Rigid tetrazine fluorophore conjugates with fluorogenic properties in the inverse electron demand Diels-Alder reaction

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.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

Readily Accessible and Brightly Fluorogenic BODIPY/NBD-Tetrazines via SNAr Reactions

We describe SNAr reactions of some commercial amino-tetrazines and halo-dyes, which give efficiently quenched BODIPY/NBD-tetrazines (ΦFl < 0.01) in high yields and, importantly, with high purities affordable via simple silica gel chromatography only. The dyes exhibit large Stokes shifts, moderate environmental sensitivity, and emission enhancements (up to 193-fold) upon Tz ligation with BCN─a strained dienophile. They successfully serve as labels for HSA protein premodified with BCN, resulting in bright blue-green emission upon ligation.

Co-aggregation as A Simple Strategy for Preparing Fluorogenic Tetrazine Probes with On-Demand Fluorogen Selection

Life science has progressed with applications of fluorescent probes-fluorophores linked to functional units responding to biological events. To meet the varied demands across experiments, simple organic reactions to connect fluorophores and functional units have been developed, enabling the on-demand selection of fluorophore-functional unit combinations. However, organic synthesis requires professional equipment and skills, standing as a daunting task for life scientists. In this study, we present a simple, fast, and convenient strategy for probe preparation: co-aggregation of hydrophobic molecules. We focused on tetrazine-a difficult-to-prepare yet useful functional unit that provides effective bioorthogonal reactivity and strong fluorogenicity. Simply mixing the tetrazine molecules and aggregation-induced emission (AIE) luminogens in water, co-aggregation is induced, and the emission of AIE luminogens is quenched. Subsequent click reaction bioorthogonally turns on the emission, identifying these coaggregates as fluorogenic probes. Thanks to this bioorthogonal fluorogenicity, we established a new time-gated fluorescence bioimaging technique to distinguish overlapping emission signals, enabling multi-organelle imaging with two same-color fluorophores. Our study showcases the potential of this co-aggregation method for the on-demand preparation of fluorescent probes as well as protocols and molecular design principles in this approach, offering an effective solution to evolving needs in life science research.

Two-Factor Fluorogenic Cyanine-Styryl Dyes with Yellow and Red Fluorescence for Bioorthogonal Labelling of DNA

An orange- and a red-emitting tetrazine-modified cyanine-styryl dyes were synthesized for bioorthogonal labelling of DNA by means of the Diels-Alder reaction with inverse electron demand. Both dyes use the concept of the "two-factor" fluorogenicity for nucleic acids: (i) The dyes are nucleic-acid sensitive by their non-covalent binding to DNA, and (ii) their covalently attached tetrazine moiety quench the fluorescence. As a result, the reaction with bicyclononyne- and spirohexene-modified DNA is significantly accelerated up to k2 =280,000 M-1  s-1 , and the fluorescence turn-on is enhanced up to 305. Both dyes are cell permeable even in low concentrations and undergo fluorogenic reactions with spirohexene-modified DNA in living HeLa cells. The fluorescence is enhanced in living cells to such an extent that washing procedures before cell imaging are not required. Their large Stokes shifts (up to 0.77 eV) also makes them well suited for imaging because the wavelength ranges for excitation and emission can be best possible separated. Furthermore, the spirohexene-modified nucleosides and DNA extend and improve the toolbox of already existing "clickable" dyes for live cell imaging.

A dark intermediate in the fluorogenic reaction between tetrazine fluorophores and trans-cyclooctene

Fluorogenic labeling via bioorthogonal tetrazine chemistry has proven to be highly successful in fluorescence microscopy of living cells. To date, trans-cyclooctene (TCO) and bicyclonyne have been found to be the most useful substrates for live-cell labeling owing to their fast labeling kinetics, high biocompatibility, and bioorthogonality. Recent kinetic studies of fluorogenic click reactions with TCO derivatives showed a transient fluorogenic effect but could not explain the reaction sequence and the contributions of different intermediates. More recently, fluorescence quenching by potential intermediates has been investigated, suggesting their occurrence in the reaction sequence. However, in situ studies of the click reaction that directly relate these observations to the known reaction sequence are still missing. In this study, we developed a single-molecule fluorescence detection framework to investigate fluorogenic click reactions. In combination with data from ultra-performance liquid chromatography-tandem mass spectrometry, this explains the transient intensity increase by relating fluorescent intermediates to the known reaction sequence of TCO with fluorogenic tetrazine dyes. More specifically, we confirm that the reaction of TCO with tetrazine rapidly forms a fluorescent 4,5-dihydropyridazine species that slowly tautomerizes to a weakly fluorescent 1,4-dihydropyridazine, explaining the observed drop in fluorescence intensity. On a much slower timescale of hours/days, the fluorescence intensity may be recovered by oxidation of the intermediate to a pyridazine. Our findings are of importance for quantitative applications in fluorescence microscopy and spectroscopy as the achieved peak intensity with TCO depends on the specific experimental settings. They clearly indicate the requirement for more robust benchmarking of click reactions with tetrazine dyes and the need for alternative dienophiles with fast reaction kinetics and stable fluorescence emission to further applications in advanced fluorescence microscopy.

Amino acids with fluorescent tetrazine ethers as bioorthogonal handles for peptide modification

A set of 3-bromo-1,2,4,5-tetrazines with three distinct substitutions have been used as reagents for late-stage functionalization of small molecules through nucleophilic aromatic substitution. Spectroscopic studies of the products obtained proved that tetrazine ethers are intrinsically fluorescent. This fluorescence is lost upon inverse Electron-Demand Diels–Alder (iEDDA) cycloaddition with strained alkenes. Tetrazine-phenol ethers are rather interesting because they can undergo rapid iEDDA reactions with a second order rate constant (k₂) compatible with bioorthogonal ligations. As a showcase, l-tyrosine was derivatized with 3-bromo-6-methyl-1,2,4,5-tetrazine and coupled to the peptide drug octreotide. This peptide was detected in cellular flow cytometry, and its fluorescence turned off through a bioorthogonal iEDDA cycloaddition with a strained alkene, showing for the first time the detection and reactivity of intrinsically fluorescent tetrazines in a biologically relevant context. The synthesis and characterization of fluorescent tetrazine ethers with bioorthogonal applicability pave the way for the generation of useful compounds for both detection and bioconjugation in vivo.

Octreotide derivatized with the fluorogenic amino acid 6-methyltetratrazinyl tryosine. Emission spectra before and after the iEDDA cycloaddition.

Acetoxymethyl-BODIPY dyes: a universal platform for the fluorescent labeling of nucleophiles

A general and robust methodology has been developed for the direct incorporation of a wide variety of C-, N-, P-, O-, S-, and halo-nucleophiles into functional BODIPY conjugates in a single reaction step.

Derivatization based on tetrazine scaffolds: synthesis of tetrazine derivatives and their biomedical applications

The recent advances in tetrazine scaffold-based derivatizations have been summarized. The advantages and limitations of derivatization methods and applications of the developed tetrazine derivatives in bioorthogonal chemistry have been highlighted.

Red-emitting fluorogenic BODIPY-tetrazine probes for biological imaging

A series of new BODIPY-tetrazine derivatives have been developed with a twist intramolecular charge transfer (TICT) state in polar solvents, which is an electron transfer process that occurs upon photoexcitation in a molecule that usually consists of an electron donor and acceptor linked by a single bond. Among them, the BODIPY-tetrazine derivative 6i was stable towards long-term storage and red-emitting with excellent performance, and was further used to image trans-cyclooctene-labeled lipids in mammalian cells and cyclopropene-labeled sugars in cancer cells under no-wash conditions.

Imaging organelle membranes in live cells at the nanoscale with lipid-based fluorescent probes

Understanding how organelles interact, exchange materials, assemble, disassemble, and evolve as a function of space, time, and environment is an exciting area at the very forefront of chemical and cell biology. Here, we bring attention to recent progress in the design and application of lipid-based tools to visualize and interrogate organelles in live cells, especially at super resolution. We highlight strategies that rely on modification of natural lipids or lipid-like small molecules ex cellula, where organelle specificity is provided by the structure of the chemically modified lipid, or in cellula using cellular machinery, where an enzyme labels the lipid in situ. We also describe recent improvements to the chemistry upon which lipid probes rely, many of which have already begun to broaden the scope of biological questions that can be addressed by imaging organelle membranes at the nanoscale.

Bio-orthogonal Red and Far-Red Fluorogenic Probes for Wash-Free Live-Cell and Super-resolution Microscopy

Small-molecule fluorophores enable the observation of biomolecules in their native context with fluorescence microscopy. Specific labeling via bio-orthogonal tetrazine chemistry combines minimal label size with rapid labeling kinetics. At the same time, fluorogenic tetrazine-dye conjugates exhibit efficient quenching of dyes prior to target binding. However, live-cell compatible long-wavelength fluorophores with strong fluorogenicity have been difficult to realize. Here, we report close proximity tetrazine-dye conjugates with minimal distance between tetrazine and the fluorophore. Two synthetic routes give access to a series of cell-permeable and -impermeable dyes including highly fluorogenic far-red emitting derivatives with electron exchange as the dominant excited-state quenching mechanism. We demonstrate their potential for live-cell imaging in combination with unnatural amino acids, wash-free multicolor and super-resolution STED, and SOFI imaging. These dyes pave the way for advanced fluorescence imaging of biomolecules with minimal label size.

Cross-Coupling Reactions of Monosubstituted Tetrazines

A Ag-mediated Pd-catalyzed cross-coupling method for 3-bromo-1,2,4,5-tetrazine with boronic acids is presented. Electronic modification of the 1,1'-bis(diphenylphosphine)ferrocene (dppf) ligand was found to be crucial for good turnover. Using this fast method, a variety of alkyl-, heteroatom-, and halide-substituted aryl- and heteroaryl-tetrazines were prepared (29 examples, up to 87% yield).

Synthesis of 3-alkyl-6-methyl-1,2,4,5-tetrazines via a Sonogashira-type cross-coupling reaction

1,2,4,5-Tetrazines have become extremely useful tools in chemical biology. However, the synthesis of some challenging substrates such as asymmetrically disubstituted alkyltetrazines is still a limitation for the widespread use of this class of compounds. Herein we describe an efficient route to these compounds based on the Sonogashira coupling of 3-bromo-6-methyl-1,2,4,5-tetrazine and 3-bromo-6-phenyl-1,2,4,5-tetrazine with terminal alkynes. The preparation of the starting reagents has also been optimized. The alkynyl products have been used as intermediates for the synthesis of dialkyl-tetrazines through a sequence of hydrogenation and re-oxidation with unprecedented yields. The synthetic applicability of this new approach is showcased through the preparation of several unnatural amino acids bearing alkynyl- and alkyl-1,2,4,5-tetrazine fragments.

Divergent Synthesis of Monosubstituted and Unsymmetrical 3,6-Disubstituted Tetrazines from Carboxylic Ester Precursors

Since tetrazines are important tools to the field of bioorthogonal chemistry, there is a need for new approaches to synthesize unsymmetrical and 3-monosubstituted tetrazines. Described here is a general, one-pot method for converting (3-methyloxetan-3-yl)methyl carboxylic esters into 3-thiomethyltetrazines. These versatile intermediates were applied to the synthesis of unsymmetrical tetrazines through Pd-catalyzed cross-coupling and in the first catalytic thioether reduction to access monosubstituted tetrazines. This method enables the development of new tetrazine compounds possessing a favorable combination of kinetics, small size, and hydrophilicity. It was applied to a broad range of aliphatic and aromatic ester precursors and to the synthesis of heterocycles including BODIPY fluorophores and biotin. In addition, a series of tetrazine probes for monoacylglycerol lipase (MAGL) were synthesized and the most reactive one was applied to the labeling of endogenous MAGL in live cells.

A Systematic Study of Coumarin-Tetrazine Light-Up Probes for Bioorthogonal Fluorescence Imaging

Fluorescent probes that light-up upon reaction with complementary bioorthogonal reagents are superior tools for no-wash fluorogenic bioimaging applications. In this work, a thorough study is presented on a set of seventeen structurally diverse coumarin-tetrazine probes that produce fluorescent dyes with exceptional turn-on ratios when reacted with trans-cyclooctene (TCO) and bicyclononyne (BCN) dienophiles. In general, formation of the fully aromatic pyridazine-containing dyes resulting from the reaction with BCN was found superior in terms of fluorogenicity. However, evaluation of the probes in cellular imaging experiments revealed that other factors, such as reaction kinetics and good cell permeability, prevail over the fluorescence turn-on properties. The best compound identified in this study showed excellent performance in live cell-labeling experiments and enabled no-wash fluorogenic imaging on a timescale of seconds.

Donor-Acceptor 1,2,4,5-Tetrazines Prepared by the Buchwald-Hartwig Cross-Coupling Reaction and Their Photoluminescence Turn-On Property by Inverse Electron Demand Diels-Alder Reaction

A facile efficient synthetic tool, Buchwald-Hartwig cross-coupling reaction, for the functionalization of 1,2,4,5-tetrazines is presented. Important factors affecting the Buchwald-Hartwig cross-coupling reaction have been optimized. Seven new donor-acceptor tetrazine molecules (TA1-TA7) were conveniently prepared in good to high yields (61-72%). They have been subsequently engaged in the inverse electron demand Diels-Alder (iEDDA) reaction with cyclooctyne. The photophysical and electrochemical properties of the new pyridazines have been studied. Some are fluorescent acting as turn-on probes. More importantly, two pyridazines (DA3 and DA6) exhibit room-temperature phosphorescence (RTP) properties.

Live-Cell Localization Microscopy with a Fluorogenic and Self-Blinking Tetrazine Probe

Recent developments in fluorescence microscopy call for novel small-molecule-based labels with multiple functionalities to satisfy different experimental requirements. A current limitation in the advancement of live-cell single-molecule localization microscopy is the high excitation power required to induce blinking. This is in marked contrast to the minimal phototoxicity required in live-cell experiments. At the same time, quality of super-resolution imaging depends on high label specificity, making removal of excess dye essential. Approaching both hurdles, we present the design and synthesis of a small-molecule label comprising both fluorogenic and self-blinking features. Bioorthogonal click chemistry ensures fast and highly selective attachment onto a variety of biomolecular targets. Along with spectroscopic characterization, we demonstrate that the probe improves quality and conditions for regular and single-molecule localization microscopy on live-cell samples.

Multifunctional, histidine-tagged polymers: antibody conjugation and signal amplification

A polymer scaffold, with multiple reactive centres, was synthesised by RAFT polymerisation and conjugated to the antibody herceptin. A hexahistidine RAFT agent enabled simple purification of polymer–protein conjugates.

A bioorthogonal time-resolved luminogenic probe for metabolic labelling and imaging of glycans

A terbium complex Tb-1 was demonstrated to undergo bioorthogonal ligation with engineered cell-surface glycans, which results in a much less efficient LRET and a 5-fold increase in long-lived terbium emission with low toxicity.

Installation of Minimal Tetrazines through Silver-Mediated Liebeskind-Srogl Coupling with Arylboronic Acids

Described is a general method for the installation of a minimal 6-methyltetrazin-3-yl group via the first example of a Ag-mediated Liebeskind-Srogl cross-coupling. The attachment of bioorthogonal tetrazines on complex molecules typically relies on linkers that can negatively impact the physiochemical properties of conjugates. Cross-coupling with arylboronic acids and a new reagent, 3-((p-biphenyl-4-ylmethyl)thio)-6-methyltetrazine (b-Tz), proceeds under mild, PdCl2(dppf)-catalyzed conditions to introduce minimal, linker-free tetrazine functionality. Safety considerations guided our design of b-Tz which can be prepared on decagram scale without handling hydrazine and without forming volatile, high-nitrogen tetrazine byproducts. Replacing conventional Cu(I) salts used in Liebeskind-Srogl cross-coupling with a Ag2O mediator resulted in higher yields across a broad library of aryl and heteroaryl boronic acids and provides improved access to a fluorogenic tetrazine-BODIPY conjugate. A covalent probe for MAGL incorporating 6-methyltetrazinyl functionality was synthesized in high yield and labeled endogenous MAGL in live cells. This new Ag-mediated cross-coupling method using b-Tz is anticipated to find additional applications for directly introducing the tetrazine subunit to complex substrates.

Biomedical applications of copper-free click chemistry: in vitro, in vivo, and ex vivo

Recently, click chemistry has provided important advances in biomedical research fields. Particularly, copper-free click chemistry including strain-promoted azide-alkyne cycloaddition (SPAAC) and inverse-electron-demand Diels-Alder (iEDDA) reactions enable fast and specific chemical conjugation under aqueous conditions without the need for toxic catalysts. Click chemistry has resulted in a change of paradigm, showing that artificial chemical reactions can occur on cell surfaces, in cell cytosol, or within the body, which is not easy with most other chemical reactions. Click chemistry in vitro allows specific labelling of cellular target proteins and studying of drug target engagement with drug surrogates in live cells. Furthermore, cellular membrane lipids and proteins could be selectively labelled with click chemistry in vitro and cells could be adhered together using click chemistry. Click chemistry in vivo enables efficient and effective molecular imaging and drug delivery for diagnosis and therapy. Click chemistry ex vivo can be used to develop molecular tools to understand tissue development, diagnosis of diseases, and therapeutic monitoring. Overall, the results from research to date suggest that click chemistry has emerged as a valuable tool in biomedical fields as well as in organic chemistry.

Bioorthogonal Fluorescence Turn-On Labeling Based on Bicyclononyne-Tetrazine Cycloaddition Reactions that Form Pyridazine Products

Fluorogenic bioorthogonal reactions enable visualization of biomolecules with excellent signal-to-noise ratio. A bicyclononyne-tetrazine ligation that produces fluorescent pyridazine products has been developed. In stark contrast to previous approaches, the formation of the dye is an inherent result of the chemical reaction and no additional fluorophores are needed in the reagents. The crucial structural elements that determine dye formation are electron-donating groups present in the starting tetrazine unit. The newly formed pyridazine fluorophores show interesting photophysical properties the fluorescence intensity increase in the reaction can reach an excellent 900-fold. Model imaging experiments demonstrate the application potential of this new fluorogenic bioorthogonal reaction.

Inverse electron demand Diels-Alder reactions in chemical biology

The emerging inverse electron demand Diels-Alder (IEDDA) reaction stands out from other bioorthogonal reactions by virtue of its unmatchable kinetics, excellent orthogonality and biocompatibility. With the recent discovery of novel dienophiles and optimal tetrazine coupling partners, attention has now been turned to the use of IEDDA approaches in basic biology, imaging and therapeutics. Here we review this bioorthogonal reaction and its promising applications for live cell and animal studies. We first discuss the key factors that contribute to the fast IEDDA kinetics and describe the most recent advances in the synthesis of tetrazine and dienophile coupling partners. Both coupling partners have been incorporated into proteins for tracking and imaging by use of fluorogenic tetrazines that become strongly fluorescent upon reaction. Selected notable examples of such applications are presented. The exceptional fast kinetics of this catalyst-free reaction, even using low concentrations of coupling partners, make it amenable for in vivo radiolabelling using pretargeting methodologies, which are also discussed. Finally, IEDDA reactions have recently found use in bioorthogonal decaging to activate proteins or drugs in gain-of-function strategies. We conclude by showing applications of the IEDDA reaction in the construction of biomaterials that are used for drug delivery and multimodal imaging, among others. The use and utility of the IEDDA reaction is interdisciplinary and promises to revolutionize chemical biology, radiochemistry and materials science.

Bioorthogonal double-fluorogenic siliconrhodamine probes for intracellular super-resolution microscopy

A series of double-fluorogenic siliconrhodamine probes were synthesized. These tetrazine-functionalized, membrane-permeable labels allowed site-specific bioorthogonal tagging of genetically manipulated intracellular proteins and subsequent imaging using super-resolution microscopy.

Design and Synthesis of Aza-Bicyclononene Dienophiles for Rapid Fluorogenic Ligations

Fluorogenic bioorthogonal reactions enable visualization of biomolecules under native conditions with excellent signal-to-noise ratio. Here, we present the design and synthesis of conformationally-strained aziridine-fused trans-cyclooctene (aza-TCO) dienophiles, which lead to the formation of fluorescent products in tetrazine ligations without the need for attachment of an extra fluorophore moiety. The presented aza-TCOs adopt the highly strained "half-chair" conformation, which was predicted computationally and confirmed by NMR measurements and X-ray crystallography. Kinetic studies revealed that the aza-TCOs belong to the most reactive dienophiles known to date. The potential of the newly developed aza-TCO probes for bioimaging applications is demonstrated by protein labeling experiments, imaging of cellular glycoconjugates and peptidoglycan imaging of live bacteria.

Alternative fluorophores designed for advanced molecular imaging

Fluorescent molecular imaging has advanced drastically over the past decade. With the development of high-resolution microscopy techniques and the ability to visualize intracellular molecular events, there is a growing need for new fluorophores to accompany these fast-developing techniques. Therefore, there has been substantial development of alternative fluorophores for single-molecule detection and molecular imaging. These rationally designed fluorophores have infinite possibilities and novel fluorophores are constantly being produced for different applications. This review focuses on the recent developments in novel fluorophores designed for molecular imaging and single-molecule detection. Here, single-molecule imaging, smart fluorescent probes, two-photon microscopy, Förster resonance energy transfer (FRET) and super-resolution microscopy are discussed in detail.

Green- to far-red-emitting fluorogenic tetrazine probes - synthetic access and no-wash protein imaging inside living cells

Fluorogenic probes for bioorthogonal labeling chemistry are highly beneficial to reduce background signal in fluorescence microscopy imaging. 1,2,4,5-Tetrazines are known substrates for the bioorthogonal inverse electron demand Diels-Alder reaction (DAinv) and tetrazine substituted fluorophores can exhibit fluorogenic properties. Herein, we report the synthesis of a palette of novel fluorogenic tetrazine dyes derived from widely-used fluorophores that cover the entire emission range from green to far-red. We demonstrate the power of the new fluorogenic probes in fixed and live cell labeling experiments and present the first example of intracellular live cell protein imaging using tetrazine-based probes under no-wash conditions.

Mechanism-Based Fluorogenic trans-Cyclooctene-Tetrazine Cycloaddition

The development of fluorogenic reactions which lead to the formation of fluorescent products from two nonfluorescent starting materials is highly desirable, but challenging. Reported herein is a new concept of fluorescent product formation upon the inverse electron-demand Diels-Alder reaction of 1,2,4,5-tetrazines with particular trans-cyclooctene (TCO) isomers. In sharp contrast to known fluorogenic reagents the presented chemistry leads to the rapid formation of unprecedented fluorescent 1,4-dihydropyridazines so that the fluorophore is built directly upon the chemical reaction. Attachment of an extra fluorophore moiety is therefore not needed. The photochemical properties of the resulting dyes can be easily tuned by changing the substitution pattern of the starting 1,2,4,5-tetrazine. We support the claim with NMR measurements and rationalize the data by computational study. Cell-labeling experiments were performed to demonstrate the potential of the fluorogenic reaction for bioimaging.

Preparation of tetrazine-containing [2 + 1] complexes of 99mTc and in vivo targeting using bioorthogonal inverse electron demand Diels–Alder chemistry

A new ^99mTc-labelled tetrazine for targeted imaging using bioorthogonal chemistry was developed and evaluated in vivo using a trans-cyclooctene derived bisphosphonate targeting regions of high bone turnover and bone lesions.

Inverse Electron-Demand Diels-Alder Bioorthogonal Reactions

Bioorthogonal reactions have been widely used over the last 10 years for imaging, detection, diagnostics, drug delivery, and biomaterials. Tetrazine reactions are a recently developed class of inverse electron-demand Diels-Alder reactions used in bioorthogonal applications. Given their rapid tunable reaction rate and highly fluorogenic properties, tetrazine bioorthogonal reactions have come to be considered highly attractive tools for elucidating biological functions and messages in vitro and in vivo. In this chapter, we present recent advances expanding the scope of precursor reactivity and we introduce new biomedical methodology based on bioorthogonal tetrazine chemistry. We specifically highlight novel applications for different kinds of biomolecules, including nucleic acid, protein, antibodies, lipids, glycans, and bioactive small molecules, in the areas of imaging, detection, and diagnostics. We also briefly present other recently developed inverse electron-demand Diels-Alder bioorthogonal reactions. Lastly, we consider future directions and potential roles that inverse electron-demand Diels-Alder reactions may play in the fields of bioorthogonal and biomedical chemistry.

Systemic Fluorescence Imaging of Zebrafish Glycans with Bioorthogonal Chemistry

Vertebrate glycans constitute a large, important, and dynamic set of post-translational modifications that are notoriously difficult to manipulate and image. Although the chemical reporter strategy has been used in conjunction with bioorthogonal chemistry to image the external glycosylation state of live zebrafish and detect tumor-associated glycans in mice, the ability to image glycans systemically within a live organism has remained elusive. Here, we report a method that combines the metabolic incorporation of a cyclooctyne-functionalized sialic acid derivative with a ligation reaction of a fluorogenic tetrazine, allowing for the imaging of sialylated glycoconjugates within live zebrafish embryos.

Design strategies for bioorthogonal smart probes

Bioorthogonal chemistry has enabled the selective labeling and detection of biomolecules in living systems. Bioorthogonal smart probes, which become fluorescent or deliver imaging or therapeutic agents upon reaction, allow for the visualization of biomolecules or targeted delivery even in the presence of excess unreacted probe. This review discusses the strategies used in the development of bioorthogonal smart probes and highlights the potential of these probes to further our understanding of biology.