Design of oligothiophene-based tetrazoles for laser-triggered photoclick chemistry in living cells

Bioorthogonally activated probes for precise fluorescence imaging

Over the past two decades, bioorthogonal chemistry has undergone a remarkable development, challenging traditional assumptions in biology and medicine. Recent advancements in the design of probes tailored for bioorthogonal applications have met the increasing demand for precise imaging, facilitating the exploration of complex biological systems. These state-of-the-art probes enable highly sensitive, low background, in situ imaging of biological species and events within live organisms, achieving resolutions comparable to the size of the biomolecule under investigation. This review provides a comprehensive examination of various categories of bioorthogonally activated in situ fluorescent labels. It highlights the intricate design and benefits of bioorthogonal chemistry for precise in situ imaging, while also discussing future prospects in this rapidly evolving field.

Visible and near-infrared light-induced photoclick reactions

Photoclick reactions combine the advantages offered by light-driven processes, that is, non-invasive and high spatiotemporal control, with classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photocrosslinking, protein labelling and bioimaging. Despite these advances, most photoclick reactions typically require near-ultraviolet (UV) and mid-UV light to proceed. UV light can trigger undesirable responses, including cellular apoptosis, and therefore, visible and near-infrared light-induced photoclick reaction systems are highly desirable. Shifting to a longer wavelength can also reduce degradation of the photoclick reagents and products. Several strategies have been used to induce a bathochromic shift in the wavelength of irradiation-initiating photoclick reactions. For instance, the extension of the conjugated π-system, triplet-triplet energy transfer, multi-photon excitation, upconversion technology, photocatalytic and photoinitiation approaches, and designs involving photocages have all been used to achieve this goal. Current design strategies, recent advances and the outlook for long wavelength-driven photoclick reactions are presented.

Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones (PQs) can efficiently react with electron-rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.

Fe3O4@SiO2@SBA-3@CPTMS@Arg-Cu: preparation, characterization, and catalytic performance in the conversion of nitriles to amides and the synthesis of 5-substituted 1H-tetrazoles

A novel, efficient, and recyclable mesoporous Fe3O4@SiO2@SBA-3@CPTMS@Arg-Cu nanocatalyst was synthesized by grafting l-arginine (with the ability to coordinate with Cu) onto a mixed phase of a magnetic mesoporous SBA-3 support. The catalyst was characterized using several techniques, including Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), X-ray diffraction (XRD) analysis, N2 adsorption-desorption analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) analysis, and atomic absorption spectroscopy (AAS). The resulting solid material possessed a surface area of 145 m2 g-1 and a total pore volume of 34 cm3 g-1. The prepared mesoporous material was studied as a practical, recyclable, and chemoselective catalyst in some organic functional group transformations such as the conversion of nitriles to amides and synthesis of 5-substituted 1H-tetrazoles. This novel magnetic nanocatalyst proved to be effective and provided the products in high to excellent yields under green solvent conditions. Meanwhile, the as-prepared Fe3O4@SiO2@SBA-3@CPTMS@Arg-Cu demonstrated excellent reusability and stability under reaction conditions, and its catalytic activity shown only a slight decrease after seven consecutive runs. Therefore, the as-synthesized magnetic Fe3O4@SiO2@SBA-3@CPTMS@Arg-Cu has broad prospects for practical applications, and offers various benefits such as simplicity, nontoxicity, low cost, simple work-up, and an environmentally benign nature.

Photo-activatable Reagents for Bioorthogonal Ligation Reactions

Light-induced bioorthogonal reactions offer spatiotemporal control over selective biomolecular labeling. This review covers the recent advances in the design of photo-activatable reagents for bioorthogonal conjugation reactions in living systems. These reagents are stable in the absence of light, but transformed into reactive species upon light illumination, which then undergo rapid ligation reactions. The light wavelength has been tuned from ultraviolet to near infrared to enable efficient photo-activation in reactions in deep tissues. The most prominent photo-activatable reagents are presented, including tetrazoles, tetrazines, 9,10-phenanthrenequinone, diarylsydnones, and others. A particular focus is on the strategies for improving reaction kinetics and biocompatibility accomplished through careful molecular engineering. The utilities of these photo-activatable reagents are illustrated through a broad range of biological applications, including in vivo protein labeling, positron emission tomography (PET) imaging, responsive hydrogels, and fluorescence microscopy. The further development and optimization of these biocompatible photo-activatable reagents should lead to new chemical biology strategies for studying biomolecular structure and function in living systems.

Metabolic labelling of DNA in cells by means of the "photoclick" reaction triggered by visible light

Two pyrene-tetrazole conjugates were synthesized as photoreactive chromophores that allow for the first time the combination of metabolic labelling of DNA in cells and subsequent bioorthogonal "photoclick" modification triggered by visible light. Two strained alkenes and three alkene-modified nucleosides were used as reactive counterparts and revealed no major differences in their "photoclick" reactivity. This is a significant advantage because it allows 5-vinyl-2'-deoxyuridine to be applied as the smallest possible alkene-modified nucleoside for metabolic labelling of DNA in cells. Both pyrene-tetrazole conjugates show fluorogenicity during the "photoclick" reactions, which is a second advantage for cellular imaging. Living HeLa cells were incubated with 5-vinyl-2'-deoxyuridine for 48 h to ensure one cell division. After fixation, the newly synthesized genomic DNA was successfully labelled by irradiation with visible light at 405 nm and 450 nm. This method is an attractive tool for the visualization of genomic DNA in cells with full spatiotemporal control by the use of visible light as a reaction trigger.

Mechanistic Understanding and Reactivity Analyses for the Photochemistry of Disubstituted Tetrazoles

The photolysis of tetrazoles has undergone extensive research. However, there are still some problems to be solved in terms of mechanistic understanding and reactivity analyses, which leaves room for theoretical calculations. Herein, multiconfiguration perturbation theory at the CASPT2//CASSCF level was employed to account for electron correction effects involved in the photolysis of four disubstituted tetrazoles. Based on calculations of vertical excitation properties and evaluations of intersystem crossing (ISC) efficiencies in the Frank-Condon region, the combination of space and electronic effects is found in maximum-absorption excitation. Two types of ISC (¹ππ* → ³nπ*, ¹ππ* → ³ππ*) are determined in disubstituted tetrazoles, and the obtained rates follow the El-Sayed rule. Through mapping three representative types of minimum energy profiles for the photolysis of 1,5-, and 2,5-disubstituted tetrazoles, a conclusion can be drawn that the photolysis of tetrazoles exhibits reactivity characteristic of bond-breaking selectivity. Kinetic evaluations show that the photogeneration of singlet imidoylnitrene operates predominately over that in the triplet state, which can be confirmed by a double-well model in the triplet potential energy surface of 1,5-disubstituted tetrazole. Similar mechanistic explorations and reactivity analyses were also applied to the photolysis of 2,5-disubstituted tetrazole to unveil fragmentation patterns of nitrile imine generation. All computational efforts allow us to better understand the photoreactions of disubstituted tetrazoles and to provide useful strategies for regulating their unique reactivity.

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.

Fluorescence turn-on by photoligation - bright opportunities for soft matter materials

Photochemical ligation has become an indispensable tool for applications that require spatially addressable functionalisation, both in biology and materials science. Interestingly, a number of photochemical ligations result in fluorescent products, enabling a self-reporting function that provides almost instantaneous visual feedback of the reaction's progress and efficiency. Perhaps no other chemical reaction system allows control in space and time to the same extent, while concomitantly providing inherent feedback with regard to reaction success and location. While photoactivable fluorescent properties have been widely used in biology for imaging purposes, the expansion of the array of photochemical reactions has further enabled its utility in soft matter materials. Herein, we concisely summarise the key developments of fluorogenic-forming photoligation systems and their emerging applications in both biology and materials science. We further summarise the current challenges and future opportunities of exploiting fluorescent self-reporting reactions in a wide array of chemical disciplines.

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.

A tetrazole-ene photoactivatable fluorophore with improved brightness and stability in protic solution

The pyrazoline fluorophore, generated by photoinduced tetrazole-ene cycloaddition, shows faint fluorescence in protic solvents. To suppress this fluorescence-quenching, we rationally designed a series of substituted diaryl tetrazoles at the N-side phenyl ring to produce a tetrazole-ene based photoactivatable fluorophore. Spectroscopic and cellular imaging studies demonstrated that the version of the fluorophore with a bis(trifluoromethyl)benzene substituent exhibited significantly enhanced brightness and photostability.

A novel docetaxel derivative exhibiting potent anti-tumor activity and high safety in preclinical animal models

As a taxoid agent, docetaxel (DTX) exhibits potent antitumor activity. However, severe toxic side effects and acquired multidrug resistance represent its clinical challenges. Herein, a novel docetaxel derivative (DTX-AI) is synthesized via the nucleophilic addition reaction of 4-acetylphenyl carbamate at the C10 position of the DTX framework. DTX-AI exhibits superior cytotoxicity and a higher apoptotic ratio in vitro against DTX-sensitive tumor cells (MCF-7, HeLa and A549 cells) and even DTX-resistant ones (HeLa/PTX cells), but displays less toxicity against normal cells (MRC-5 and L929 cells) compared with DTX. DTX-AI can effectively suppress the growth of HeLa-tumor xenografts in vivo and even induce complete tumor regression. Furthermore, DTX-AI shows sustained effects on the inhibition of A549-tumor xenograft growth and no obvious recurrence, even after the drug administration was stopped for 30 d. More importantly, DTX-AI has significantly reduced long-term and short-term animal toxicity and extended the survival of mice (100%) compared with DTX (0%). DTX-AI is expected to be a promising 'me-better' anti-tumor drug with higher efficiency and lower toxicity for improved chemotherapy in the clinic.

Lights on 2,5-diaryl tetrazoles: applications and limits of a versatile photoclick reaction

Recently, photoclick chemistry emerged as a powerful tool employed in several research fields, from medicinal chemistry and biology to material sciences. The growing interest in this type of chemical process is justified by the possibility to produce complex molecular systems using mild reaction conditions. However, the elevated spatio-temporal control offered by photoclick chemistry is highly intriguing, as it expands the range of applications. In this context, the light-triggered reaction of 2,5-diaryl tetrazoles with dipolarophiles emerged for its interesting features: excellent stability of the substrates, fast reaction kinetic, and the formation of a highly fluorescent product, fundamental for sensing applications. In the last years, 2,5-diaryl tetrazoles have been extensively employed, especially for bioorthogonal ligations, to label biomolecules and nucleic acids. In this review, we summarized recent applications of this interesting photoclick reaction, with a particular focus on biological fields. Moreover, we described the main limits that affect this system and current strategies proposed to overcome these issues. The general discussion here presented could prompt further optimization of the process and pave the way for the development of new original structures and innovative applications.

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Action Plots in Action: In-Depth Insights into Photochemical Reactivity

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Molecular platforms based on biocompatible photoreactions for photomodulation of biological targets

Photoirradiation provides a convenient and biocompatible approach for spatiotemporal modulation of biological systems with photoresponsive components. The construction of molecular platforms with a photoresponse to be integrated into biomolecules for photomodulation has been of great research interest in optochemical biology. In this review, we summarize typical molecular platforms that are integratable with biomolecules for photomodulation purposes. We categorize these molecular platforms according to their excitation light source, namely ultraviolet (UV), visible (Vis) or near-infrared (NIR) light. The protype chemistry of these molecular platforms is introduced along with an overview of their most recent applications for spatiotemporal regulation of biomolecular function in living cells or mice models. Challenges and the outlook are also presented. We hope this review paper will contribute to further progress in the development of molecular platforms and their biomedical use.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Visible-light induced photo-click and release strategy between monoarylsydnone and phenoxylfumarate

We report a visible-light induced photo-click and release platform between monoarylsydnone (MASyd) and phenoxylfumarates. The pyrazoline produced by the cycloaddition undergoes a photo-aromatization to form a fluorescent pyrazole. Meanwhile, the photo-aromatization also serves as the driving force to release fluorophores that are quenched in the form of phenoxylfumarates.

Recent Developments and Strategies for Mutually Orthogonal Bioorthogonal Reactions

Over the past decade, several different metal-free bioorthogonal reactions have been developed to enable simultaneous double-click labeling with minimal-to-no competing cross-reactivities; such transformations are termed 'mutually orthogonal'. More recently, several examples of successful triple ligation strategies have also been described. In this minireview, we discuss selected aspects of the development of orthogonal bioorthogonal reactions over the past decade, including general strategies to drive future innovations to achieve simultaneous, mutually orthogonal click reactions in one pot.

Photoactivated In Situ Generation of Near Infrared Cyanines for Spatiotemporally Controlled Fluorescence Imaging in Living Cells

Photoactivated trimerization of 2,3,3-trimethyl-3H-indole derivatives created near infrared fluorophore Cy5. The synthetic method is air-tolerant, photosensitizer free, metal free, and condensation agent free. Living cells make Cy5 on a time scale of minutes under white light irradiation at a low power intensity, with the monomer as the only exogenous agent. The new method is promising to find applications in cell studies for in situ spatiotemporally controlled fluorescence imaging in living cells.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Laser Photodissociation Action Spectroscopy for the Wavelength-Dependent Evaluation of Photoligation Reactions

The nitrile imine-mediated tetrazole-ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N-ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

Green-light induced cycloadditions

We introduce a red-shifted tetrazole that is able to undergo efficient nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) under blue and green light irradiation. We provide a detailed wavelength-dependent reactivity map, and employ a number of LEDs for high-conversion small molecule and polymer end-group modification.

Selective analysis of newly synthesized proteins by combining fluorescence correlation spectroscopy with bioorthogonal non-canonical amino acid tagging

FCS with the BONCAT strategy is a promising approach for analysis of newly synthesized proteins and also be extended to further application for studying physiological or pathological processes related to proteins or other metabolic molecular changes.

Transition-Metal-Free Coupling of 1,3-Dipoles and Boronic Acids as a Sustainable Approach to C-C Bond Formation

The need for alternative, complementary approaches to enable C-C bond formation within organic chemistry is an on-going challenge in the area. Of particular relevance are transformations that proceed in the absence of transition-metal reagents. In the current study, we report a comprehensive investigation of the coupling of nitrile imines and aryl boronic acids as an approach towards sustainable C-C bond formation. In situ generation of the highly reactive 1,3-dipole facilitates a Petasis-Mannich-type coupling via a nucleophilic boronate complex. The introduction of hydrazonyl chlorides as a complementary nitrile imine source to the 2,5-tetrazoles previously reported by our laboratory further broadens the scope of the approach. Additionally, we exemplify for the first time the extension of this protocol into another 1,3-dipole, through the synthesis of aryl ketone oximes from aryl boronic acids and nitrile N-oxides.

Developing bioorthogonal probes to span a spectrum of reactivities

Bioorthogonal chemistries enable researchers to interrogate biomolecules in living systems. These reactions are highly selective and biocompatible and can be performed in many complex environments. However, like any organic transformation, there is no perfect bioorthogonal reaction. Choosing the "best fit" for a desired application is critical. Correspondingly, there must be a variety of chemistries-spanning a spectrum of rates and other features-to choose from. Over the past few years, significant strides have been made towards not only expanding the number of bioorthogonal chemistries, but also fine-tuning existing reactions for particular applications. In this Review, we highlight recent advances in bioorthogonal reaction development, focusing on how physical organic chemistry principles have guided probe design. The continued expansion of this toolset will provide more precisely tuned reagents for manipulating bonds in distinct environments.

Photo-controllable bioorthogonal chemistry for spatiotemporal control of bio-targets in living systems

The establishment of bioorthogonal chemistry is one of the most significant advances in chemical biology using exogenous chemistry to perturb and study biological processes. Photo-modulation of biological systems has realized temporal and spatial control on biomacromolecules in living systems. The combination of photo-modulation and bioorthogonal chemistry is therefore emerging as a new direction to develop new chemical biological tools with spatiotemporal resolution. This minireview will focus on recent development of bioorthogonal chemistry subject to spatiotemporal control through photo-irradiation. Different strategies to realize photo-control on bioorthogonal bond-forming reactions and biological applications of photo-controllable bioorthogonal reactions will be summarized to give a perspective on how the innovations on photo-chemistry can contribute to the development of optochemical biology. Future trends to develop more optochemical tools based on novel photochemistry will also be discussed to envision the development of chemistry-oriented optochemical biology.

Direct ring-strain loading for visible-light accelerated bioorthogonal ligation via diarylsydnone-dibenzo[b,f ][1,4,5]thiadiazepine photo-click reactions

Ultra-fast and selective covalent-bond forming reactions with spatiotemporal controllability are foundational for developing a bioorthogonal approach with high manipulability. However, it is challenging to exploit a reporter functional group to achieve these requirements simultaneously. Here, 11H-Dibenzo[c,f][1,2]diazepine and a set of heterocyclic analogues are investigated for both their photo-switching natures and their ability to serve as dipolarophiles in photo-click reactions with diarylsydnone. Sulfur-containing dibenzothiadiazepine (DBTD) is discovered to be an excellent chemical reporter in cycloaddition with visible-light excitation for in-situ ring-strain loading via its (Z) → (E) photo-isomerization. The bioorthogonal utility of the DBTD tag in spatiotemporally controlled ligation for protein modifications on live cells is also demonstrated.

Sterically shielded tetrazoles for a fluorogenic photoclick reaction: tuning cycloaddition rate and product fluorescence

A panel of sterically shielded tetrazoles with different N-aryl groups were synthesized and subsequently evaluated in the photoinduced tetrazole-alkene cycloaddition reaction. It was found that increase in the HOMO energy of the corresponding nitrile imines leads to a faster cycloaddition reaction along with a red shift in the fluorescence emission of the pyrazoline cycloadduct.

Modular activatable bioorthogonal reagents

The bioorthogonal reaction toolbox contains approximately two-dozen unique chemistries that permit selective tagging and probing of biomolecules. Over the past two decades, significant effort has been devoted to optimizing and discovering bioorthogonal reagents that are faster, fluorogenic, and orthogonal to the already existing bioorthogonal repertoire. Conversely, efforts to explore bioorthogonal reagents whose reactivity can be controlled in space and/or time are limited. The "activatable" bioorthogonal reagents that do exist are often unimodal, meaning that their reagent's activation method cannot be easily modified to enable activation with red-shifted wavelengths, enzymes, or metabolic-byproducts and ions like H₂O₂ or Fe³⁺. Here, we summarize the available activatable bioorthogonal reagents with a focus on our recent addition: modular caged cyclopropenes. We designed caged cyclopropenes to be unreactive to their bioorthogonal partner until they are activated through the removal of the cage by light, an enzyme, or another reaction partner. To accomplish this, their structure includes a nitrogen atom at the cyclopropene C3 position that is decorated with the desired caging group through a carbamate linkage. This 3-N cyclopropene system can allow control of cyclopropene reactivity using a multitude of already available photo- and enzyme-caging groups. Additionally, this cyclopropene scaffold can enable metabolic-byproduct or ion activation of bioorthogonal reactions.

Photoinducible Oncometabolite Detection

Dysregulated metabolism can fuel cancer by altering the production of bioenergetic building blocks and directly stimulating oncogenic gene-expression programs. However, relatively few optical methods for the direct study of metabolites in cells exist. To address this need and facilitate new approaches to cancer treatment and diagnosis, herein we report an optimized chemical approach to detect the oncometabolite fumarate. Our strategy employs diaryl tetrazoles as cell-permeable photoinducible precursors to nitrileimines. Uncaging these species in cells and cell extracts enables them to undergo 1,3-dipolar cycloadditions with endogenous dipolarophile metabolites such as fumarate to form pyrazoline cycloadducts that can be readily detected by their intrinsic fluorescence. The ability to photolytically uncage diaryl tetrazoles provides greatly improved sensitivity relative to previous methods, and enables the facile detection of dysregulated fumarate metabolism through biochemical activity assays, intracellular imaging, and flow cytometry. Our studies showcase an intersection of bioorthogonal chemistry and metabolite reactivity that can be applied for biological profiling, imaging, and diagnostics.

Fluorogenic "Photoclick" Labeling and Imaging of DNA with Coumarin-Fused Tetrazole in Vivo

Photoclickable fluorogenic probes will enable visualization of specific biomolecules with precise spatiotemporal control in their native environment. However, the fluorogenic tagging of DNA with current photocontrolled clickable probes is still challenging. Herein, we demonstrated the fast (19.5 ± 2.5 M^-1 s^-1) fluorogenic labeling and imaging of DNA in vitro and in vivo with rationally designed coumarin-fused tetrazoles under UV LED photoirradiation. With a water-soluble, nuclear-specific coumarin-fused tetrazole (CTz-SO₃), the metabolically synthesized DNA in cultured cells was effectively labeled and visualized, without fixation, via "photoclick" reaction. Moreover, the photoclickable CTz-SO₃ enabled real-time, spatially controlled imaging of DNA in live zebrafish.

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

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

Sterically Shielded, Stabilized Nitrile Imine for Rapid Bioorthogonal Protein Labeling in Live Cells

In pursuit of fast bioorthogonal reactions, reactive moieties have been increasingly employed for selective labeling of biomolecules in living systems, posing a challenge in attaining reactivity without sacrificing selectivity. To address this challenge, here we report a bioinspired strategy in which molecular shape controls the selectivity of a transient, highly reactive nitrile imine dipole. By tuning the shape of structural pendants attached to the ortho position of the N-aryl ring of diaryltetrazoles-precursors of nitrile imines, we discovered a sterically shielded nitrile imine that favors the 1,3-dipolar cycloaddition over the competing nucleophilic addition. The photogenerated nitrile imine exhibits an extraordinarily long half-life of 102 s in aqueous medium, owing to its unique molecular shape that hinders the approach of a nucleophile as shown by DFT calculations. The utility of this sterically shielded nitrile imine in rapid (∼1 min) bioorthogonal labeling of glucagon receptor in live mammalian cells was demonstrated.