Advances in Tetrazine Bioorthogonal Chemistry Driven by the Synthesis of Novel Tetrazines and Dienophiles

Cyclopropenes as Chemical Reporters for Dual Bioorthogonal and Orthogonal Metabolic Labeling of DNA

Dual bioorthogonal labeling enables the investigation and understanding of interactions in the biological environment that are not accessible by a single label. However, applying two bioorthogonal reactions in the same environment remains challenging due to cross-reactivity. We developed a pair of differently modified 2'-deoxynucleosides that solved this issue for dual and orthogonal labeling of DNA. Inverse-electron demand Diels-Alder and photoclick reactions were combined to attach two different fluorogenic labels to genomic DNA in cells. Using a small synthetic library of 1- and 3-methylcyclopropenyl-modified 2'-deoxynucleosides, two 2'-deoxyuridines were identified to be the fastest-reacting ones for each of the two bioorthogonal reactions. Their orthogonal reactivity could be evidenced in vitro. Primer extension experiments were performed with both 2'-deoxyuridines investigating their replication properties as substitutes for thymidine and evaluating subsequent labeling reactions on the DNA level. Finally, dual, orthogonal and metabolic fluorescent labeling of genomic DNA was demonstrated in HeLa cells. An experimental procedure was developed combining intracellular transport and metabolic DNA incorporation of the two 2'-deoxyuridines with the subsequent dual bioorthogonal labeling using a fluorogenic cyanine-styryl tetrazine and a fluorogenic pyrene-tetrazole. These results are fundamental for advanced metabolic labeling strategies for nucleic acids in the future, especially for live cell experiments.

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.

An all-in-one tetrazine reagent for cysteine-selective labeling and bioorthogonal activable prodrug construction

The prodrug design strategy offers a potent solution for improving therapeutic index and expanding drug targets. However, current prodrug activation designs are mainly responsive to endogenous stimuli, resulting in unintended drug release and systemic toxicity. In this study, we introduce 3-vinyl-6-oxymethyl-tetrazine (voTz) as an all-in-one reagent for modular preparation of tetrazine-caged prodrugs and chemoselective labeling peptides to produce bioorthogonal activable peptide-prodrug conjugates. These stable prodrugs can selectively bind to target cells, facilitating cellular uptake. Subsequent bioorthogonal cleavage reactions trigger prodrug activation, significantly boosting potency against tumor cells while maintaining exceptional off-target safety for normal cells. In vivo studies demonstrate the therapeutic efficacy and safety of this prodrug design approach. Given the broad applicability of functional groups and labeling versatility with voTz, we foresee that this strategy will offer a versatile solution to enhance the therapeutic range of cytotoxic agents and facilitate the development of bioorthogonal activatable biopharmaceuticals and biomaterials.

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.

Tetrazine-Isonitrile Bioorthogonal Fluorogenic Reactions Enable Multiplex Labeling and Wash-Free Bioimaging of Live Cells

Developing fluorogenic probes for simultaneous live cell labeling of multiple targets is crucial for understanding complex cellular events. The emerging [4+1] cycloaddition between tetrazine and isonitriles holds promise as a bioorthogonal tool, yet existing tetrazine probes lack reactivity and fluorogenicity. Here, we present the development of a series of tetrazine-functionalized bioorthogonal probes. By incorporating pyrazole adducts into the fluorophore scaffolds, the post-reacted probes displayed remarkable fluorescence turn-on ratios, up to 3184-fold. Moreover, these modifications are generalizable to various fluorophores, enabling a broad emission range from 473 to 659 nm. Quantum chemical calculations further elucidate the turn-on mechanisms. These probes enable the simultaneous labeling of multiple targets in live cells, without the need for a washing step. Consequently, our findings pave the way for advanced multiplex imaging and detection techniques for cellular studies.

What is the future of click chemistry in drug discovery and development?

INTRODUCTION

The concept of click chemistry was introduced in 2001 as an effective, efficient, and sustainable approach to making functional groups harnessing the thermodynamic properties of a set of known chemical reactions that are based on nature. Some of the most common examples include reactions that produce 1,2,3-triazoles, which have been used with great success in drug discovery and development, and in chemical biology. The reactions unite two molecules quickly and irreversibly, and the reactions can be performed inside living cells, without harming the cell.

AREAS COVERED

The main focus of this perspective is the future of click chemistry in drug discovery and development, exemplified by novel click chemistry approaches and other aspects of the drug development enterprise, like SPAAC and analogous techniques, PROTACs, as well as diversity-oriented click chemistry.

EXPERT OPINION

Drug discovery and development has benefited enormously from the amazing advances that have been made in the field of click chemistry since 2001. The methods most likely to have the most future applications include metal-catalyzed azide-alkyne cycloadditions giving 1,2,3-triazoles, SPAAC for medical diagnostics and vaccine development, other congeners, Sulfur-Fluoride Exchange (SuFEx) and Diversity-Oriented Clicking (DOC), a concept with diverse molecular methodology with the potential for obtaining extensive molecular diversity.

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.

Discovery of new tetrazines for bioorthogonal reactions with strained alkenes via computational chemistry

Tetrazines are widely employed reagents in bioorthogonal chemistry, as they react readily with strained alkenes in inverse electron demand Diels-Alder reactions, allowing for selective labeling of biomacromolecules. For optimal performance, tetrazine reagents have to react readily with strained alkenes, while remaining inert against nucleophiles like thiols. Balancing these conditions is a challenge, as reactivity towards strained alkenes and nucleophiles is governed by the same factor - the energy of unoccupied orbitals of tetrazine. Herein, we utilize computational chemistry to screen a set of tetrazine derivatives, aiming to identify structural elements responsible for a better ratio of reactivity with strained alkenes vs. stability against nucleophiles. This advantageous trait is present in sulfone- and sulfoxide-substituted tetrazines. In the end, the distortion/interaction model helped us to identify that the reason behind this enhanced reactivity profile is a secondary orbital interaction between the strained alkene and sulfone-/sulfoxide-substituted tetrazine. This insight can be used to design new tetrazines for bioorthogonal chemistry with improved reactivity/stability profiles.

Bioorthogonal Bond Cleavage Chemistry for On-demand Prodrug Activation: Opportunities and Challenges

Time- and space-resolved drug delivery is highly demanded for cancer treatment, which, however, can barely be achieved with a traditional prodrug strategy. In recent years, the prodrug strategy based on a bioorthogonal bond cleavage chemistry has emerged with the advantages of high temporospatial resolution over drug activation and homogeneous activation irrespective of individual heterogeneity. In the past five years, tremendous progress has been witnessed in this field with one such bioorthogonal prodrug entering Phase II clinical trials. This Perspective aims to highlight these new advances (2019-2023) and critically discuss their pros and cons. In addition, the remaining challenges and potential strategic directions for future progress will also be included.

Multiconfigurational photodynamics simulations reveal the mechanism of photodecarbonylations of cyclopropenones in explicit aqueous environments

Gas-evolving photochemical reactions use light and mild conditions to access strained organic compounds irreversibly. Cyclopropenones are a class of light-responsive molecules used in bioorthogonal photoclick reactions; their excited-state decarbonylation reaction mechanisms are misunderstood due to their ultrafast (<100 femtosecond) lifetimes. We have combined multiconfigurational quantum mechanical (QM) calculations and non-adiabatic molecular dynamics (NAMD) simulations to uncover the excited-state mechanism of cyclopropenone and a photoprotected cyclooctyne-(COT)-precursor in gaseous and explicit aqueous environments. We explore the role of H-bonding with fully quantum mechanical explicitly solvated NAMD simulations for the decarbonylation reaction. The cyclopropenones pass through asynchronous conical intersections and have dynamically concerted photodecarbonylation mechanisms. The COT-precursor has a higher quantum yield of 55% than cyclopropenone (28%) because these trajectories prefer to break a σCC bond to avoid the strained trans-cyclooctene geometries. Our solvated simulations show an increased quantum yield (58%) for the systems studied here.

Rational Design of Bioorthogonally Activatable PROTAC for Tumor-Targeted Protein Degradation

Protein degradation mediated by the proteolysis-targeting chimera (PROTAC) has emerged as an efficient strategy to accurately control intracellular protein levels. However, the development of PROTACs is limited by their systemic toxicity. Herein, we report a bioorthogonally activatable prodrug (BT-PROTAC) strategy to accurately control the activity of PROTACs. As a proof of concept, we introduced the highly reactive trans-cyclooctene into PROTAC molecule MZ1, the structure-acitivity relationships of which were well characterized previously, to construct the bioorthogonally activatable prodrug BT-PROTAC. Compared with MZ1, BT-PROTAC is incapable of degradation of BRD4 protein. However, BT-PROTAC can be activated by highly active tetrazine compound BODIPY-TZ in vitro. Furthermore, we could selectively degrade BRD4 protein in tumor tissue enabled by tumor-targeted tetrazine compound IR808-TZ. This strategy may represent an alternative to existing strategies and may be widely applied in the design of BT-PROTAC targeting other proteins.

Crystal engineering of high explosives through lone pair-π interactions: Insights for improving thermal safety

In this high-risk/high-reward study, we prepared complexes of a high explosive anion (picrate) with potentially explosive s-tetrazine-based ligands with the sole purpose of advancing the understanding of one of the weakest supramolecular forces: the lone pair-π interaction. This is a proof-of-concept study showing how lone pair-π contacts can be effectively used in crystal engineering, even of high explosives, and how the supramolecular architecture of the resulting crystalline phases influences their experimental thermokinetic properties. Herein we present XRD structures of 4 novel detonating compounds, all showcasing lone pair-π interactions, their thermal characterization (DSC, TGA), including the correlation of experimental thermokinetic parameters with crystal packing, and in silico explosion properties. This last aspect is relevant for improving the safety of high-energy materials.

A Tetrazine-Caged Carbon-Dipyrromethene as a Bioorthogonally Activatable Fluorescent Probe

A water-soluble 1,2,4,5-tetrazine-substituted carbon-dipyrromethene (C-DIPY) was synthesized from the previously reported carbonyl pyrrole dimer through a two-step procedure. Owing to the presence of a tetrazine moiety, the fluorescence emission of this compound was largely quenched in phosphate-buffered saline at pH 7.4. Upon addition of a bicyclo[6.1.0]non-4-yne (BCN) derivative, the tetrazine-based quenching component of the compound was disrupted through the inverse electron-demand Diels-Alder reaction to restore the fluorescence in up to 6.6-fold. This bioorthogonal activation was also demonstrated using U-87 MG human glioblastoma cells, in which the fluorescence intensity of this C-DIPY could be enhanced by 8.7-fold upon post-incubation with the BCN derivative. The results showed that this tetrazine-caged C-DIPY can serve as a bioorthogonally activatable fluorescent probe for bioimaging. The compound, however, was found to reside preferentially in the lysosomes instead of the mitochondria of the cells as predicted based on its cationic character, which could be attributed to its energy-dependent endocytic cellular uptake pathway, for which lysosomes are the end station.

Orthogonal Inverse-Electron-Demand Cycloaddition Reactions Controlled by Frontier Molecular Orbital Interactions

Chemoselective pairs of bioorthogonal reactants enable the simultaneous labeling of several biomolecules. Here, we access orthogonal click reactions by exploiting differences in frontier molecular orbital interaction energies in transition states. We establish that five-membered cyclic dienes are inert to isonitriles but readily react with strained alkynes, while tetrazines with bulky substituents readily react with isonitriles. Strained alkynes show an opposite reactivity pattern. The approach was demonstrated by orthogonally labeling two proteins with different fluorophores.

A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C-N, C-O and C-S bonds

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(μ2-Me)(AlMe3)2(μ4-CH) (1) was synthesized via C-H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza-allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C[double bond, length as m-dash]S double bond was cleaved, followed by C-H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

The Application of Bio-orthogonality for In Vivo Animal Imaging

The application of bio-orthogonality has greatly facilitated numerous aspects of biological studies in recent years. In particular, bio-orthogonal chemistry has transformed biological research, including in vitro conjugate chemistry, target identification, and biomedical imaging. In this review, we highlighted examples of bio-orthogonal in vivo imaging published in recent years. We grouped the references into two major categories: bio-orthogonal chemistry-related imaging and in vivo imaging with bio-orthogonal nonconjugated pairing. Lastly, we discussed the challenges and opportunities of bio-orthogonality for in vivo imaging.

Thiomethyltetrazines Are Reversible Covalent Cysteine Warheads Whose Dynamic Behavior can be "Switched Off" via Bioorthogonal Chemistry Inside Live Cells

Electrophilic small molecules that can reversibly modify proteins are of growing interest in drug discovery. However, the ability to study reversible covalent probes in live cells can be limited by their reversible reactivity after cell lysis and in proteomic workflows, leading to scrambling and signal loss. We describe how thiomethyltetrazines function as reversible covalent warheads for cysteine modification, and this dynamic labeling behavior can be "switched off" via bioorthogonal chemistry inside live cells. Simultaneously, the tetrazine serves as a bioorthogonal reporter enabling the introduction of tags for fluorescent imaging or affinity purification. Thiomethyltetrazines can label isolated proteins, proteins in cellular lysates, and proteins in live cells with second-order rate constants spanning 2 orders of magnitude (k2, 1-100 M-1 s-1). Reversible modification by thiomethyltetrazines can be switched off upon the addition of trans-cyclooctene in live cells, converting the dynamic thiomethyltetrazine tag into a Diels-Alder adduct which is stable to lysis and proteomic workflows. Time-course quenching experiments were used to demonstrate temporal control over electrophilic modification. Moreover, it is shown that "locking in" the tag through Diels-Alder chemistry enables the identification of protein targets that are otherwise lost during sample processing. Three probes were further evaluated to identify unique pathways in a live-cell proteomic study. We anticipate that discovery efforts will be enabled by the trifold function of thiomethyltetrazines as electrophilic warheads, bioorthogonal reporters, and switches for "locking in" stability.

Integrated Imaging and Proteomic Sensors Resolve Proteome Aggregation in Liver Caused by Non-steroidal Anti-inflammatory Drug Overdose

Given the extreme heterogeneity and the loss of defined protein structures, misfolded and aggregated proteins are technically challenging to visualize and analyze. Herein, we assembled an integrated sensor system to resolve aggregated proteome in live cells and animal liver tissues that are overdosed by non-steroidal anti-inflammatory drugs (NSAIDs). A fluorogenic protein aggregation sensor (AggStain) first discovered the presence of aggregated proteome upon overdosing liver cells with NSAIDs. A solvatochromic protein aggregation sensor (AggRetina) further quantified the compactness (polarity) inside these cellular aggregates. Importantly, we exploited a proteomic sensor (AggLink) to selectively capture aggregated proteins upon NSAID overdose and profile their composition, revealing global collapse of cellular protein homeostasis. Finally, we detected subtle proteome aggregation in mouse liver tissue without obvious acute injury at a low NSAID dosage. Overall, we demonstrated an integrated sensor toolset for proteome aggregation studies and unveiled for the first time that NSAID overdose can cause proteome aggregation in liver cells and tissues.

Tetrazo[1,2-b]indazoles: Straightforward Access to Nitrogen-Rich Polyaromatics from s-Tetrazines

The straightforward access to a new class of aza-polyaromatics is reported. Starting from readily available fluorinated s-tetrazine, a cyclization process with azide leads to the formation of an unprecedented tetrazo[1,2-b]indazole or a bis-tetrazo[1,2-b]indazole (cis and trans conformers). Based on the new nitrogen core, further N-directed palladium-catalyzed ortho-C-H bond functionalization allows the introduction of halides or acetates. The physicochemical properties of these compounds were studied by a joint experimental/theoretical approach. The tetrazo[1,2-b]indazoles display solid-state π-stacking, low reduction potential, absorption in the visible range up to the near-infrared, and intense fluorescence, depending on the molecular structure.

The Efficiency of Metabolic Labeling of DNA by Diels-Alder Reactions with Inverse Electron Demand: Correlation with the Size of Modified 2'-Deoxyuridines

A selection of four different 2'-deoxyuridines with three different dienophiles of different sizes was synthesized. Their inverse electron demand Diels-Alder reactivity increases from k₂ = 0.15 × 10^-2 M^-1 s^-1 to k₂ = 105 × 10^-2 M^-1 s^-1 with increasing ring strain of the dienophiles. With a fluorogenic tetrazine-modified cyanine-styryl dye as reactive counterpart the fluorescence turn-on ratios lie in the range of 21-48 suitable for wash-free cellular imaging. The metabolic DNA labeling was visualized by a dot blot on a semiquantitative level and by confocal fluorescence microscopy on a qualitative level. A clear correlation between the steric demand of the dienophiles and the incorporation efficiency of the modified 2'-deoxyuridines into cellular DNA was observed. Even 2'-deoxyuridines with larger dienophiles, such as norbornene and cyclopropene, were incorporated to a detectable level into the nascent genomic DNA. This was achieved by an optimized way of cell culturing. This expands the toolbox of modified nucleosides for metabolic labeling of nucleic acids in general.

DNA labelling in live cells via visible light-induced [2+2] photocycloaddition

We introduce a visible light-driven (λ_max = 451 nm) photo-chemical strategy for labelling of DNA in living HeLa cells via the [2+2] cycloaddition of a styrylquinoxaline moiety, which we incorporate into both the DNA and the fluorescent label. Our methodology offers advanced opportunities for the mild remote labelling of DNA in water while avoiding UV light activation.

Mutual leveraging of proximity effects and click chemistry in chemical biology

Nature has the remarkable ability to realize reactions under physiological conditions that normally would require high temperature and other forcing conditions. In doing so, often proximity effects such as simultaneous binding of two reactants in the same pocket and/or strategic positioning of catalytic functional groups are used as ways to achieve otherwise kinetically challenging reactions. Though true biomimicry is challenging, there have been many beautiful examples of how to leverage proximity effects in realizing reactions that otherwise would not readily happen under near-physiological conditions. Along this line, click chemistry is often used to endow proximity effects, and proximity effects are also used to further leverage the facile and bioorthogonal nature of click chemistry. This review brings otherwise seemingly unrelated topics in chemical biology and drug discovery under one unifying theme of mutual leveraging of proximity effects and click chemistry and aims to critically analyze the biomimicry use of such leveraging effects as powerful approaches in chemical biology and drug discovery. We hope that this review demonstrates the power of employing mutual leveraging proximity effects and click chemistry and inspires the development of new strategies that will address unmet needs in chemistry and biology.

Two-Factor Fluorogenicity of Tetrazine-Modified Cyanine-Styryl Dyes for Bioorthogonal Labelling of DNA

Two green fluorescent tetrazine-modified cyanine-styryl dyes were synthesized for bioorthogonal labelling of DNA by means of the Diels-Alder reaction with inverse electron demand. With DNA as target biopolymer the fluorescence of these dyes is released by two factors: (i) sterically by their interaction with DNA, and (ii) structurally via the conjugated tetrazine as quencher moiety. As a result, the reaction with bicyclononyne-modified DNA is significantly accelerated up to ≥284,000 M^-1  s^-1 , and the fluorescence turn-on is enhanced up to 560 by the two-factor fluorogenicity. These dyes are cell permeable even in low concentrations and undergo fluorogenic reactions with BCN-modified DNA in living HeLa cells. The two-factor fluorescence release improves the signal-to-noise ratio such that washing procedures prior to cell imaging are not needed, which is a great advantage for live cell imaging of DNA and RNA in the future.

Tetrazine bioorthogonal chemistry makes nanotechnology a powerful toolbox for biological applications

Bioorthogonal chemistry enables researchers to manipulate bioactive molecules in living systems. These highly selective and biocompatible reactions can be carried out in various complex environments. Over the past two decades, a considerable number of strides have been made to expand the capacities of bioorthogonal chemistry coupled with the aim to fine-tune present reactions for specific applications. The good points of bioorthogonal chemistry have pushed material chemists to integrate bioorthogonal chemistry with nanotechnologies to broaden the biological applications of nanomaterials. Notably, bioorthogonal nanotechnologies fundamentally rely on, more than half, according to our investigation, tetrazine bioorthogonal chemistry (TBC) to function as bioorthogonal handles to react with target agents owing to the extremely rapid kinetics and high selectivities of TBC. Its utilization in combination with nanotechnologies has led to developments in various areas of biomedicine, such as in situ drug activation and targeted delivery, bioimaging and biosensing, and the understanding of cell-biomolecule interactions. Given the fantastic past achievements and the rapid developments in tetrazine bioorthogonal technologies, the future is certainly very bright.

A tetrazine-responsive isonitrile-caged photosensitiser for site-specific photodynamic therapy

We report herein a versatile and efficient bioorthogonal strategy to actualise targeted delivery and site-specific activation of photosensitisers for precise antitumoural photodynamic therapy. The strategy involved the use of an isonitrile-caged distyryl boron dipyrromethene-based photosensitiser, labelled as NC-DSBDP, of which the photoactivities could be specifically activated upon conversion of the meso ester substituent to carboxylate initiated by the [4 + 1] cycloaddition with a tetrazine derivative. By using two tetrazines conjugated with a galactose moiety or the GE11 peptide, labelled as gal-Tz and GE11-Tz, we could selectively label the cancer cells overexpressed with the asialoglycoprotein receptor and the epidermal growth factor receptor respectively. Upon encountering the internalised NC-DSBDP, these tetrazines triggered the "ester-to-carboxylate" transformation of this compound, activating its fluorescence and reactive oxygen species generation inside the target cells. The bioorthogonal activation was also demonstrated in vivo, leading to effective photo-eradication of the tumour in nude mice.

1,2,3,5-Tetrazines: A General Synthesis, Cycloaddition Scope, and Fundamental Reactivity Patterns

Despite the explosion of interest in heterocyclic azadienes, 1,2,3,5-tetrazines remain unexplored. Herein, the first general synthesis of this new class of heterocycles is disclosed. Its use in the preparation of a series of derivatives, and the first study of substituent effects on their cycloaddition reactivity, mode, and regioselectivity provide the foundation for future use. Their reactions with amidine, electron-rich, and strained dienophiles reveal unique fundamental reactivity patterns (4,6-dialkyl-1,2,3,5-tetrazines > 4,6-diaryl-1,2,3,5-tetrazines for amidines but slower with strained dienophiles), an exclusive C4/N1 mode of cycloaddition, and dominant alkyl versus aryl control on regioselectivity. An orthogonal reactivity of 1,2,3,5-tetrazines and the well-known isomeric 1,2,4,5-tetrazines is characterized, and detailed kinetic and mechanistic investigations of the remarkably fast reaction of 1,2,3,5-tetrazines with amidines, especially 4,6-dialkyl-1,2,3,5-tetrazines, established the mechanistic origins underlying the reactivity patterns and key features needed for future applications.

Ru- and Ir-complex decorated periodic mesoporous organosilicas as sensitizers for artificial photosynthesis

A versatile and facile strategy based on an inverse electron demand Diels-Alder reaction between 5-norbornen-2-yltriethoxysilane and a tetrazine derivative has been established for the synthesis of a new triethoxysilane precursor containing dipyridylpyridazine units. Such a precursor has been incorporated into the mesostructure of an ethylene-bridged periodic mesoporous organosilica (PMO) material through a one-pot synthesis via a co-condensation method. Upon attachment of Ru- and Ir-complexes to the pendant N-chelating heterocyclic ligands, the resulting decorated PMOs have acted as photosensitizers in artificial photosynthetic systems. The deposition of Pt on these PMOs has allowed us to obtain efficient photocatalytic materials for the hydrogen evolution reaction as a result of electron transfer from the light harvesting Ru- and Ir-complexes to the supported Pt nanoparticles through methyl viologen as an electron relay. They have exhibited total turnover number values of 573 and 846, respectively, under visible light irradiation. The role played by each component and the stability of the photocatalytic systems have been discussed. The present approach paves the way to the synthesis of different materials with coordination sites capable of forming surface complexes to be applied as sensitizers and catalysts.

Tetrazine bioorthogonal chemistry derived in vivo imaging

Bioorthogonal chemistry represents plenty of highly efficient and biocompatible reactions that proceed selectively and rapidly in biological situations without unexpected side reactions towards miscellaneous endogenous functional groups. Arise from the strict demands of physiological reactions, bioorthogonal chemical reactions are natively selective transformations that are rarely found in biological environments. Bioorthogonal chemistry has long been applied to tracking and real-time imaging of biomolecules in their physiological environments. Thereinto, tetrazine bioorthogonal reactions are particularly important and have increasing applications in these fields owing to their unique properties of easily controlled fluorescence or radiation off-on mechanism, which greatly facilitate the tracking of real signals without been disturbed by background. In this mini review, tetrazine bioorthogonal chemistry for in vivo imaging applications will be attentively appraised to raise some guidelines for prior tetrazine bioorthogonal chemical studies.

Acyclic and Heterocyclic Azadiene Diels-Alder Reactions Promoted by Perfluoroalcohol Solvent Hydrogen Bonding: Comprehensive Examination of Scope

Herein, the first use of perfluoroalcohol H-bonding in accelerating acyclic azadiene inverse electron demand cycloaddition reactions is described, and its use in the promotion of heterocyclic azadiene cycloaddition reactions is generalized through examination of a complete range of azadienes. The scope of dienophiles was comprehensively explored; relative reactivity trends and solvent compatibilities were established with respect to the dienophile as well as azadiene; H-bonding solvent effects that lead to rate enhancements, yield improvements, and their impact on regioselectivity and mode of cycloaddition are defined; new viable diene/dienophile reaction partners in the cycloaddition reactions are disclosed; and key comparison rate constants are reported. The perfluoroalcohol effectiveness at accelerating an inverse electron demand Diels-Alder cycloaddition is directly correlated with its H-bond potential (pKa). Not only are the reactions of electron-rich dienophiles accelerated but those of strained and even unactivated alkenes and alkynes are improved, including representative bioorthogonal click reactions.

Bio-orthogonally activated tetraphenylene-tetrazine aggregation-induced emission fluorogenic probes

Tetrazine-based bio-orthogonally activated fluorogenic probes have drawn great attention due to their excellent performance in bioimaging; however, most of them suffer from aggregation-caused quenching (ACQ) problems. Herein, we developed a set of novel tetrazine-modified tetraphenylenes (TPEs) as bio-orthogonally activated aggregation-induced emission (AIE) fluorogenic probes. Both the fluorescence and AIE features are quenched by tetrazine, which is mediated by the through-bond energy-transfer (TBET) mechanism, and are activated upon converting tetrazine to pyridazine via the inverse electron-demand Diels-Alder (iEDDA) reaction. The activated cycloadducts displayed a notable fluorescence enhancement, a large Stokes shift, a high fluorescence quantum yield, and evident AIE-active features. Manipulating the length and position of the π-linker enables fine-tuning of the photophysical properties of the probes, while an overlong planar π-linker leads to AIE-to-ACQ transformation. We also designed bi-tetrazyl-substituted probes, which exhibited a higher turn-on ratio than the mono-tetrazyl analogs owing to the 'double-quenched' function. When they reacted with double-clickable linkers, fluorescent macrocycles were obtained because of the restriction of the free rotation of the phenyl rings of TPE. Using an organelle-pretargeting strategy, we succeeded in applying these probes for mitochondria-specific bio-orthogonal imaging in live cells under no-wash conditions, which is expected to provide a powerful tool for biomedical applications.

Bioconjugation Strategies for Revealing the Roles of Lipids in Living Cells

The structural boundaries of living cells are composed of numerous membrane-forming lipids. Lipids not only are crucial for the cellular compartmentalization but also are involved in cell signaling as well as energy storage. Abnormal lipid levels have been linked to severe human diseases such as cancer, multiple sclerosis, neurodegenerative diseases, as well as lysosomal storage disorders. Given their biological significance, there is immense interest in studying lipids and their effect on cells. However, limiting factors include the low solubility of lipids, their structural complexity, and the challenge of using genetic techniques to directly manipulate lipid structure. Current methods to study lipids rely mostly on lipidomics, which analyzes the composition of lipid extracts using mass spectrometry. Although, these efforts have successfully catalogued and profiled a great number of lipids in cells, many aspects about their exact functional role and subcellular distribution remain enigmatic.In this Account, we outline how our laboratory developed and applied different bioconjugation strategies to study the role of lipids and lipid modifications in cells. Inspired by our ongoing work on developing lipid bioconjugation strategies to generate artificial cell membranes, we developed a ceramide synthesis method in live cells using a salicylaldehyde ester that readily reacts with sphingosine in form of a traceless ceramide ligation. Our study not only confirmed existing knowledge about the association of ceramides with cell death, but also gave interesting new findings about the structure-function relationship of ceramides in apoptosis. Our initial efforts led us to investigate probes that detect endogenous sphingolipids using live cell imaging. We describe the development of a fluorogenic probe that reacts chemoselectively with sphingosine in living cells, enabling the detection of elevated endogenous levels of this biomarker in human disease. Building on our interest in the fluorescence labeling of lipids, we have also explored the use of bioorthogonal reactions to label chemically synthesized lipid probes. We discuss the development of photocaged dihydrotetrazine lipids, where the initiation of the bioorthogonal reaction can be triggered by visible light, allowing for live cell modification of membranes with spatiotemporal control.Finally, proteins are often post-translationally modified by lipids, which have important effects on protein subcellular localization and function. Controlling lipid modifications with small molecule probes could help reveal the function of lipid post-translational modifications and could potentially inspire novel therapeutic strategies. We describe how our previous studies on synthetic membrane formation inspired us to develop an amphiphilic cysteine derivative that depalmitoylates membrane-bound S-acylated proteins in live cells. Ultimately, we applied this amphiphile mediated depalmitoylation (AMD) in studies investigating the palmitoylation of cancer relevant palmitoylated proteins in healthy and diseased cells.

Sensitive Imaging of Cellular RNA via Cascaded Proximity-Induced Fluorogenic Reactions

Owing to its important biological functions, RNA has become a promising molecular biomarker of various diseases. With a dynamic change in its expression level and a relatively low amount within the complicated biological matrix, signal amplification detection based on DNA probes has been put forward, which is helpful for early diagnosis and prognostic prediction. However, conventional methods are confined to cell lysates or dead cells and are not only time-consuming in sample preparation but also inaccessible to the spatial-temporal information of target RNAs. To achieve live-cell imaging of specific RNAs, both the detection sensitivity and intracellular delivery issues should be addressed. Herein, a new cascaded fluorogenic system based on the combination of hybridization chain reactions (HCRs) and proximity-induced bioorthogonal chemistry is developed, in which a bioorthogonal reaction pair (a tetrazine-quenched dye and its complementary dienophile) is brought into spatial proximity upon target RNA triggering the HCR to turn on and amplify the fluorescence in one step, sensitively indicating the cellular distribution of RNA with minimal false positive results caused by unspecific degradation. Facilitated by a biodegradable carrier based on black phosphorus with high loading capacity and excellent biocompatibility, the resulting imaging platform allows wash-free tracking of target RNAs inside living cells.

Rational Design for High Bioorthogonal Fluorogenicity of Tetrazine-Encoded Green Fluorescent Proteins

The development of bioorthogonal fluorogenic probes constitutes a vital force to advance life sciences. Tetrazine-encoded green fluorescent proteins (GFPs) show high bioorthogonal reaction rate and genetic encodability, but suffer from low fluorogenicity. Here, we unveil the real-time fluorescence mechanisms by investigating two site-specific tetrazine-modified superfolder GFPs via ultrafast spectroscopy and theoretical calculations. Förster resonance energy transfer (FRET) is quantitatively modeled and revealed to govern the fluorescence quenching; for GFP150-Tet with a fluorescence turn-on ratio of ~9, it contains trimodal subpopulations with good (P1), random (P2), and poor (P3) alignments between the transition dipole moments of protein chromophore (donor) and tetrazine tag (Tet-v2.0, acceptor). By rationally designing a more free/tight environment, we created new mutants Y200A/S202Y to introduce more P2/P1 populations and improve the turn-on ratios to ~14/31, making the fluorogenicity of GFP150-Tet-S202Y the highest among all up-to-date tetrazine-encoded GFPs. In live eukaryotic cells, the GFP150-Tet-v3.0-S202Y mutant demonstrates notably increased fluorogenicity, substantiating our generalizable design strategy.

An Alliance of Polynitrogen Heterocycles: Novel Energetic Tetrazinedioxide-Hydroxytetrazole-Based Materials

Energetic materials constitute one of the most important subtypes of functional materials used for various applications. A promising approach for the construction of novel thermally stable high-energy materials is based on an assembly of polynitrogen biheterocyclic scaffolds. Herein, we report on the design and synthesis of a new series of high-nitrogen energetic salts comprising the C-C linked 6-aminotetrazinedioxide and hydroxytetrazole frameworks. Synthesized materials were thoroughly characterized by IR and multinuclear NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction and differential scanning calorimetry. As a result of a vast amount of the formed intra- and intermolecular hydrogen bonds, prepared ammonium and amino-1,2,4-triazolium salts are thermally stable and have good densities of 1.75–1.78 g·cm⁻³. All synthesized compounds show high detonation performance, reaching that of benchmark RDX. At the same time, as compared to RDX, investigated salts are less friction sensitive due to the formed net of hydrogen bonds. Overall, reported functional materials represent a novel perspective subclass of secondary explosives and unveil further opportunities for an assembly of biheterocyclic next-generation energetic materials.

Seeking Citius: Photochemical Access of Reactive Intermediates for Faster Bioorthogonal Reactions

Fast bioorthogonal reactions are sought after because of their superior performance in labeling low-abundance biomolecules in native cellular environments. An attractive strategy to increase reaction kinetics is to access the reactive intermediates through photochemical activation. To this end, significant progress was made in the last few years in harnessing two highly reactive intermediates-nitrile imine and tetrazine-generated through photoinduced ring rupture and catalytic photooxidation, respectively. The efficient capture of these reactive intermediates by their cognate reaction partners has enabled bioorthogonal fluorescent labeling of biomolecules in live cells.

Photoactivatable Fluorogenic Azide-Alkyne Click Reaction: A Dual-Activation Fluorescent Probe

Aryl azide and diaryl tetrazole are both photoactive molecules, which can form nitrene and nitrile imine intermediates respectively by photolysis. Depending on the new finding that the azide can suppress the photolysis of tetrazole in the azide-tetrazole conjugated system, we developed aryl azide-tetrazole probes for the photoactivatable fluorogenic azide alkyne click (PFAAC) reaction, in which the aryl azide-tetrazole probes were not phoroactivatable fluorogenic itself, but the triazole products after click reaction were prefluorophore that can be activated by light. Therefore, in PFAAC chemistry, the fluorescent probes can be activated by two orthogonal events: azide-alkyne click reaction and light, which leads to spatiotemporal resolution and high signal-to-noise ratio. This PFAAC process was proved in vitro by copper catalyzed or strain-promoted azide-alkyne reactions and in live cells by spatiotemporally controlled organelle imaging. By incorporation a linker to the azide-tetrazole conjugate, this PFAAC chemistry could covalently label extra probes to the biomolecules and spatiotemporally detecting this process by photoinduced fluorescence.

Functionalized Triazines and Tetrazines: Synthesis and Applications

The molecules possessing triazine and tetrazine moieties belong to a special class of heterocyclic compounds. Both triazines and tetrazines are building blocks and have provided a new dimension to the design of biologically important organic molecules. Several of their derivatives with fine-tuned electronic properties have been identified as multifunctional, adaptable, switchable, remarkably antifungal, anticancer, antiviral, antitumor, cardiotonic, anti-HIV, analgesic, anti-protozoal, etc. The objective of this review is to comprehensively describe the recent developments in synthesis, coordination properties, and various applications of triazine and tetrazine molecules. The rich literature demonstrates various synthetic routes for a variety of triazines and tetrazines through microwave-assisted, solid-phase, metal-based, [4+2] cycloaddition, and multicomponent one-pot reactions. Synthetic approaches contain linear, angular, and fused triazine and tetrazine heterocycles through a combinatorial method. Notably, the triazines and tetrazines undergo a variety of organic transformations, including electrophilic addition, coupling, nucleophilic displacement, and intramolecular cyclization. The mechanistic aspects of these heterocycles are discussed in a detailed way. The bioorthogonal application of these polyazines with various strained alkenes and alkynes provides a new prospect for investigations in chemical biology. This review systematically encapsulates the recent developments and challenges in the synthesis and possible potential applications of various triazine and tetrazine systems.

Bioorthogonally activatable cyanine dye with torsion-induced disaggregation for in vivo tumor imaging

Advancement of bioorthogonal chemistry in molecular optical imaging lies in expanding the repertoire of fluorophores that can undergo fluorescence signal changes upon bioorthogonal ligation. However, most available bioorthogonally activatable fluorophores only emit shallow tissue-penetrating visible light via an intramolecular charge transfer mechanism. Herein, we report a serendipitous “torsion-induced disaggregation (TIDA)” phenomenon in the design of near-infrared (NIR) tetrazine (Tz)-based cyanine probe. The TIDA of the cyanine is triggered upon Tz-transcyclooctene ligation, converting its heptamethine chain from S-trans to S-cis conformation. Thus, after bioorthogonal reaction, the tendency of the resulting cyanine towards aggregation is reduced, leading to TIDA-induced fluorescence enhancement response. This Tz-cyanine probe sensitively delineates the tumor in living mice as early as 5 min post intravenous injection. As such, this work discovers a design mechanism for the construction of bioorthogonally activatable NIR fluorophores and opens up opportunities to further exploit bioorthogonal chemistry in in vivo imaging.

Expanding the responsive dyes repertoire is currently a developing field in biorthogonal chemistry. In this article, the authors develop fluorophores that turn on their near-infrared fluorescence upon biorthogonal reaction based on a “torsion-induced disaggregation” approach, allowing for sensitive in vivo imaging of tumors.

Biosynthesis of Isonitrile- and Alkyne-Containing Natural Products

Natural products are a diverse class of biologically produced compounds that participate in fundamental biological processes such as cell signaling, nutrient acquisition, and interference competition. Unique triple-bond functionalities like isonitriles and alkynes often drive bioactivity and may serve as indicators of novel chemical logic and enzymatic machinery. Yet, the biosynthetic underpinnings of these groups remain only partially understood, constraining the opportunity to rationally engineer biomolecules with these functionalities for applications in pharmaceuticals, bioorthogonal chemistry, and other value-added chemical processes. Here, we focus our review on characterized biosynthetic pathways for isonitrile and alkyne functionalities, their bioorthogonal transformations, and prospects for engineering their biosynthetic machinery for biotechnological applications.

Development of a multi faceted platform containing a tetrazine, fluorophore and chelator: synthesis, characterization, radiolabeling, and immuno-SPECT imaging

BACKGROUND

Combining optical (fluorescence) imaging with nuclear imaging has the potential to offer a powerful tool in personal health care, where nuclear imaging offers in vivo functional whole-body visualization, and the fluorescence modality may be used for image-guided tumor resection. Varying chemical strategies have been exploited to fuse both modalities into one molecular entity. When radiometals are employed in nuclear imaging, a chelator is typically inserted into the molecule to facilitate radiolabeling; the availability of the chelator further expands the potential use of these platforms for targeted radionuclide therapy if a therapeutic radiometal is employed. Herein, a novel mixed modality scaffold which contains a tetrazine (Tz)--for biomolecule conjugation, fluorophore-for optical imaging, and chelator-for radiometal incorporation, in one construct is presented. The novel platform was characterized for its fluorescence properties, radiolabeled with single-photon emission computed tomography (SPECT) isotope indium-111 (¹¹¹In³⁺) and therapeutic alpha emitter actinium-225 (²²⁵Ac³⁺). Both radiolabels were conjugated in vitro to trans-cyclooctene (TCO)-modified trastuzumab; biodistribution and immuno-SPECT imaging of the former conjugate was assessed.

RESULTS

Key to the success of the platform synthesis was incorporation of a 4,4'-dicyano-BODIPY fluorophore. The route gives access to an advanced intermediate where final chelator-incorporated compounds can be easily accessed in one step prior to radiolabeling or biomolecule conjugation. The DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) conjugate was prepared, displayed good fluorescence properties, and was successfully radiolabeled with ¹¹¹In & ²²⁵Ac in high radiochemical yield. Both complexes were then separately conjugated in vitro to TCO modified trastuzumab through an inverse electron demand Diels-Alder (IEDDA) reaction with the Tz. Pilot small animal in vivo immuno-SPECT imaging with [¹¹¹In]In-DO3A-BODIPY-Tz-TCO-trastuzumab was also conducted and exhibited high tumor uptake (21.2 ± 5.6%ID/g 6 days post-injection) with low uptake in non-target tissues.

CONCLUSIONS

The novel platform shows promise as a multi-modal probe for theranostic applications. In particular, access to an advanced synthetic intermediate where tailored chelators can be incorporated in the last step of synthesis expands the potential use of the scaffold to other radiometals. Future studies including validation of ex vivo fluorescence imaging and exploiting the pre-targeting approach available through the IEDDA reaction are warranted.

A Chemical Biology Primer for NMR Spectroscopists

Among structural biology techniques, NMR spectroscopy offers unique capabilities that enable the atomic resolution studies of dynamic and heterogeneous biological systems under physiological and native conditions. Complex biological systems, however, often challenge NMR spectroscopists with their low sensitivity, crowded spectra or large linewidths that reflect their intricate interaction patterns and dynamics. While some of these challenges can be overcome with the development of new spectroscopic approaches, chemical biology can also offer elegant and efficient solutions at the sample preparation stage. In this tutorial, we aim to present several chemical biology tools that enable the preparation of selectively and segmentally labeled protein samples, as well as the introduction of site-specific spectroscopic probes and post-translational modifications. The four tools covered here, namely cysteine chemistry, inteins, native chemical ligation, and unnatural amino acid incorporation, have been developed and optimized in recent years to be more efficient and applicable to a wider range of proteins than ever before. We briefly introduce each tool, describe its advantages and disadvantages in the context of NMR experiments, and offer practical advice for sample preparation and analysis. We hope that this tutorial will introduce beginning researchers in the field to the possibilities chemical biology can offer to NMR spectroscopists, and that it will inspire new and exciting applications in the quest to understand protein function in health and disease.

Overcoming Spectral Dependence: A General Strategy for Developing Far-Red and Near-Infrared Ultra-Fluorogenic Tetrazine Bioorthogonal Probes

Bioorthogonal fluorogenic dyes are indispensable tools in wash-free bioimaging of specific biological targets. However, the fluorogenicity of existing tetrazine-based bioorthogonal probes deteriorates as the emission wavelength shifts towards the NIR window, greatly limiting their applications in live cells and tissues. Herein, we report a generalizable molecular design strategy to construct ultra-fluorogenic dyes via a simple substitution at the meso-positions of various far-red and NIR fluorophores. Our probes demonstrate significant fluorescence turn-on ratios (10² -10³ -fold) in the range 586-806 nm. These results will greatly expand the applications of bioorthogonal chemistry in NIR bioimaging and biosensing.

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.

Tuning the Physicochemical Properties of BODIPY for Bioimaging via meso-Amino Acylation

A series of BODIPY probes with a wide emission range were prepared via aminoacylation at the meso-position. Functional moieties were also introduced to induce bathochromic shifts in emission, improve water solubility, increase Stokes shifts, and construct bioorthogonal turn-on probes. The developed analogues were successfully used in live-cell imaging, suggesting that the described strategy can be used to prepare probes with improved bioimaging potential.

Fluorogenic Cyclopropenones for Multicomponent, Real-Time Imaging

Fluorogenic bioorthogonal reactions enable biomolecule visualization in real time. These reactions comprise reporters that "light up" upon reaction with complementary partners. While the spectrum of fluorogenic chemistries is expanding, few transformations are compatible with live cells due to cross-reactivities or insufficient signal turn-on. To address the need for more suitable chemistries for cellular imaging, we developed a fluorogenic reaction featuring cyclopropenone reporters and phosphines. The transformation involves regioselective activation and cyclization of cyclopropenones to form coumarin products. With optimal probes, the reaction provides >1600-fold signal turn-on, one of the highest fluorescence enhancements reported to date. The bioorthogonal motifs were evaluated in vitro and in cells. The reaction was also found to be compatible with other common fluorogenic transformations, enabling multicomponent, real-time imaging. Collectively, these data suggest that the cyclopropenone-phosphine reaction will bolster efforts to track biomolecule targets in their native settings.