Cu-free click cycloaddition reactions in chemical biology

The Amazing Transglycosylation Activity of Endo-β-N-acetylglucosaminidases

Major advances have been made in exploring the transglycosylation activity of endo-β-N-acetylglucosaminidases (ENGases) for synthetic purpose. The exploration of synthetic sugar oxazolines as donor substrates for the ENGase-catalyzed transglycosylation has expanded the substrate availability and significantly enhanced the overall transglycosylation efficiency. On the other hand, site-directed mutagenesis in combination with activity screening has led to the discovery of the first generation ENGase-based glycosynthases that can use highly active sugar oxazolines as substrates for transglycosylation but lack hydrolytic activity on the ground-state products. ENGases have shown amazing flexibility in transglycosylation and possess much broader substrate specificity than previously thought. Now the ENGase-based chemoenzymatic method has been extended to the synthesis of a range of complex carbohydrates, including homogeneous glycopeptides, glycoproteins carrying well-defined glycans, novel oligosaccharide clusters, unusually glycosylated natural products, and even polysaccharides. This article highlights recent advances related to ENGase-catalyzed transglycosylation with a focus on their synthetic potential.

Chemical synthesis and functionalization of clickable glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositol (GPI) anchorage is a common posttranslational modification of eukaryotic proteins. Chemical synthesis of structurally defined GPIs and GPI derivatives is a necessary step toward understanding the properties and functions of these molecules in biological systems. In this work, the synthesis of several functionalized GPI anchors was accomplished using the para-methoxybenzyl (PMB) group for permanent hydroxyl protection, which allowed the incorporation of functionalities that are incompatible with permanent protecting groups traditionally used in carbohydrate synthesis. A flexible convergent-divergent assembly strategy enabled efficient access to a diverse set of target structures, including "clickable" Alkynyl-GPIs 1 and 2 and Azido-GPI 3. For global deprotection, a one-pot reaction was employed to afford the target GPIs in excellent yields (85-97%). Fully deprotected clickable GPIs 2 and 3 were readily conjugated to imaging and affinity probes via Cu(I)-catalyzed and Cu-free strain-promoted [3+2] cycloaddition, respectively, resulting in GPI-Fluor 4 and GPI-Biotin 5.

Development and evaluation of new cyclooctynes for cell surface glycan imaging in cancer cells

Two reagents have been synthesized for selective labeling of cell surface azidoglycans, an unusually stable version of a dibenzocyclooctyne (TMDIBO) and a third-generation difluorinated cyclooctyne (DIFO3). Both syntheses are efficient with minimal purification, and the dibenzocyclooctyne is stable under basic and acidic conditions. Flow cytometric measurements with azidosugar labeled cancer cells, in which these reagents were linked to the fluorophore Alexa Fluor 647, gave a signal-to-background ratio of up to 35 with TMDIBO as compared to ≈10 for DIFO3 and ≈5 for a phosphine reagent. TMDIBO-based probes should have applications in molecular imaging of cell surface glycans in vivo.

Metal-free sequential [3 + 2]-dipolar cycloadditions using cyclooctynes and 1,3-dipoles of different reactivity

Although metal-free cycloadditions of cyclooctynes and azides to give stable 1,2,3-triazoles have found wide utility in chemical biology and material sciences, there is an urgent need for faster and more versatile bioorthogonal reactions. We have found that nitrile oxides and diazocarbonyl derivatives undergo facile 1,3-dipolar cycloadditions with cyclooctynes. Cycloadditions with diazocarbonyl derivatives exhibited similar kinetics as compared to azides, whereas the reaction rates of cycloadditions with nitrile oxides were much faster. Nitrile oxides could conveniently be prepared by direct oxidation of the corresponding oximes with BAIB, and these conditions made it possible to perform oxime formation, oxidation, and cycloaddition as a one-pot procedure. The methodology was employed to functionalize the anomeric center of carbohydrates with various tags. Furthermore, oximes and azides provide an orthogonal pair of functional groups for sequential metal-free click reactions, and this feature makes it possible to multifunctionalize biomolecules and materials by a simple synthetic procedure that does not require toxic metal catalysts.

Solid phase strain promoted "click" modification of DNA via [3+2]-nitrile oxide-cyclooctyne cycloadditions

Rapid, catalyst free, solid phase modification of DNA by strain promoted cyclooctyne-nitrile oxide click chemistry is reported; the reaction is characterised by mild conditions, occurring in an aqueous environment under atmospheric conditions at room temperature and is complete in 10 minutes.

Convergent assembly and surface modification of multifunctional dendrimers by three consecutive click reactions

Multifunctional dendrimers bearing two or more surface functionalities have the promise to provide smart drug delivery devices that can for example combine tissue targeting and imaging or be directed more precisely to a specific tissue or cell type. We have developed a concise synthetic methodology for efficient dendrimer assembly and heterobifunctionalization based on three sequential azide-alkyne cycloadditions. The methodology is compatible with biologically important compounds rich in chemical functionalities such as peptides, carbohydrates, and fluorescent tags. In the approach, a strain-promoted azide-alkyne cycloaddition (SPAAC) between polyester dendrons modified at the focal point with an azido and 4-dibenzocyclooctynol (DIBO) moiety provided dendrimers bearing terminal and TMS-protected (TMS=trimethylsilyl) alkynes at the periphery. The terminal alkynes were outfitted with azido-modified polyethylene glycol (PEG) chains or galactosyl residues by using Cu(I) -catalyzed azide-alkyne cycloadditions (CuAAC). Next, a one-pot TMS deprotection and second click reaction of the resulting terminal alkyne with azido-containing compounds gave multifunctional dendrimers bearing complex biologically active moieties at the periphery.

Biocompatible copper(I) catalysts for in vivo imaging of glycans

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is the standard method for bioorthogonal conjugation. However, current Cu(I) catalyst formulations are toxic, hindering their use in living systems. Here we report that BTTES, a tris(triazolylmethyl)amine-based ligand for Cu(I), promotes the cycloaddition reaction rapidly in living systems without apparent toxicity. This catalyst allows, for the first time, noninvasive imaging of fucosylated glycans during zebrafish early embryogenesis. We microinjected embryos with alkyne-bearing GDP-fucose at the one-cell stage and detected the metabolically incorporated unnatural sugars using the biocompatible click chemistry. Labeled glycans could be imaged in the enveloping layer of zebrafish embryos between blastula and early larval stages. This new method paves the way for rapid, noninvasive imaging of biomolecules in living organisms.

Tetrazine-trans-cyclooctene ligation for the rapid construction of 18F labeled probes

A radiolabeling method for bioconjugation based on the Diels-Alder reaction between 3,6-diaryl-s-tetrazines and an (18)F-labeled trans-cyclooctene is described. The reaction proceeds with exceptionally fast rates, making it an effective conjugation method within seconds at low micromolar concentrations.

Difluorobenzocyclooctyne: synthesis, reactivity, and stabilization by beta-cyclodextrin

Highly reactive cyclooctynes have been sought as substrates for Cu-free cycloaddition reactions with azides in biological systems. To elevate the reactivities of cyclooctynes, two strategies, LUMO lowering through propargylic fluorination and strain enhancement through fused aryl rings, have been explored. Here we report the facile synthesis of a difluorobenzocyclooctyne (DIFBO) that combines these modifications. DIFBO was so reactive that it spontaneously trimerized to form two asymmetric products that we characterized by X-ray crystallography. However, we were able to trap DIFBO by forming a stable inclusion complex with beta-cyclodextrin in aqueous media. This complex could be stored as a lyophilized powder and then dissociated in organic solvents to produce free DIFBO for in situ kinetic and spectroscopic analysis. Using this procedure, we found that the rate constant for the cycloaddition reaction of DIFBO with an azide exceeds those for difluorinated cyclooctyne (DIFO) and dibenzocyclooctyne (DIBO). Cyclodextrin complexation is therefore a promising approach for stabilizing compounds that possess the high intrinsic reactivities desired for Cu-free click chemistry.

Synthesis of a DOTA--biotin conjugate for radionuclide chelation via Cu-free click chemistry

A strain-induced copper-free click reaction mediated by a new and easily prepared cyclooctyne derivative was used to efficiently assemble a DOTA-biotin adduct capable of radionuclide ((68)Ga) uptake. This synthetic strategy offers a potentially general and convenient means of preparing targeted radiolabeling and radiotherapeutic agents.

Strain-promoted double-click reaction for chemical modification of azido-biomolecules

The strain-promoted "double-click" (SPDC) reaction using Sondheimer diyne, a novel convergent method conjugating three molecules spontaneously, has enabled us to readily modify an azido-biomolecule with a small reporter azido-molecule.

Rapid Cu-free click chemistry with readily synthesized biarylazacyclooctynones

Bioorthogonal chemical reactions, those that do not interact or interfere with biology, have allowed for exploration of numerous biological processes that were previously difficult to study. The reaction of azides with strained alkynes, such as cyclooctynes, readily forms a triazole product without the need for a toxic catalyst. Here we describe a biarylazacyclooctynone (BARAC) that has exceptional reaction kinetics and whose synthesis is designed to be both modular and scalable. We employed BARAC for live cell fluorescence imaging of azide-labeled glycans. The high signal-to-background ratio obtained using nanomolar concentrations of BARAC obviated the need for washing steps. Thus, BARAC is a promising reagent for in vivo imaging.

A DOTA-peptide conjugate by copper-free click chemistry

Attachment of DOTA to a novel monofluoro-cyclooctyne facilitates bioconjugation to an azide-modified peptide via Cu-free click chemistry. The resulting conjugate was radiolabeled with (111)In to afford a potential targeted molecular imaging agent with high specific activity and an excellent radiochemical purity.