Diazo compounds for the bioreversible esterification of proteins

Generation of a selective senolytic platform using a micelle-encapsulated Sudan Black B conjugated analog

The emerging field of senolytics is centered on eliminating senescent cells to block their contribution to the progression of age-related diseases, including cancer, and to facilitate healthy aging. Enhancing the selectivity of senolytic treatments toward senescent cells stands to reduce the adverse effects associated with existing senolytic interventions. Taking advantage of lipofuscin accumulation in senescent cells, we describe here the development of a highly efficient senolytic platform consisting of a lipofuscin-binding domain scaffold, which can be conjugated with a senolytic drug via an ester bond. As a proof of concept, we present the generation of GL392, a senolytic compound that carries a dasatinib senolytic moiety. Encapsulation of the GL392 compound in a micelle nanocarrier (termed mGL392) allows for both in vitro and in vivo (in mice) selective elimination of senescent cells via targeted release of the senolytic agent with minimal systemic toxicity. Our findings suggest that this platform could be used to enhance targeting of senotherapeutics toward senescent cells.

Target Bioconjugation of Protein Through Chemical, Molecular Dynamics, and Artificial Intelligence Approaches

Covalent modification of proteins at specific, predetermined sites is essential for advancing biological and biopharmaceutical applications. Site-selective labeling techniques for protein modification allow us to effectively track biological function, intracellular dynamics, and localization. Despite numerous reports on modifying target proteins with functional chemical probes, unique organic reactions that achieve site-selective integration without compromising native functional properties remain a significant challenge. In this review, we delve into site-selective protein modification using synthetic probes, highlighting both chemical and computational methodologies for chemo- and regioselective modifications of naturally occurring amino acids, as well as proximity-driven protein-selective chemical modifications. We also underline recent traceless affinity labeling strategies that involve exchange/cleavage reactions and catalyst tethering modifications. The rapid development of computational infrastructure and methods has made the bioconjugation of proteins more accessible, enabling precise predictions of structural changes due to protein modifications. Hence, we discuss bioconjugational computational approaches, including molecular dynamics and artificial intelligence, underscoring their potential applications in enhancing our understanding of cellular biology and addressing current challenges in the field.

Oxadiazolines as Photoreleasable Labels for Drug Target Identification

Photoaffinity labeling is a widely used technique for studying ligand-protein and protein-protein interactions. Traditional photoaffinity labels utilize nonspecific C-H bond insertion reactions mediated by a highly reactive intermediate. Despite being the most widely used photoaffinity labels, diazirines exhibit limited compatibility with downstream organic reactions and suffer from storage stability concerns. This study introduces oxadiazolines as innovative and complementary photoactivatable labels for addition to the toolbox and demonstrates their application in vitro and through in cellulo labeling experiments. Oxadiazolines can be easily synthesized from ketone moieties and cleaved with 302-330 nm light to cleanly liberate a diazo reactive moiety that can covalently modify nucleophilic amino acid residues. Notably, oxadiazolines are compatible with various organic reaction conditions and functional groups, allowing for the exploration of a large chemical space. Several known inhibitors featuring the oxadiazoline functionality were prepared without affecting their binding affinity. Furthermore, we confirmed the ability of oxadiazolines to form covalent bonds with proteins upon UV-irradiation, both in vitro and in cellulo, yielding comparable results to those of the matched diazirine compounds.

Chemoselective Reagents for the Traceless Bioreversible Modification of Native Proteins

Nature utilizes bioreversible post-translational modifications (PTMs) to spatiotemporally diversify protein function. Mimicking Nature's approach, chemists have developed a variety of chemoselective regents for traceless, bioreversible modification of native proteins. These strategies have found utility in the development of reversible covalent inhibitors and degraders as well as the synthesis of functional protein conjugates for delivery into cells. This Viewpoint provides a snapshot of such tools, which currently cover Cys, Ser, Thr, Lys, Asp, and Glu residues and the N terminus. Additionally, we explore how bioreversible reagents, originally developed by research communities with differing objectives, can be utilized synergistically. Looking forward, we discuss the need for developing bioreversible reagents for labeling His, Tyr, Arg, Trp, Asn, Gln, and Met residues and the C-terminus as well as the installation of dynamic PTMs. Finally, to broaden the applicability of these tools, we point out the importance of developing modular release scaffolds with tunable release times and responsiveness to multiple endogenous triggers. We anticipate that this Viewpoint will catalyze further research and technological breakthroughs in this rapidly evolving field.

Strain-release alkylation of Asp12 enables mutant selective targeting of K-Ras-G12D

K-Ras is the most commonly mutated oncogene in human cancer. The recently approved non-small cell lung cancer drugs sotorasib and adagrasib covalently capture an acquired cysteine in K-Ras-G12C mutation and lock it in a signaling-incompetent state. However, covalent inhibition of G12D, the most frequent K-Ras mutation particularly prevalent in pancreatic ductal adenocarcinoma, has remained elusive due to the lack of aspartate-targeting chemistry. Here we present a set of malolactone-based electrophiles that exploit ring strain to crosslink K-Ras-G12D at the mutant aspartate to form stable covalent complexes. Structural insights from X-ray crystallography and exploitation of the stereoelectronic requirements for attack of the electrophile allowed development of a substituted malolactone that resisted attack by aqueous buffer but rapidly crosslinked with the aspartate-12 of K-Ras in both GDP and GTP state. The GTP-state targeting allowed effective suppression of downstream signaling, and selective inhibition of K-Ras-G12D-driven cancer cell proliferation in vitro and xenograft growth in mice.

Mechanistic differences between linear vs. spirocyclic dialkyldiazirine probes for photoaffinity labeling

Dialkyldiazirines have emerged as a photo-reactive group of choice for interactome mapping in live cell experiments. Upon irradiation, 'linear' dialkyldiazirines produce dialkylcarbenes which are susceptible to both intramolecular reactions and unimolecular elimination processes, as well as diazoalkanes, which also participate in intermolecular labeling. Cyclobutylidene has a nonclassical bonding structure and is stable enough to be captured in bimolecular reactions. Cyclobutanediazirines have more recently been studied as photoaffinity probes based on cyclobutylidene, but the mechanism, especially with respect to the role of putative diazo intermediates, was not fully understood. Here, we show that photolysis (365 nm) of cyclobutanediazirines can produce cyclobutylidene intermediates as evidenced by formation of their expected bimolecular and unimolecular products, including methylenecyclopropane derivatives. Unlike linear diazirines, cyclobutanediazirine photolysis in the presence of tetramethylethylene produces a [2 + 1] cycloaddition adduct. By contrast, linear diazirines produce diazo compounds upon low temperature photolysis in THF, whereas diazo compounds are not detected in similar photolyses of cyclobutanediazirines. Diazocyclobutane, prepared by independent synthesis, is labile, reactive toward water and capable of protein alkylation. The rate of diazocyclobutane decomposition is not affected by 365 nm light, suggesting that the photochemical conversion of diazocyclobutane to cyclobutylidene is not an important pathway. Finally, chemical proteomic studies revealed that a likely consequence of this primary conversion to a highly reactive carbene is a marked decrease in labeling by cyclobutanediazirine-based probes relative to linear diazirine counterparts both at the individual protein and proteome-wide levels. Collectively, these observations are consistent with a mechanistic picture for cyclobutanediazirine photolysis that involves carbene chemistry with minimal formation of diazo intermediates, and contrasts with the photolyses of linear diazirines where alkylation by diazo intermediates plays a more significant role.

Mutually Orthogonal Bioorthogonal Reactions: Selective Chemistries for Labeling Multiple Biomolecules Simultaneously

Bioorthogonal click chemistry has played a transformative role in many research fields, including chemistry, biology, and medicine. Click reactions are crucial to produce increasingly complex bioconjugates, to visualize and manipulate biomolecules in living systems and for various applications in bioengineering and drug delivery. As biological (model) systems grow more complex, researchers have an increasing need for using multiple orthogonal click reactions simultaneously. In this review, we will introduce the most common bioorthogonal reactions and discuss their orthogonal use on the basis of their mechanism and electronic or steric tuning. We provide an overview of strategies to create reaction orthogonality and show recent examples of mutual orthogonal chemistry used for simultaneous biomolecule labeling. We end by discussing some considerations for the type of chemistry needed for labeling biomolecules in a system of choice.

Derivatization of carboxylic groups prior to their LC analysis - A review

Carboxylic acids (CAs) represent a large group of important molecules participating in various biologically significant processes. Analytical study of these compounds is typically performed by liquid chromatography (LC) combined with various types of detection. However, their analysis is often accompanied by a wide variety of problems depending on used separation system or detection method. The dominant ones are: i) poor chromatographic behavior of the CAs in reversed-phase LC; ii) absence of a chromophore (or fluorophore); iii) weak ionization in mass spectrometry (MS). To overcome these problems, targeted chemical modification, and derivatization, come into play. Therefore, derivatization still plays an important and, in many cases, irreplaceable role in sample preparation, and new derivatization methods of CAs are constantly being developed. The most commonly used type of reaction for CAs derivatization is amidation. In recent years, an increased interest in the isotopic labeling derivatization method has been observed. In this review, we comprehensively summarize the possibilities and actual trends in the derivatization of CAs that have been published over the past decade.

α-Silylated Diazoalkynes: New Tools for Bioconjugation of Proteins

α-Silylated diazoalkynes are stabilized diazo compounds that can selectively react with carboxylic residues in buffered aqueous media. In-situ fluoride induced desilylation increases this reactivity, leading to a very fast reaction. Application to the selective functionalization of RNase A, followed by post-functionalization using click chemistry, is described. These new reagents expand the toolbox for native protein modification at carboxylic residues.

Rapid and Bifunctional Chemoselective Metabolome Analysis of Liver Disease Plasma Using the Reagent 4-Nitrophenyl-2H-azirine

Primary sclerosing cholangitis (PSC) is a chronic inflammatory disease of the bile ducts that has been associated with diverse metabolic carboxylic acids. Mass spectrometric techniques are the method of choice for their analysis. However, the broad investigation of this metabolite class remains challenging. Derivatization of carboxylic acids represents a strategy to overcome these limitations but available methods suffer from diverse analytical challenges. Herein, we have designed a novel strategy introducing 4-nitrophenyl-2H-azirine as a new chemoselective moiety for the first time for carboxylic acid metabolites. This moiety was selected as it rapidly forms a stable amide bond and also generates a new ketone, which can be analyzed by our recently developed quant-SCHEMA method specific for carbonyl metabolites. Optimization of this new method revealed a high reproducibility and robustness, which was utilized to validate 102 metabolic carboxylic acids using authentic synthetic standard conjugates in human plasma samples including nine metabolites that were newly detected. Using this sequential analysis of the carbonyl- and carboxylic acid-metabolomes revealed alterations of the ketogenesis pathway, which demonstrates the vast benefit of our unique methodology. We anticipate that the developed azirine moiety with rapid functional group transformation will find broad application in diverse chemical biology research fields.

Amphos-Mediated Conversion of Alkyl Azides to Diazo Compounds and One-Pot Azide-Site Selective Transient Protection, Click Conjugation, and Deprotective Transformation

A one-pot conversion of alkyl azides to diazo compounds is outlined. After the reaction of α-azidocarbonyl compounds with Amphos, treatment of the resulting phosphazides with silica gel in a wet solvent afforded α-diazo carbonyl products. Through the azido group protection property of Amphos, inter- and intramolecular azide-site selective reactions of azido group protection, click functionalization, and deprotection of the diazo group have been demonstrated in one pot.

Pinpointing Acidic Residues in Proteins

It is of great importance to pinpoint specific residues or sites of a protein in biological contexts to enable desired mechanism of action for small molecules or to precisely control protein function. In this regard, acidic residues including aspartic acid (Asp) and glutamic acid (Glu) hold great potential due to their great prevalence and unique function. To unlock the largely untapped potential, great efforts have been made recently by synthetic chemists, chemical biologists and pharmacologists. Herein, we would like to highlight the remarkable progress and particularly introduce the electrophiles that exhibit reactivity to carboxylic acids, the light-induced reactivities to carboxylic acids and the genetically encoded noncanonical amino acids that allow protein manipulations at acidic residues. We also comment on certain unresolved challenges, hoping to draw more attention to this rapidly developing area.

Photocatalytic Activation of Aryl(trifluoromethyl) Diazos to Carbenes for High-Resolution Protein Labeling with Red Light

State-of-the-art methods in photoproximity labeling center on the targeted generation and capture of short-lived reactive intermediates to provide a snapshot of local protein environments. Diazirines are the current gold standard for high-resolution proximity labeling, generating short-lived aryl(trifluoromethyl) carbenes. Here, we present a method to access aryl(trifluoromethyl) carbenes from a stable diazo source via tissue-penetrable, deep red to near-infrared light (600-800 nm). The operative mechanism of this activation involves Dexter energy transfer from photoexcited osmium(II) photocatalysts to the diazo, thus revealing an aryl(trifluoromethyl) carbene. The labeling preferences of the diazo probe with amino acids are studied, showing high reactivity toward heteroatom-H bonds. Upon the synthesis of a biotinylated diazo probe, labeling studies are conducted on native proteins as well as proteins conjugated to the Os photocatalyst. Finally, we demonstrate that the conjugation of a protein inhibitor to the photocatalyst also enables selective protein labeling in the presence of spectator proteins and achieves specific labeling of a membrane protein on the surface of mammalian cells via a two-antibody photocatalytic system.

Chemical technology principles for selective bioconjugation of proteins and antibodies

Proteins are multifunctional large organic compounds that constitute an essential component of a living system. Hence, control over their bioconjugation impacts science at the chemistry-biology-medicine interface. A chemical toolbox for their precision engineering can boost healthcare and open a gateway for directed or precision therapeutics. Such a chemical toolbox remained elusive for a long time due to the complexity presented by the large pool of functional groups. The precise single-site modification of a protein requires a method to address a combination of selectivity attributes. This review focuses on guiding principles that can segregate them to simplify the task for a chemical method. Such a disintegration systematically employs a multi-step chemical transformation to deconvolute the selectivity challenges. It constitutes a disintegrate (DIN) theory that offers additional control parameters for tuning precision in protein bioconjugation. This review outlines the selectivity hurdles faced by chemical methods. It elaborates on the developments in the perspective of DIN theory to demonstrate simultaneous regulation of reactivity, chemoselectivity, site-selectivity, modularity, residue specificity, and protein specificity. It discusses the progress of such methods to construct protein and antibody conjugates for biologics, including antibody-fluorophore and antibody-drug conjugates (AFCs and ADCs). It also briefs how this knowledge can assist in developing small molecule-based covalent inhibitors. In the process, it highlights an opportunity for hypothesis-driven routes to accelerate discoveries of selective methods and establish new targetome in the precision engineering of proteins and antibodies.

Small-molecule probes from bench to bedside: advancing molecular analysis of drug-target interactions toward precision medicine

Over the past decade, remarkable advances have been witnessed in the development of small-molecule probes. These molecular tools have been widely applied for interrogating proteins, pathways and drug-target interactions in preclinical research. While novel structures and designs are commonly explored in probe development, the clinical translation of small-molecule probes remains limited, primarily due to safety and regulatory considerations. Recent synergistic developments - interfacing novel chemical probes with complementary analytical technologies - have introduced and expedited diverse biomedical opportunities to molecularly characterize targeted drug interactions directly in the human body or through accessible clinical specimens (e.g., blood and ascites fluid). These integrated developments thus offer unprecedented opportunities for drug development, disease diagnostics and treatment monitoring. In this review, we discuss recent advances in the structure and design of small-molecule probes with novel functionalities and the integrated development with imaging, proteomics and other emerging technologies. We further highlight recent applications of integrated small-molecule technologies for the molecular analysis of drug-target interactions, including translational applications and emerging opportunities for whole-body imaging, tissue-based measurement and blood-based analysis.

R, Kumar M, Molla R, V. B. U, Rai V. istegrate (DIN) Theory Enabling Precision Engineering of Proteins

The chemical toolbox for the selective modification of proteins has witnessed immense interest in the past few years. The rapid growth of biologics and the need for precision therapeutics have fuelled this growth further. However, the broad spectrum of selectivity parameters creates a roadblock to the field's growth. Additionally, bond formation and dissociation are significantly redefined during the translation from small molecules to proteins. Understanding these principles and developing theories to deconvolute the multidimensional attributes could accelerate the area. This outlook presents a disintegrate (DIN) theory for systematically disintegrating the selectivity challenges through reversible chemical reactions. An irreversible step concludes the reaction sequence to render an integrated solution for precise protein bioconjugation. In this perspective, we highlight the key advancements, unsolved challenges, and potential opportunities.

Reactive fragments targeting carboxylate residues employing direct to biology, high-throughput chemistry

We present a carboxylate-targeting reactive fragment screening platform using 2-aryl-5-carboxytetrazole (ACT) as the photoreactive functionality. This work will provide a simple accessible method to rapidly discover tool molecules to interrogate important biological targets.

O-Methylation of carboxylic acids with streptozotocin

The clinically used DNA-alkylating drug streptozotocin (STZ) was investigated using a simple work-up as an O-methylating agent to transform various carboxylic acids, sulfonic acids and phosphorous acids into corresponding methyl esters, and did so with yields of up to 97% in 4 h at room temperature. Good substrate tolerance was observed, and benefited from the mild conditions and compatibility of the reaction with water.

Unlocking novel reaction pathways of diazoalkanes with visible light

Photochemistry has recently attracted the interest of synthetic chemists to conduct photolysis reactions of diazoalkanes. In this feature article, we provide a concise overview on this field, starting with discoveries...

Recent advances in pyrazole synthesis employing diazo compounds and synthetic analogues

This review summarizes the recent developments in the construction of pyrazoles using diazo compounds, nitrile imines and their synthetic equivalents.

Protein Modifications: From Chemoselective Probes to Novel Biocatalysts

Chemical reactions can be performed to covalently modify specific residues in proteins. When applied to native enzymes, these chemical modifications can greatly expand the available set of building blocks for the development of biocatalysts. Nucleophilic canonical amino acid sidechains are the most readily accessible targets for such endeavors. A rich history of attempts to design enhanced or novel enzymes, from various protein scaffolds, has paved the way for a rapidly developing field with growing scientific, industrial, and biomedical applications. A major challenge is to devise reactions that are compatible with native proteins and can selectively modify specific residues. Cysteine, lysine, N-terminus, and carboxylate residues comprise the most widespread naturally occurring targets for enzyme modifications. In this review, chemical methods for selective modification of enzymes will be discussed, alongside with examples of reported applications. We aim to highlight the potential of such strategies to enhance enzyme function and create novel semisynthetic biocatalysts, as well as provide a perspective in a fast-evolving topic.

Photoaffinity labelling strategies for mapping the small molecule-protein interactome

Nearly all FDA approved drugs and bioactive small molecules exert their effects by binding to and modulating proteins. Consequently, understanding how small molecules interact with proteins at an molecular level is a central challenge of modern chemical biology and drug development. Complementary to structure-guided approaches, chemoproteomics has emerged as a method capable of high-throughput identification of proteins covalently bound by small molecules. To profile noncovalent interactions, established chemoproteomic workflows typically incorporate photoreactive moieties into small molecule probes, which enable trapping of small molecule-protein interactions (SMPIs). This strategy, termed photoaffinity labelling (PAL), has been utilized to profile an array of small molecule interactions, including for drugs, lipids, metabolites, and cofactors. Herein we describe the discovery of photocrosslinking chemistries, including a comparison of the strengths and limitations of implementation of each chemotype in chemoproteomic workflows. In addition, we highlight key examples where photoaffinity labelling has enabled target deconvolution and interaction site mapping.

Self-assembly of protein-polymer conjugates for drug delivery

Protein-polymer conjugates are a class of molecules that combine the stability of polymers with the diversity, specificity, and functionality of biomolecules. These bioconjugates can result in hybrid materials that display properties not found in their individual components and can be particularly relevant for drug delivery applications. Engineering amphiphilicity into these bioconjugate materials can lead to phase separation and the assembly of high-order structures. The assembly, termed self-assembly, of these hierarchical structures entails multiple levels of organization: at each level, new properties emerge, which are, in turn, influenced by lower levels. Here, we provide a critical review of protein-polymer conjugate self-assembly and how these materials can be used for therapeutic applications and drug delivery. In addition, we discuss central bioconjugate design questions and propose future perspectives for the field of protein-polymer conjugate self-assembly.

Novel N-transfer reagent for converting α-amino acid derivatives to α-diazo compounds

A novel universal N-transfer reagent for direct and effective transformation of α-amino ketones, acetamides, and esters to the corresponding α-diazo products under mild basic conditions has been developed. This one-step synthetic approach not only allows for generation of α-substituted-α-diazo carbonyl compounds from α-amino acid derivatives but also permits preparation of α-diazo dipeptides from N-terminal dipeptides (32 examples, up to 91%).

Probing the Mechanism of Photoaffinity Labeling by Dialkyldiazirines through Bioorthogonal Capture of Diazoalkanes

Dialkyldiazirines have emerged as reagents of choice for biological photoaffinity labeling studies. The mechanism of crosslinking has dramatic consequences for biological applications where instantaneous labeling is desirable, as carbene insertions display different chemoselectivity and are much faster than competing mechanisms involving diazo or ylide intermediates. Here, deuterium labeling and diazo compound trapping experiments are employed to demonstrate that both carbene and diazo mechanisms operate in the reactions of a dialkyldiazirine motif that is commonly utilized for biological applications. For the fraction of intermolecular labeling that does involve a carbene mechanism, direct insertion is not necessarily involved, as products derived from a carbonyl ylide are also observed. We demonstrate that a strained cycloalkyne can intercept diazo compound intermediates and serve as a bioorthogonal probe for studying the contribution of the diazonium mechanism of photoaffinity labeling on a model protein under aqueous conditions.

A Visible and Near-Infrared Light Activatable Diazocoumarin Probe for Fluorogenic Protein Labeling in Living Cells

Chemical modification of proteins in living cells permits valuable glimpses into the molecular interactions that underpin dynamic cellular events. While genetic engineering methods are often preferred, selective labeling of endogenous proteins in a complex intracellular milieu with chemical approaches represents a significant challenge. In this study, we report novel diazocoumarin compounds that can be photoactivated by visible (430-490 nm) and near-infrared light (800 nm) irradiation to photo-uncage reactive carbene intermediates, which could subsequently undergo an insertion reaction with concomitant fluorescence "turned on". With these new molecules in hand, we have developed a new approach for rapid, selective, and fluorogenic labeling of endogenous protein in living cells. By using CA-II and eDHFR as model proteins, we demonstrated that subcellular localization of proteins can be precisely visualized by live-cell imaging and protein levels can be reliably quantified in multiple cell types using flow cytometry. Dynamic protein regulations such as hypoxia-induced CA-IX accumulation can also be detected. In addition, by two-photon excitation with an 800 nm laser, cell-selective labeling can also be achieved with spatially controlled irradiation. Our method circumvents the cytotoxicity of UV light and obviates the need for introducing external reporters with "click chemistries". We believe that this approach of fluorescence labeling of endogenous protein by bioorthogonal photoirradiation opens up exciting opportunities for discoveries and mechanistic interrogation in chemical biology.

2H-Azirine-Based Reagents for Chemoselective Bioconjugation at Carboxyl Residues Inside Live Cells

Protein modification by chemical reagents has played an essential role in the treatment of human diseases. However, the reagents currently used are limited to the covalent modification of cysteine and lysine residues. It is thus desirable to develop novel methods that can covalently modify other residues. Despite the fact that the carboxyl residues are crucial for maintaining the protein function, few selective labeling reactions are currently available. Here, we describe a novel reactive probe, 3-phenyl-2H-azirine, that enables chemoselective modification of carboxyl groups in proteins under both in vitro and in situ conditions with excellent efficiency. Furthermore, proteome-wide profiling of reactive carboxyl residues was performed with a quantitative chemoproteomic platform.

Chemical methods for modification of proteins

The field of protein bioconjugation draws attention from stakeholders in chemistry, biology, and medicine. This review provides an overview of the present status, challenges, and opportunities for organic chemists.

Covalently modified carboxyl side chains on cell surface leads to a novel method toward topology analysis of transmembrane proteins

The research on transmembrane proteins (TMPs) is quite widespread due to their biological importance. Unfortunately, only a little amount of structural data is available of TMPs. Since technical difficulties arise during their high-resolution structure determination, bioinformatics and other experimental approaches are widely used to characterize their low-resolution structure, namely topology. Experimental and computational methods alone are still limited to determine TMP topology, but their combination becomes significant for the production of reliable structural data. By applying amino acid specific membrane-impermeable labelling agents, it is possible to identify the accessible surface of TMPs. Depending on the residue-specific modifications, new extracellular topology data is gathered, allowing the identification of more extracellular segments for TMPs. A new method has been developed for the experimental analysis of TMPs: covalent modification of the carboxyl groups on the accessible cell surface, followed by the isolation and digestion of these proteins. The labelled peptide fragments and their exact modification sites are identified by nanoLC-MS/MS. The determined peptides are mapped to the primary sequences of TMPs and the labelled sites are utilised as extracellular constraints in topology predictions that contribute to the refined low-resolution structure data of these proteins.

Bioinspired Thiophosphorodichloridate Reagents for Chemoselective Histidine Bioconjugation

Site-selective bioconjugation to native protein residues is a powerful tool for protein functionalization, with cysteine and lysine side chains being the most common points for attachment owing to their high nucleophilicity. We now report a strategy for histidine modification using thiophosphorodichloridate reagents that mimic post-translational histidine phosphorylation, enabling fast and selective labeling of protein histidines under mild conditions where various payloads can be introduced via copper-assisted alkyne-azide cycloaddition (CuAAC) chemistry. We establish that these reagents are particularly effective at covalent modification of His-tags, which are common motifs to facilitate protein purification, as illustrated by selective attachment of polyarginine cargoes to enhance the uptake of proteins into living cells. This work provides a starting point for probing and enhancing protein function using histidine-directed chemistry.

Unlocking a Diazirine Long-Lived Nuclear Singlet State via Photochemistry: NMR Detection and Lifetime of an Unstabilized Diazo-Compound

Diazirines are important for photoaffinity labeling, and their photoisomerization is relatively well-known. This work shows how hyperpolarized NMR spectroscopy can be used to characterize an unstable diazo-compound formed via photoisomerization of a ¹⁵N₂-labeled silyl-ether-substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen. The active hyperpolarization catalyst is characterized to provide insight into the mechanism. The photochemical isomerization of the diazirine into the diazo-analogue allows the NMR invisible nuclear singlet state of the parent compound to be probed. The identity of the diazo-species is confirmed by trapping with N-phenyl maleimide via a cycloaddition reaction to afford bicyclic pyrazolines that also show singlet state character. The presence of singlet states in the diazirine and the diazo-compound is validated by comparison of experimental nutation behavior with theoretical simulation. The magnetic state lifetime of the diazo-compound is determined as 12 ± 1 s in CD₃OD solution at room temperature, whereas its chemical lifetime is measured as 100 ± 5 s by related hyperpolarized NMR studies. Indirect evidence for the generation of the photoproduct para-N₂ is presented.

Blue light-promoted photolysis of aryldiazoacetates

Aryldiazoacetates can undergo photolysis under blue light irradiation (460-490 nm) at room temperature and under air in the presence of numerous trapping agents, such as styrene, carboxylic acids, amines, alkanes and arenes, thus providing a straighforward and general platform for their mild functionalization.