Selective Engineering of Linkage-Specific α2,6-N-Linked Sialoproteins Using Sydnone-Modified Sialic Acid Bioorthogonal Reporters

Non-natural sialic acid derivatives with o-nitrobenzyl alcohol substituents for light-mediated protein conjugation and cell imaging

We have synthesized two sialic acid derivatives substituted with an ortho-nitrobenzyl alcohol (o-NBA) group that can undergo light-mediated conjugation with primary amines at their 5- or 9-carbon position. The o-NBA derivatives were shown to react with multiple lysine residues of human serum albumin (HSA) when exposed to 365 nm light irradiation within 10 min. The resulting sugar conjugates were characterized by mass spectroscopy and used for fluorescence-based cell imaging.

Sydnone-based prosthetic groups for radioiodination

The potential of Strained-Promoted Sydnone-Alkyne Cycloaddition (SPSAC) for radioiodination was evaluated with model cyclooctyne-conjugated peptides. Starting with a series of sydnones with varying N3 and C4 substitution, a preliminary kinetic study with non-radioactive iodinated compounds highlighted the superiority of an arylsydnone substituted by a chlorine atom in C4 position. Interestingly, reaction rate up to 11 times higher than using an azide was achieved with the best system. Access to 125I-labelled sydnones was granted with high efficiency from arylboronic acid precursors by copper catalyzed nucleophilic substitution. Application of SPSAC on the model peptide in radiotracer conditions showed the same trend than in non-radioactive kinetic study and complete reactions could be achieved within less than an hour for the best systems. These results are favorable for use in the production of radiopharmaceuticals with heavy halogens and increase the diversity of available bioorthogonal reaction for nuclear imaging and therapy.

A Kinetic Trapping Approach for Facile Access to 3FaxNeu5Ac and a Photo-Cross-Linkable Sialyltransferase Probe

Sialic acid (Neu5Ac) is installed onto glycoconjugates by sialyltransferases (STs) using cytidine monophosphate-Neu5Ac (CMP-β-d-Neu5Ac) as their donor. The only class of cell-active ST inhibitors are those based on a 3FaxNeu5Ac scaffold, which is metabolically converted into CMP-3FaxNeu5Ac within cells. It is essential for the fluorine to be axial, yet stereoselective installation of fluorine in this specific orientation is challenging. Sialic acid aldolase can convert 3-fluoropyruvate and 2-acetamido-2-deoxy-d-mannopyranose (ManNAc) to 3FNeu5Ac, but stereocontrol of the fluorine in the product has not been possible. We hypothesized that the 3Fax kinetic product of a sialic acid aldolase reaction could be trapped by coupling with CMP-sialic acid synthetase to yield CMP-3FaxNeu5Ac. Here, we report that highly active CMP-sialic acid synthetase and short reaction times produce exclusively CMP-3FaxNeu5Ac. Removal of CMP from CMP-3FaxNeu5Ac under acidic conditions unexpectedly led to 3-fluoro-β-d-Neu5Ac 2-phosphate (3FaxNeu5Ac-2P). Alkaline phosphatase successfully converted 3FaxNeu5Ac-2P to 3FaxNeu5Ac, enabling stereochemically controlled access to 3FaxNeu5Ac, which is effective in lowering the sialoglycan ligands for Siglecs on cells. Moreover, our kinetic trapping approach could be used to access CMP-3FaxNeu5Ac with modifications at the C5, C9, or both positions, which enabled the chemoenzymatic synthesis of a photo-cross-linkable version of CMP-3FaxNeu5Ac that selectively photo-cross-linked to ST6GAL1 over two other STs.

One-Step Selective Labeling of Native Cell Surface Sialoglycans by Exogenous α2,8-Sialylation

Exo-enzymatic glycan labeling strategies have emerged as versatile tools for efficient and selective installation of terminal glyco-motifs onto live cell surfaces. Through employing specific enzymes and nucleotide-sugar probes, cells can be equipped with defined glyco-epitopes for modulating cell function or selective visualization and enrichment of glycoconjugates. Here, we identifyCampylobacter jejunisialyltransferase Cst-II I53S as a tool for cell surface glycan modification, expanding the exo-enzymatic labeling toolkit to include installation of α2,8-disialyl epitopes. Labeling with Cst-II was achieved with biotin- and azide-tagged CMP-Neu5Ac derivatives on a model glycoprotein and native sialylated cell surface glycans across a panel of cell lines. The introduction of modified Neu5Ac derivatives onto cells by Cst-II was also retained on the surface for 6 h. By examining the specificity of Cst-II on cell surfaces, it was revealed that the α2,8-sialyltransferase primarily labeled N-glycans, with O-glycans labeled to a lesser extent, and there was an apparent preference for α2,3-linked sialosides on cells. This approach thus broadens the scope of tools for selective exo-enzymatic labeling of native sialylated glycans and is highly amenable for the construction of cell-based arrays.

Facile Synthesis of Clickable Unnatural Sugars in the Unprotected and 1,6-Di-O-Acylated Forms for Metabolic Glycan Labeling

Clickable unnatural sugars have been widely used in studying glycosylation in living systems via the metabolic glycan labelling (MGL) strategy. Partial protection of unnatural sugars by 1,6-di-O-acylation increases the labelling efficiency while avoiding the non-specific S-glyco-modification. Herein, we report the facile synthesis of a series of clickable unnatural sugars in both the unprotected and 1,6-di-O-acylated forms at the ten-gram scale. By evaluation of the labelling specificity, efficiency, and biocompatibility of various 1,6-di-O-acylated sugars for MGL in cell lines and living mice, we demonstrate that 1,6-di-O-propionylated unnatural sugars are optimal chemical reporters for glycan labelling. The synthetic routes developed in this work should facilitate the widespread use of MGL with no artificial S-glyco-modification for investigating the functional roles of glycans.

Investigation of mitochondrial targeting ability of sydnones and sydnonimines and mitochondria-targeted delivery of celecoxib

Mitochondria are considered to be a promising target in cancer diagnosis and therapeutics. Recently, sydnone and sydnonimine, as mesoionic bioorthogonal reagents, have been used in cell labeling and drug delivery. Here we investigated the mitochondrial targeting ability of sydnones and sydnonimines for the first time. Experimental results show that sydnone and sydnonimine themselves have high mitochondrial distribution. However, the introduction of a phenyl group into the C₄ position of sydnone dramatically decreases the mitochondrial affinity. In addition, we took advantage of mitochondrial targeting ability and click-and-release reaction of sydnonimine to evaluate anticancer activities of in-mitochondria delivery of celecoxib against HeLa and HepG2 cells, indicating that celecoxib-induced cancer cell death may not involve mitochondria-related pathway.

Recombinant mucin biotechnology and engineering

Mucins represent a largely untapped class of polymeric building block for biomaterials, therapeutics, and other biotechnology. Because the mucin polymer backbone is genetically encoded, sequence-specific mucins with defined physical and biochemical properties can be fabricated using recombinant technologies. The pendent O-glycans of mucins are increasingly implicated in immunomodulation, suppression of pathogen virulence, and other biochemical activities. Recent advances in engineered cell production systems are enabling the scalable synthesis of recombinant mucins with precisely tuned glycan side chains, offering exciting possibilities to tune the biological functionality of mucin-based products. New metabolic and chemoenzymatic strategies enable further tuning and functionalization of mucin O-glycans, opening new possibilities to expand the chemical diversity and functionality of mucin building blocks. In this review, we discuss these advances, and the opportunities for engineered mucins in biomedical applications ranging from in vitro models to therapeutics.

Precise assembly of inside-out cell membrane camouflaged nanoparticles via bioorthogonal reactions for improving drug leads capturing

Cell membrane camouflaged nanoparticles have been widely used in the field of drug leads discovery attribute to their unique biointerface targeting function. However, random orientation of cell membrane coating does not guarantee effective and appropriate binding of drugs to specific sites, especially when applied to intracellular regions of transmembrane proteins. Bioorthogonal reactions have been rapidly developed as a specific and reliable method for cell membrane functionalization without disturbing living biosystem. Herein, inside-out cell membrane camouflaged magnetic nanoparticles (IOCMMNPs) were accurately constructed via bioorthogonal reactions to screen small molecule inhibitors targeting intracellular tyrosine kinase domain of vascular endothelial growth factor recptor-2. Azide functionalized cell membrane acted as a platform for specific covalently coupling with alkynyl functionalized magnetic Fe₃O₄ nanoparticles to prepare IOCMMNPs. The inside-out orientation of cell membrane was successfully verified by immunogold staining and sialic acid quantification assay. Ultimately, two compounds, senkyunolide A and ligustilidel, were successfully captured, and their potential antiproliferative activities were further testified by pharmacological experiments. It is anticipated that the proposed inside-out cell membrane coating strategy endows tremendous versatility for engineering cell membrane camouflaged nanoparticles and promotes the development of drug leads discovery platforms.

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

Attenuation of Polysialic Acid Biosynthesis in Cells by the Small Molecule Inhibitor 8-Keto-sialic acid

Sialic acids are key mediators of cell function, particularly with regard to cellular interactions with the surrounding environment. Reagents that modulate the display of specific sialyl glycoforms at the cell surface would be useful biochemical tools and potentially allow for therapeutic intervention in numerous challenging chronic diseases. While multiple strategies are being explored for the control of cell surface sialosides, none that shows high selectivity between sialyltransferases or that targets a specific sialyl glycoform has yet to emerge. Here, we describe a strategy to block the formation of α2,8-linked sialic acid chains (oligo- and polysialic acid) through the use of 8-keto-sialic acid as a chain-terminating metabolic inhibitor that, if incorporated, cannot be elongated. 8-Keto-sialic acid is nontoxic at effective concentrations and serves to block polysialic acid synthesis in cancer cell lines and primary immune cells, with minimal effects on other sialyl glycoforms.

Switching azide and alkyne tags on bioorthogonal reporters in metabolic labeling of sialylatedglycoconjugates: a comparative study

Sialylation of cell surface glycans plays an essential role in cell-cell interaction and communication of cells with their microenvironment. Among the tools that have been developed for the study of sialylation in living cells, metabolic oligosaccharide engineering (MOE) exploits the biosynthetic pathway of sialic acid (Sia) to incorporate unnatural monosaccharides into nascent sialylatedglycoconjugates, followed by their detection by a bioorthogonal ligation of a molecular probe. Among bioorthogonal reactions, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is the only ligation where both reactive tags can be switched on the chemical reporter or on the probe, making this reaction very flexible and adaptable to various labeling strategies. Azide- and alkyne-modified ManNAc and Sia reporters have been widely used, but per-O-acetylated ManNAz (Ac₄ManNAz) remains the most popular choice so far for tracking intracellular processing of sialoglycans and cell surface sialylation in various cells. Taking advantage of CuAAC, we compared the metabolic incorporation of ManNAl, ManNAz, SiaNAl, SiaNAz and Ac₄ManNAz in the human colon cell lines CCD841CoN, HT29 and HCT116, and in the two gold standard cell lines, HEK293 and HeLa. Using complementary approaches, we showed marked differences in the efficiency of labeling of sialoglycoproteins between the different chemical reporters in a given cell line, and that switching the azide and alkyne bioorthogonal tags on the analogs highly impacted their metabolic incorporation in the human colon cell lines. Our results also indicated that ManNAz was the most promiscuous metabolized reporter to study sialylation in these cells.

Pathogen-sugar interactions revealed by universal saturation transfer analysis

Many pathogens exploit host cell-surface glycans. However, precise analyses of glycan ligands binding with heavily modified pathogen proteins can be confounded by overlapping sugar signals and/or compounded with known experimental constraints. Universal saturation transfer analysis (uSTA) builds on existing nuclear magnetic resonance spectroscopy to provide an automated workflow for quantitating protein-ligand interactions. uSTA reveals that early-pandemic, B-origin-lineage severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike trimer binds sialoside sugars in an "end-on" manner. uSTA-guided modeling and a high-resolution cryo-electron microscopy structure implicate the spike N-terminal domain (NTD) and confirm end-on binding. This finding rationalizes the effect of NTD mutations that abolish sugar binding in SARS-CoV-2 variants of concern. Together with genetic variance analyses in early pandemic patient cohorts, this binding implicates a sialylated polylactosamine motif found on tetraantennary N-linked glycoproteins deep in the human lung as potentially relevant to virulence and/or zoonosis.

A Clickable Bioorthogonal Sydnone-Aglycone for the Facile Preparation of a Core 1 O-Glycan-Array

Protein-O-glycosylation has been shown to be essential for many biological processes. However, determining the exact relationship between O-glycan structures and their biological activity remains challenging. Here we report that, unlike azides, sydnones can be incorporated as an aglycon into core 1 O-glycans early-on in their synthesis since it is compatible with carbohydrate chemistry and enzymatic glycosylations, allowing us to generate a small library of sydnone-containing core 1 O-glycans by chemoenzymatic synthesis. The sydnone-aglycon was then employed for the facile preparation of an O-glycan array, via bioorthogonal strain-promoted sydnone-alkyne cycloaddition click reaction, and in turn was utilized for the high-throughput screening of O-glycan-lectin interactions. This sydnone-aglycon, particularly adapted for O-glycomics, is a valuable chemical tool that complements the limited technologies available for investigating O-glycan structure-activity relationships.

Fast and Efficient Postsynthetic DNA Labeling in Cells by Means of Strain-Promoted Sydnone-Alkyne Cycloadditions

Sydnones are highly stable mesoionic 1,3-dipoles that react with cyclooctynes through strain-promoted sydnone-alkyne cycloaddition (SPSAC). Although sydnones have been shown to be valuable bioorthogonal chemical reporters for the labeling of proteins and complex glycans, nucleic acids have not yet been tagged by SPSAC. Evaluation of SPSAC kinetics with model substrates showed fast reactions with cyclooctyne probes (up to k=0.59 M-1  s-1 ), and two different sydnones were effectively incorporated into both 2'-deoxyuridines at position 5, and 7-deaza-2'-deoxyadenosines at position 7. These modified nucleosides were synthetically incorporated into single-stranded DNAs, which were successfully postsynthetically labeled with cyclooctyne probes both in vitro and in cells. These results show that sydnones are versatile bioorthogonal tags and have the premise to become essential tools for tracking DNA and potentially RNA in living cells.

Impacting Bacterial Sialidase Activity by Incorporating Bioorthogonal Chemical Reporters onto Mammalian Cell-Surface Sialosides

Bioorthogonal chemical reporters, in synergy with click chemistry, have emerged as a key technology for tagging complex glycans in living cells. This strategy relies on the fact that bioorthogonal chemical reporters are highly reactive species while being biologically noninvasive. Here, we report that chemical reporters and especially sydnones may have, on the contrary, enormous impact on biomolecule processing enzymes. More specifically, we show that editing cell-surface sialic acid-containing glycans (sialosides) with bioorthogonal chemical reporters can significantly affect the activity of bacterial sialidases, enzymes expressed by bacteria during pathogenesis for cleaving sialic acid sugars from mammalian cell-surface glycans. Upon screening various chemical reporters, as well as their position on the sialic acid residue, we identified that pathogenic bacterial sialidases were unable to cleave sialosides displaying a sydnone at the 5-position of sialic acids in vitro as well as in living cells. This study highlights the importance of investigating more systematically the metabolic fate of glycoconjugates modified with bioorthogonal reporters.

Dynamic tracing of sugar metabolism reveals the mechanisms of action of synthetic sugar analogs

Synthetic sugar analogs are widely applied in metabolic oligosaccharide engineering (MOE) and as novel drugs to interfere with glycoconjugate biosynthesis. However, mechanistic insights on their exact cellular metabolism over time are mostly lacking. We combined ion-pair ultrahigh performance liquid chromatography–triple quadrupole mass spectrometry mass spectrometry using tributyl- and triethylamine buffers for sensitive analysis of sugar metabolites in cells and organisms and identified low abundant nucleotide sugars, such as UDP-arabinose in human cell lines and CMP-sialic acid (CMP-NeuNAc) in Drosophila. Furthermore, MOE revealed that propargyloxycarbonyl (Poc)-labeled ManNPoc was metabolized to both CMP-NeuNPoc and UDP-GlcNPoc. Finally, time-course analysis of the effect of antitumor compound 3F_ax-NeuNAc by incubation of B16-F10 melanoma cells with N-acetyl-D-[UL-¹³C6]glucosamine revealed full depletion of endogenous ManNAc 6-phosphate and CMP-NeuNAc within 24 h. Thus, dynamic tracing of sugar metabolic pathways provides a general approach to reveal time-dependent insights into the metabolism of synthetic sugars, which is important for the rational design of analogs with optimized effects.

Cellular and Molecular Engineering of Glycan Sialylation in Heterologous Systems

Glycans have been shown to play a key role in many biological processes, such as signal transduction, immunogenicity, and disease progression. Among the various glycosylation modifications found on cell surfaces and in biomolecules, sialylation is especially important, because sialic acids are typically found at the terminus of glycans and have unique negatively charged moieties associated with cellular and molecular interactions. Sialic acids are also crucial for glycosylated biopharmaceutics, where they promote stability and activity. In this regard, heterogenous sialylation may produce variability in efficacy and limit therapeutic applications. Homogenous sialylation may be achieved through cellular and molecular engineering, both of which have gained traction in recent years. In this paper, we describe the engineering of intracellular glycosylation pathways through targeted disruption and the introduction of carbohydrate active enzyme genes. The focus of this review is on sialic acid-related genes and efforts to achieve homogenous, humanlike sialylation in model hosts. We also discuss the molecular engineering of sialyltransferases and their application in chemoenzymatic sialylation and sialic acid visualization on cell surfaces. The integration of these complementary engineering strategies will be useful for glycoscience to explore the biological significance of sialic acids on cell surfaces as well as the future development of advanced biopharmaceuticals.

Sydnone Methides-A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N-Heterocyclic Carbenes

Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π-electron rich anionic N-heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C-C coupling reactions with 1-fluoro-4-iodobenzene under Pd(PPh3 )4 and CuBr catalysis. 77 Se NMR resonance frequencies and 1 JC4-Se as well as 1 JC4-H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N-heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13 C NMR spectroscopy at -40 °C which is extremely shifted upfield in comparison to classical N-heterocyclic carbenes.

Glycan Labeling and Analysis in Cells and In Vivo

As one of the major types of biomacromolecules in the cell, glycans play essential functional roles in various biological processes. Compared with proteins and nucleic acids, the analysis of glycans in situ has been more challenging. Herein we review recent advances in the development of methods and strategies for labeling, imaging, and profiling of glycans in cells and in vivo. Cellular glycans can be labeled by affinity-based probes, including lectin and antibody conjugates, direct chemical modification, metabolic glycan labeling, and chemoenzymatic labeling. These methods have been applied to label glycans with fluorophores, which enables the visualization and tracking of glycans in cells, tissues, and living organisms. Alternatively, labeling glycans with affinity tags has enabled the enrichment of glycoproteins for glycoproteomic profiling. Built on the glycan labeling methods, strategies enabling cell-selective and tissue-specific glycan labeling and protein-specific glycan imaging have been developed. With these methods and strategies, researchers are now better poised than ever to dissect the biological function of glycans in physiological or pathological contexts.

Toward Intracellular Bioconjugation Using Transition-Metal-Free Techniques

Bioconjugation is the chemical strategy of covalent modification of biomolecules, using either an external reagent or other biomolecules. Since its inception in the twentieth century, the technique has grown by leaps and bounds, and has a variety of applications in chemical biology. However, it is yet to reach its full potential in the study of biochemical processes in live cells, mainly because the bioconjugation strategies conflict with cellular processes. This has mostly been overcome by using transition metal catalysts, but the presence of metal centers limit them to in vitro use, or to the cell surface. These hurdles can potentially be circumvented by using metal-free strategies. However, the very modifications that are necessary to make such metal-free reactions proceed effectively may impact their biocompatibility. This is because biological processes are easily perturbed and greatly depend on the prevailing inter- and intracellular environment. With this taken into consideration, this review analyzes the applicability of the transition-metal-free strategies reported in this decade to the study of biochemical processes in vivo.

Click and Bio-Orthogonal Reactions with Mesoionic Compounds

Click and bio-orthogonal reactions are dominated by cycloaddition reactions in general and 1,3-dipolar cycloadditions in particular. Among the dipoles routinely used for click chemistry, azides, nitrones, isonitriles, and nitrile oxides are the most popular. This review is focused on the emerging click chemistry that uses mesoionic compounds as dipole partners. Mesoionics are a very old family of molecules, but their use as reactants for click and bio-orthogonal chemistry is quite recent. The facility to derivatize these dipoles and to tune their reactivity toward cycloaddition reactions makes mesoionics an attractive opportunity for future click chemistry development. In addition, some compounds from this family are able to undergo click-and-release reactions, finding interesting applications in cells, as well as in animals. This review covers the synthetic access to main mesoionics, their reaction with dipolarophiles, and recent applications in chemical biology and heterocycle synthesis.

Chemical reporters to study mammalian O-glycosylation

Glycans play essential roles in a range of cellular processes and have been shown to contribute to various pathologies. The diversity and dynamic nature of glycan structures and the complexities of glycan biosynthetic pathways make it challenging to study the roles of specific glycans in normal cellular function and disease. Chemical reporters have emerged as powerful tools to characterise glycan structures and monitor dynamic changes in glycan levels in a native context. A variety of tags can be introduced onto specific monosaccharides via the chemical modification of endogenous glycan structures or by metabolic or enzymatic incorporation of unnatural monosaccharides into cellular glycans. These chemical reporter strategies offer unique opportunities to study and manipulate glycan functions in living cells or whole organisms. In this review, we discuss recent advances in metabolic oligosaccharide engineering and chemoenzymatic glycan labelling, focusing on their application to the study of mammalian O-linked glycans. We describe current barriers to achieving glycan labelling specificity and highlight innovations that have started to pave the way to overcome these challenges.

Glycoengineering: scratching the surface

At the surface of many cells is a compendium of glycoconjugates that form an interface between the cell and its surroundings; the glycocalyx. The glycocalyx serves several functions that have captivated the interest of many groups. Given its privileged residence, this meshwork of sugar-rich biomolecules is poised to transmit signals across the cellular membrane, facilitating communication with the extracellular matrix and mediating important signalling cascades. As a product of the glycan biosynthetic machinery, the glycocalyx can serve as a partial mirror that reports on the cell's glycosylation status. The glycocalyx can also serve as an information-rich barrier, withholding the entry of pathogens into the underlying plasma membrane through glycan-rich molecular messages. In this review, we provide an overview of the different approaches devised to engineer glycans at the cell surface, highlighting considerations of each, as well as illuminating the grand challenges that face the next era of ‘glyco-engineers’. While we have learned much from these techniques, it is evident that much is left to be unearthed.

Sydnone Ribosides as a Platform for the Synthesis of Pyrazole C-Nucleosides: A Unified Synthesis of Formycin B and Pyrazofurin

The C-nucleoside natural products formycin B and pyrazofurin were synthesized in seven steps employing a sydnone riboside as common intermediate. Sydnone ribosides were synthesized via a direct Lewis acid catalyzed dehydrative glycosylation reaction. We demonstrated that these can be used for the diversity-oriented synthesis of pyrazole C-nucleoside analogues via thermal 1,3-dipolar cycloaddition reactions with various alkynes, giving access to the pyrazole C-nucleoside natural products, as well as opening new avenues for exploring nucleoside chemical space.

Bioorthogonal Ligations and Cleavages in Chemical Biology

Bioorthogonal reactions including the bioorthogonal ligations and cleavages have become an active field of research in chemical biology, and they play important roles in chemical modification and functional regulation of biomolecules. This review summarizes the developments and applications of the representative bioorthogonal reactions including the Staudinger reactions, the metal-mediated bioorthogonal reactions, the strain-promoted cycloadditions, the inverse electron demand Diels-Alder reactions, the light-triggered bioorthogonal reactions, and the reactions of chloroquinoxalines and ortho-dithiophenols.

Target-Amplified Drug Delivery of Polymer Micelles Bearing Staudinger Ligation

Bioorthogonal chemistry together with biomarker-installing techniques is very promising in the amplification of the tumor targeting efficiency of nanomedicine. In this work, we newly synthesized an amphiphilic block copolymer polyoxazoline-block-polycaprolactone (POX-PCL) in which a certain number of amino groups were dangled in the side chain of the POX block and then functionalized into triarylphosphine groups for active tumor targeting via Staudinger ligation. By using the block copolymer self-assembly, the Staudinger ligation reagent-containing and drug-loaded reactive micelles were prepared with a hydrodynamic diameter of ∼74 nm. Such drug-loaded reactive POX-PCL micelles exhibited significant tumor target ability through the Staudinger ligation between the micelles and the tumors metabolically labeled with azide group, as demonstrated by a series of in vitro and in vivo evaluations. In this work, a novel method was proposed for the application of Staudinger ligation in the nanomedicine for amplifying the tumor targeting ability and antitumor activity of nanodrugs.

Photo-accelerated “click” reaction between diarylsydnones and ring-strained alkynes for bioorthogonal ligation

A novel photo-click ligation reaction between diarylsydnones and ring-strained alkynes, exhibiting decent bioorthogonality, was established under 405 nm light irradiation.

Synthesis and evaluation of photo-activatable β-diarylsydnone-l-alanines for fluorogenic photo-click cyclization of peptides

β-Diarylsydnone-l-alanines were designed and introduced into peptides allowing photo-cyclization only in phosphate containing buffer with concomitant fluorescence generation in live cells.