Palladium-Protein Oxidative Addition Complexes from Cysteine Residues

Few chemical methods exist for the covalent conjugation of two proteins. We report the preparation of site-specific protein-protein conjugates that arise from the sequential cross-coupling of cysteine residues on two different proteins. The method involves the synthesis of stable palladium-protein oxidative addition complexes (Pd-protein OACs), a process that converts nucleophilic cysteine residues into an electrophilic S-aryl-Pd-X unit by taking advantage of an intramolecular oxidative addition strategy. This process is demonstrated on proteins up to 83 kDa in size and can be conveniently carried out in water and open to air. The resulting Pd-protein OACs can cross-couple with other thiol-containing proteins to arrive at homogeneous protein-protein bioconjugates.

18F-Trifluoromethanesulfinate Enables Direct C-H 18F-Trifluoromethylation of Native Aromatic Residues in Peptides

18F labeling strategies for unmodified peptides with [18F]fluoride require 18F-labeled prosthetics for bioconjugation more often with cysteine thiols or lysine amines. Here we explore selective radical chemistry to target aromatic residues applying C-H 18F-trifluoromethylation. We report a one-step route to [18F]CF3SO2NH4 from [18F]fluoride and its application to direct [18F]CF3 incorporation at tryptophan or tyrosine residues using unmodified peptides as complex as recombinant human insulin. The fully automated radiosynthesis of octreotide[Trp(2-CF218F)] enables in vivo positron emission tomography imaging.

Ex situ gas generation for lab scale organic synthesis

This review discusses recent examples of ex situ generated gaseous reagents, and their use in organic synthesis.

Electrochemical oxidation induced selective tyrosine bioconjugation for the modification of biomolecules

Directly introducing a beneficial functional group into biomolecules under mild, clean and easy-to-handle conditions is of great importance in the field of chemical biology and pharmacology. Herein, we described an electrochemical strategy to perform the bioconjugation of tyrosine residues with phenothiazine derivatives in a rapid and simple manner. In this electrochemical system, various polypeptides and proteins were successfully labelled with excellent site- and chemo-selectivity, and metals, oxidants or additives were also avoided.

Carbon Isotope Labeling Strategy for β-Amino Acid Derivatives via Carbonylation of Azanickellacycles

A series of 4-membered azametallacycles have been prepared by the oxidative addition of Ni(0) with aziridines. Stoichiometric ¹³C-labeled carbon monoxide could be efficiently incorporated via Ni-C bond insertion to generate air stable and isolable cyclic Ni-acyl complexes. Upon subjection to a range of C-, N-, O-, and S-nucleophiles, ¹³C-labeled β-amino acids and derivatives thereof, as well as β-aminoketones, could be rapidly accessed. The methodology proved highly adaptable for the synthesis of the antidiabetic drug, sitagliptin, with a single carbon isotope label.

HCN on Tap: On-Demand Continuous Production of Anhydrous HCN for Organic Synthesis

A continuous process for the on-demand generation, separation, and reaction of hydrogen cyanide (HCN) using membrane separation technology was developed. The inner tube of the reactor is manufactured from a gas-permeable, hydrophobic fluoropolymer (Teflon AF-2400) membrane. HCN is formed from aqueous reagents within the inner tube and then diffuses through the membrane into an outer tubing containing organic solvent. This technique enabled the safe handling of HCN for three different organic transformations without the need for distillation.

Mechanochemistry allows carrying out sensitive organometallic reactions in air: glove-box-and-Schlenk-line-free synthesis of oxidative addition complexes from aryl halides and palladium(0)

Organic reactions that employ moisture- and/or oxygen-sensitive reagents or intermediates usually involve the use of glove-box or Schlenk-line techniques as well as dry and degassed solvents. Unfortunately, these requirements may greatly reduce the utility of the targeted organic molecules. Herein, we demonstrate that solvent-free mechanochemical synthetic techniques allow using highly oxygen-sensitive palladium(0) species in air for the stoichiometric oxidative addition of aryl halides. The low diffusion efficiency of gaseous oxygen in crystalline or amorphous solid-state reaction mixtures should be the main reason for the low impact of the presence of atmospheric oxygen on the sensitive oxidative addition reactions under the applied conditions. This study thus illustrates the outstanding potential of mechanochemistry to serve as an operationally simple, glove-box-and-Schlenk-line-free synthetic route to organometallic compounds and other valuable synthetic targets, even when sensitive reagents or intermediates are involved.

Arylation Chemistry for Bioconjugation

Bioconjugation chemistry has been used to prepare modified biomolecules with functions beyond what nature intended. Central to these techniques is the development of highly efficient and selective bioconjugation reactions that operate under mild, biomolecule compatible conditions. Methods that form a nucleophile-sp2 carbon bond show promise for creating bioconjugates with new modifications, sometimes resulting in molecules with unparalleled functions. Here we outline and review sulfur, nitrogen, selenium, oxygen, and carbon arylative bioconjugation strategies and their applications to modify peptides, proteins, sugars, and nucleic acids.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

Late-Stage Diversification through Manganese-Catalyzed C-H Activation: Access to Acyclic, Hybrid, and Stapled Peptides

Bioorthogonal C-H allylation with ample scope was accomplished through a versatile manganese(I)-catalyzed C-H activation for the late-stage diversification of structurally complex peptides. The unique robustness of the manganese(I) catalysis manifold was reflected by full tolerance of sensitive functional groups, such as iodides, esters, amides, and OH-free hydroxy groups, thereby setting the stage for the racemization-free synthesis of C-H fused peptide hybrids featuring steroids, drug molecules, natural products, nucleobases, and saccharides.

Rapid nickel(ii)-promoted cysteine S-arylation with arylboronic acids

Nickel salts catalyze fast cysteine arylation with 2-nitroarylboronic acids. The process uses cheap, readily-available reagents and allows introduction of diverse chemical handles.

Bioconjugated arylpalladium complexes on solid supports for a convenient last-step synthesis of 11C-labelled tracers for positron emission tomography

Bioconjugated arylpalladium complexes anchored onto polystyrene beads provided [¹¹C]CO-labelled compounds with excellent radiochemical purities after a simple filtration.

Site-Specific Deoxyfluorination of Small Peptides with [18 F]Fluoride

Radiolabeled receptor-binding peptides are an important class of positron emission tomography tracers owing to achievable high binding affinities and their rapid blood clearance. Herein, a method to introduce a 4-[¹⁸ F]fluoro-phenylalanine residue into peptide sequences is reported, by chemoselective radio-deoxyfluorination of a tyrosine residue using a traceless activating group. The replacement of only one hydrogen atom with [¹⁸ F]fluoride results in minimal structural perturbation of the peptide, which is desirable in the labeling of tracer candidates.

Site-Selective, Late-Stage C-H 18 F-Fluorination on Unprotected Peptides for Positron Emission Tomography Imaging

Peptides are often ideal ligands for diagnostic molecular imaging due to their ease of synthesis and tuneable targeting properties. However, labelling unmodified peptides with ¹⁸ F for positron emission tomography (PET) imaging presents a number of challenges. Here we show the combination of photoactivated sodium decatungstate and [¹⁸ F]-N-fluorobenzenesulfonimide effects site-selective ¹⁸ F-fluorination at the branched position in leucine residues in unprotected and unaltered peptides. This streamlined process provides a means to directly convert native peptides into PET imaging agents under mild aqueous conditions, enabling rapid discovery and development of peptide-based molecular imaging tools.

Late-Stage Isotopic Carbon Labeling of Pharmaceutically Relevant Cyclic Ureas Directly from CO2

A robust, click-chemistry-inspired procedure for radiolabeling of cyclic ureas was developed. This protocol, suitable for all carbon isotopes (11 C, 13 C, 14 C), is based on the direct functionalization of carbon dioxide: the universal building block for carbon radiolabeling. The strategy is operationally simple and reproducible in different radiochemistry centers, exhibits remarkably wide substrate scope with short reaction times, and demonstrates superior reactivity as compared to previously reported systems. With this procedure, a variety of pharmaceuticals and an unprotected peptide were labeled with high radiochemical efficiency.

Aryl transition metal chemical warheads for protein bioconjugation

The past seven years have witnessed the burgeoning of protein bioconjugation reactions highlighting aryl transition metal reagents as coupling partners. This new bioorthogonal organometallic chemistry, which sets the scene for stoichiometric processes in place of the catalytic procedures that developed in parallel, already enabled the forging of C-S and C-C bonds onto protein substrates, respectively in their native state or equipped with pre-installed non-natural terminal alkene or alkyne appendages. Although not yet applied to proteins, related transformations pointing to the creation of C-N bonds have, in addition, just been disclosed by targeting peptide lysine residues. Central to this research was the selection of ligands attached to the transition metal, in order to confer to metal complexes, not only their stability in aqueous medium, but also the desired chemoselectivity. We summarize here this body of work, which has already put in the limelight elaborated palladium and gold complexes equipped with biologically relevant appendages, such as fluorescent and affinity tags, as well as drug molecules. This research holds much promise, not only for the study of proteins themselves, but also for the design of new protein-based biotherapeutics, such as protein-drug conjugates or constrained analogs resulting from macrocyclisation reactions.

Palladium Oxidative Addition Complexes for Peptide and Protein Cross-linking

A new method for cysteine-lysine cross-linking in peptides and proteins using palladium oxidative addition complexes is presented. First, a biarylphosphine-supported palladium reagent is used to transfer an aryl group bearing an O-phenyl carbamate substituent to a cysteine residue. Next, this carbamate undergoes chemoselective acyl substitution by a proximal lysine to form a cross-link. The linkage so formed is stable toward acid, base, oxygen, and external thiol nucleophiles. This method was applied to cross-link cysteine with nearby lysines in sortase A*. Furthermore, we used this method for the intermolecular cross-linking between a peptide and a protein based on the p53-MDM2 interaction. These studies demonstrate the potential for palladium-mediated methods to serve as a platform for the development of future cross-linking techniques for peptides and proteins with natural amino acid residues.

18F-Trifluoromethylation of Unmodified Peptides with 5-18F-(Trifluoromethyl)dibenzothiophenium Trifluoromethanesulfonate

The 18F-labeling of 5-(trifluoromethyl)-dibenzothiophenium trifluoromethanesulfonate, commonly referred to as the Umemoto reagent, has been accomplished applying a halogen exchange 18F-fluorination with 18F-fluoride, followed by oxidative cyclization with Oxone and trifluoromethanesulfonic anhydride. This new 18F-reagent allows for the direct chemoselective 18F-labeling of unmodified peptides at the thiol cysteine residue.

Palladium(II)-Mediated C-H Tritiation of Complex Pharmaceuticals

Tritium-labeled molecules are critical tools for elucidating the binding and metabolic properties of bioactive compounds, particularly during pharmaceutical discovery. Direct tritiation of inert C-H bonds with T₂ gas is an ideal approach for tritium labeling, but significant gaps remain for direct tritiation of structurally complex molecules with diverse functional groups. Here we report the first application of palladium(II) C-H activation chemistry for tritiation with T₂ gas. This practical transformation exhibits novel substrate scope and greater functional group tolerance compared to previous state of the art tritiation methods, and has been applied to directly tritiate 9 complex pharmaceuticals and an unprotected dipeptide. The isolated tritium-labeled products exhibit >15 Ci mmol^-1 specific activity, exceeding the typical requirements for application in studies of molecular interaction and metabolism.

Selective modification of natural nucleophilic residues in peptides and proteins using arylpalladium complexes

The present review outlines the recent methodologies for selective arylation of natural nucleophilic residues within unprotected peptides and proteins promoted by arylpalladium complexes, which demonstrate the advantages and potential of organometallic palladium complexes in bioconjugation.