Design and synthesis of highly reactive dienophiles for the tetrazine-trans-cyclooctene ligation

RNA-peptide conjugate synthesis by inverse-electron demand Diels-Alder reaction

Here we report an efficient method for the synthesis of RNA-peptide conjugates by inverse-electron demand Diels-Alder reaction. Various dienophiles were enzymatically incorporated into RNA and reacted with a chemically synthesized diene-modified peptide. The Diels-Alder reaction proceeds with near-quantitative yields in aqueous solution with stoichiometric amounts of reactants, even at low micromolar concentrations.

Systematic Evaluation of Bioorthogonal Reactions in Live Cells with Clickable HaloTag Ligands: Implications for Intracellular Imaging

Bioorthogonal reactions, including the strain-promoted azide–alkyne cycloaddition (SPAAC) and inverse electron demand Diels–Alder (iEDDA) reactions, have become increasingly popular for live-cell imaging applications. However, the stability and reactivity of reagents has never been systematically explored in the context of a living cell. Here we report a universal, organelle-targetable system based on HaloTag protein technology for directly comparing bioorthogonal reagent reactivity, specificity, and stability using clickable HaloTag ligands in various subcellular compartments. This system enabled a detailed comparison of the bioorthogonal reactions in live cells and informed the selection of optimal reagents and conditions for live-cell imaging studies. We found that the reaction of sTCO with monosubstituted tetrazines is the fastest reaction in cells; however, both reagents have stability issues. To address this, we introduced a new variant of sTCO, Ag-sTCO, which has much improved stability and can be used directly in cells for rapid bioorthogonal reactions with tetrazines. Utilization of Ag complexes of conformationally strained trans-cyclooctenes should greatly expand their usefulness especially when paired with less reactive, more stable tetrazines.

Genetically encoded unstrained olefins for live cell labeling with tetrazine dyes

A number of non-canonical amino acids (NCAAs) with unstrained olefins are genetically encoded using mutant pyrrolysyl-tRNA synthetase-tRNA(Pyl)(CUA) pairs. These NCAAs readily undergo inverse electron-demand Diels-Alder cycloadditions with tetrazine dyes, leading to selective labeling of proteins bearing these NCAAs in live cells.

AlCl₃-Catalyzed Ring Expansion Cascades of Bicyclic Cyclobutenamides Involving Highly Strained Cis,Trans-Cycloheptadienone Intermediates

We report the first experimental evidence for the generation of highly strained cis,trans-cycloheptadienones by electrocyclic ring opening of 4,5-fused cyclobutenamides. In the presence of AlCl3, the cyclobutenamides rearrange to [2.2.1]-bicyclic ketones; DFT calculations provide evidence for a mechanism involving torquoselective 4π-electrocyclic ring opening to a cis,trans-cycloheptadienone followed by a Nazarov-like recyclization and a 1,2-alkyl shift. Similarly, 4,6-fused cyclobutenamides undergo AlCl3-catalyzed rearrangements to [3.2.1]-bicyclic ketones through cis,trans-cyclooctadienone intermediates. The products can be further elaborated via facile cascade reactions to give complex tri- and tetracyclic molecules.

3,6-Substituted-1,2,4,5-tetrazines: tuning reaction rates for staged labeling applications

Cycloaddition reactions involving tetrazines have proven to be powerful bioorthogonal tools for various applications. Conceivably, sequential and selective labeling using tetrazine-based reactions can be achieved by tuning the reaction rate. By varying the substituents on tetrazines, cycloaddition rate variations of over 200 fold have been achieved with the same dienophile. Upon coupling with different dienophiles, such as norbornene, the reaction rate difference can be over 14,000 fold. These substituted tetrazines can be very useful for selective labeling under different conditions.

Modular and orthogonal synthesis of hybrid polymers and networks

Biomaterials scientists strive to develop polymeric materials with distinct chemical make-up, complex molecular architectures, robust mechanical properties and defined biological functions by drawing inspirations from biological systems. Salient features of biological designs include (1) repetitive presentation of basic motifs; and (2) efficient integration of diverse building blocks. Thus, an appealing approach to biomaterials synthesis is to combine synthetic and natural building blocks in a modular fashion employing novel chemical methods. Over the past decade, orthogonal chemistries have become powerful enabling tools for the modular synthesis of advanced biomaterials. These reactions require building blocks with complementary functionalities, occur under mild conditions in the presence of biological molecules and living cells and proceed with high yield and exceptional selectivity. These chemistries have facilitated the construction of complex polymers and networks in a step-growth fashion, allowing facile modulation of materials properties by simple variations of the building blocks. In this review, we first summarize features of several types of orthogonal chemistries. We then discuss recent progress in the synthesis of step growth linear polymers, dendrimers and networks that find application in drug delivery, 3D cell culture and tissue engineering. Overall, orthogonal reactions and modulular synthesis have not only minimized the steps needed for the desired chemical transformations but also maximized the diversity and functionality of the final products. The modular nature of the design, combined with the potential synergistic effect of the hybrid system, will likely result in novel hydrogel matrices with robust structures and defined functions.

Improved metabolic stability for 18F PET probes rapidly constructed via tetrazine trans-cyclooctene ligation

The fast kinetics and bioorthogonal nature of the tetrazine trans-cyclooctene (TCO) ligation makes it a unique tool for PET probe construction. In this study, we report the development of an (18)F-labeling system based on a CF3-substituted diphenyl-s-tetrazine derivative with the aim of maintaining high reactivity while increasing in vivo stability. c(RGDyK) was tagged by a CF3-substituted diphenyl-s-tetrazine derivative via EDC-mediated coupling. The resulting tetrazine-RGD conjugate was combined with a (19)F-labeled TCO derivative to give HPLC standards. The analogous (18)F-labeled TCO derivative was combined with the diphenyl-s-tetrazine-RGD at μM concentration. The resulting tracer was subjected to in vivo metabolic stability assessment, and microPET studies in murine U87MG xenograft models. The diphenyl-s-tetrazine-RGD combines with an (18)F-labeled TCO in high yields (>97% decay-corrected on the basis of TCO) using only 4 equiv of tetrazine-RGD relative to the (18)F-labeled TCO (concentration calculated based on product's specific activity). The radiochemical purity of the (18)F-RGD peptides was >95% and the specific activity was 111 GBq/μmol. Noninvasive microPET experiments demonstrated that (18)F-RGD had integrin-specific tumor uptake in subcutaneous U87MG glioma. In vivo metabolic stability of (18)F-RGD in blood, urine, and major organs showed two major peaks: one corresponded to the Diels-Alder conjugate and the other was identified as the aromatized analog. A CF3-substituted diphenyl-s-tetrazine displays excellent speed and efficiency in (18)F-PET probe construction, providing nearly quantitative (18)F labeling within minutes at low micromolar concentrations. The resulting conjugates display improved in vivo metabolic stability relative to our previously described system.

Harnessing the bioorthogonal inverse electron demand Diels-Alder cycloaddition for pretargeted PET imaging

Due to their exquisite affinity and specificity, antibodies have become extremely promising vectors for the delivery of radioisotopes to cancer cells for PET imaging. However, the necessity of labeling antibodies with radionuclides with long physical half-lives often results in high background radiation dose rates to non-target tissues. In order to circumvent this issue, we have employed a pretargeted PET imaging strategy based on the inverse electron demand Diels-Alder cycloaddition reaction. The methodology decouples the antibody from the radioactivity and thus exploits the positive characteristics of antibodies, while eschewing their pharmacokinetic drawbacks. The system is composed of four steps: (1) the injection of a mAb-trans-cyclooctene (TCO) conjugate; (2) a localization time period during which the antibody accumulates in the tumor and clears from the blood; (3) the injection of the radiolabeled tetrazine; and (4) the in vivo click ligation of the components followed by the clearance of excess radioligand. In the example presented in the work at hand, a (64)Cu-NOTA-labeled tetrazine radioligand and a trans-cyclooctene-conjugated humanized antibody (huA33) were successfully used to delineate SW1222 colorectal cancer tumors with high tumor-to-background contrast. Further, the pretargeting methodology produces high quality images at only a fraction of the radiation dose to non-target tissue created by radioimmunoconjugates directly labeled with (64)Cu or (89)Zr. Ultimately, the modularity of this protocol is one of its greatest assets, as the trans-cyclooctene moiety can be appended to any non-internalizing antibody, and the tetrazine can be attached to a wide variety of radioisotopes.

Enhancing reactivity for bioorthogonal pretargeting by unmasking antibody-conjugated trans-cyclooctenes

The bioorthogonal cycloaddition reaction between tetrazine and trans-cyclooctene (TCO) is rapidly growing in use for molecular imaging and cell-based diagnostics. We have surprisingly uncovered that the majority of TCOs conjugated to monoclonal antibodies using standard amine-coupling procedures are nonreactive. We show that antibody-bound TCOs are not inactivated by trans-cis isomerization and that the bulky cycloaddition reaction is not sterically hindered. Instead, TCOs are likely masked by hydrophobic interactions with the antibody. We show that introducing TCO via hydrophilic poly(ethylene glycol) (PEG) linkers can fully preserve reactivity, resulting in >5-fold enhancement in functional density without affecting antibody binding. This is accomplished using a novel dual bioorthogonal approach in which heterobifunctional dibenzylcyclooctyne (DBCO)-PEG-TCO molecules are reacted with azido-antibodies. Improved imaging capabilities are demonstrated for different cancer biomarkers using tetrazine-modified fluorophore and quantum dot probes. We believe that the PEG linkers prevent TCOs from burying within the antibody during conjugation, which could be relevant to other bioorthogonal tags and biomolecules. We expect the improved TCO reactivity obtained using the reported methods will significantly advance bioorthogonal pretargeting applications.

Post-synthesis DNA modifications using a trans-cyclooctene click handle

Post-synthesis DNA modification is a very useful method for DNA functionalization. This is achieved by using a modified NTP, which has a handle for further modifications, replacing the corresponding natural NTP in polymerase-catalyzed DNA synthesis. Subsequently, the handle can be used for further functionalization after PCR, preferably through a very fast reaction. Herein we describe polymerase-mediated incorporation of trans-cyclooctene modified thymidine triphosphate (TCO-TTP). Subsequently, the trans-cyclooctene group was reacted with a tetrazine tethered to other functional groups through a very fast click reaction. The utility of this DNA functionalization method was demonstrated with the incorporation of a boronic acid group and a fluorophore. The same approach was also successfully used in modifying a known aptamer for fluorescent labelling applications.

Bioorthogonal tetrazine-mediated transfer reactions facilitate reaction turnover in nucleic acid-templated detection of microRNA

Tetrazine ligations have proven to be a powerful bioorthogonal technique for the detection of many labeled biomolecules, but the ligating nature of these reactions can limit reaction turnover in templated chemistry. We have developed a transfer reaction between 7-azabenzonorbornadiene derivatives and fluorogenic tetrazines that facilitates turnover amplification of the fluorogenic response in nucleic acid-templated reactions. Fluorogenic tetrazine-mediated transfer (TMT) reaction probes can be used to detect DNA and microRNA (miRNA) templates to 0.5 and 5 pM concentrations, respectively. The endogenous oncogenic miRNA target mir-21 could be detected in crude cell lysates and detected by imaging in live cells. Remarkably, the technique is also able to differentiate between miRNA templates bearing a single mismatch with high signal to background. We imagine that TMT reactions could find wide application for amplified fluorescent detection of clinically relevant nucleic acid templates.

Design strategies for bioorthogonal smart probes

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

Self-assembly of random copolymers

Self-assembly of random copolymers has attracted considerable attention recently. In this feature article, we highlight the use of random copolymers to prepare nanostructures with different morphologies and to prepare nanomaterials that are responsive to single or multiple stimuli. The synthesis of single-chain nanoparticles from random copolymers and their potential applications are also discussed in some detail. We aim to draw more attention to these easily accessible copolymers, which are likely to play an important role in translational polymer research.

Click chemistry in complex mixtures: bioorthogonal bioconjugation

The selective chemical modification of biological molecules drives a good portion of modern drug development and fundamental biological research. While a few early examples of reactions that engage amine and thiol groups on proteins helped establish the value of such processes, the development of reactions that avoid most biological molecules so as to achieve selectivity in desired bond-forming events has revolutionized the field. We provide an update on recent developments in bioorthogonal chemistry that highlights key advances in reaction rates, biocompatibility, and applications. While not exhaustive, we hope this summary allows the reader to appreciate the rich continuing development of good chemistry that operates in the biological setting.

Selective chemical protein modification

Chemical modification of proteins is an important tool for probing natural systems, creating therapeutic conjugates and generating novel protein constructs. Site-selective reactions require exquisite control over both chemo- and regioselectivity, under ambient, aqueous conditions. There are now various methods for achieving selective modification of both natural and unnatural amino acids--each with merits and limitations--providing a 'toolkit' that until 20 years ago was largely limited to reactions at nucleophilic cysteine and lysine residues. If applied in a biologically benign manner, this chemistry could form the basis of true Synthetic Biology.

Two rapid catalyst-free click reactions for in vivo protein labeling of genetically encoded strained alkene/alkyne functionalities

Detailed kinetic analyses of inverse electron-demand Diels–Alder cycloaddition and nitrilimine-alkene/alkyne 1,3-diploar cycloaddition reactions were conducted and the reactions were applied for rapid protein bioconjugation. When reacted with a tetrazine or a diaryl nitrilimine, strained alkene/alkyne entities including norbornene, trans-cyclooctene, and cyclooctyne displayed rapid kinetics. To apply these “click” reactions for site-specific protein labeling, five tyrosine derivatives that contain a norbornene, trans-cyclooctene, or cyclooctyne entity were genetically encoded into proteins in Escherichia coli using an engineered pyrrolysyl-tRNA synthetase-tRNA_CUA^Pyl pair. Proteins bearing these noncanonical amino acids were successively labeled with a fluorescein tetrazine dye and a diaryl nitrilimine both in vitro and in living cells.

Kinetic studies of inverse electron demand Diels-Alder reactions (iEDDA) of norbornenes and 3,6-dipyridin-2-yl-1,2,4,5-tetrazine

Inverse electron demand Diels-Alder additions (iEDDA) between 1,2,4,5-tetrazines and olefins have recently found widespread application as a novel 'click chemistry' scheme and as a mild technique for the modification of materials. Norbornenes are, due to their straightforward synthetic availability, especially interesting in the latter context. Therefore, the reactivity of different norbornene-based compounds was compared with unsubstituted norbornene and other alkenes using UV-vis measurements for the determination of reaction rates under pseudo first order conditions. Thereby, exo,exo-5-norbornene-2,3-dimethanol was found to be almost as reactive as unsubstituted norbornene whereas (±)-endo,exo-dimethyl-5-norbornene-2,3-dicarboxylate reacted only insignificanty faster than unstrained alkenes.

Interfacial Bioorthogonal Cross-Linking

Described herein is interfacial bioorthogonal cross-linking, the use of bioorthogonal chemistry to create and pattern biomaterials through diffusion-controlled gelation at the liquid-gel interface. The basis is a rapid (k₂ 284000 M^–1 s^–1) reaction between strained trans-cyclooctene (TCO) and tetrazine (Tz) derivatives. Syringe delivery of Tz-functionalized hyaluronic acid (HA-Tz) to a bath of bis-TCO cross-linker instantly creates microspheres with a cross-linked shell through which bis-TCO diffuses freely to introduce further cross-linking at the interface. Tags can be introduced with 3D resolution without external triggers or templates. Water-filled hydrogel channels were prepared by simply reversing the order of addition. Prostate cancer cells encapsulated in the microspheres have 99% viability, proliferate readily, and form aggregated clusters. This process is projected to be useful in the fabrication of cell-instructive matrices for in vitro tissue models.

Potential of cycloaddition reactions to generate cytotoxic metal drugs in vitro

Severe general toxicity issues blight many chemotherapeutics utilized in the treatment of cancers, resulting in the need for more selective drugs able to exert their biological activity at only the required location(s). Toward this aim, we report the development of an organometallic ruthenium compound, functionalized through a η(6)-bound arene ligand with a bicyclononyne derivative, able to participate in strain-promoted cycloaddition reactions with tetrazines. We show that combination of the ruthenium compound with a ditetrazine in biological media results in the in situ formation of a dinuclear molecule that is more cytotoxic toward cancer cells than the starting mononuclear ruthenium compound and tetrazine components. Such an approach may be extended to in vivo applications to construct a cytotoxic metallodrug at a tumor site, providing a novel approach toward the turn-on cytotoxicity of metallodrugs in the treatment of cancer.

Torquoselective ring opening of fused cyclobutenamides: evidence for a cis,trans-cyclooctadienone intermediate

Electrocyclic ring opening of 4,6-fused cyclobutenamides 1 under thermal conditions leads to cis,trans-cyclooctadienones 2-E,E as transient intermediates, en route to 5,5-bicyclic products 3. Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving cis,trans-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.

Development of a (18) F-labeled tetrazine with favorable pharmacokinetics for bioorthogonal PET imaging

A low-molecular-weight (18) F-labeled tetrazine derivative was developed as a highly versatile tool for bioorthogonal PET imaging. Prosthetic groups and undesired carrying of (18) F through additional steps were evaded by direct (18) F-fluorination of an appropriate tetrazine precursor. Reaction kinetics of the cycloaddition with trans-cyclooctenes were investigated by applying quantum chemical calculations and stopped-flow measurements in human plasma; the results indicated that the labeled tetrazine is suitable as a bioorthogonal probe for the imaging of dienophile-tagged (bio)molecules. In vitro and in vivo investigations revealed high stability and PET/MRI in mice showed fast homogeneous biodistribution of the (18) F-labeled tetrazine that also passes the blood-brain barrier. An in vivo click experiment confirmed the bioorthogonal behavior of this novel tetrazine probe. Due to favorable chemical and pharmacokinetic properties this bioorthogonal agent should find application in bioimaging and biomedical research.

Exploiting the tetrazine-norbornene reaction for single polymer chain collapse

Single chain polymer nanoparticles (SCNPs) have been formed using polystyrenes decorated with pendent norbornenes and a bifunctional tetrazine crosslinker. Characterisation by size exclusion chromatography and (1)H NMR gives evidence for the formation of SCNPs by the tetrazine-norbornene reaction, whilst light scattering, neutron scattering, transmission electron microscopy and atomic force microscopy show that discrete well-defined nanoparticles are formed and their size in solution calculated.

The inverse electron demand Diels-Alder click reaction in radiochemistry

The inverse electron-demand Diels-Alder (IEDDA) cycloaddition between 1,2,4,5-tetrazines and strained alkene dienophiles is an emergent variety of catalyst-free 'click' chemistry that has the potential to have a transformational impact on the synthesis and development of radiopharmaceuticals. The ligation is selective, rapid, high-yielding, clean, and bioorthogonal and, since its advent in 2008, has been employed in a wide variety of chemical settings. In radiochemistry, the reaction has proven particularly useful with (18)  F and has already been utilized to create a number of (18)  F-labeled agents, including the PARP1-targeting small molecule (18)  F-AZD2281, the αv β3 integrin-targeting peptide (18)  F-RGD, and the GLP-1-targeting peptide (18)  F-exendin. The inherent flexibility of the ligation has also been applied to the construction of radiometal-based probes, specifically the development of a modular strategy for the synthesis of radioimmunoconjugates that effectively eliminates variability in the construction of these agents. Further, the exceptional speed and biorthogonality of the reaction have made it especially promising in the realm of in vivo pretargeted imaging and therapy, and pretargeted imaging strategies based on the isotopes (111) In, (18)  F, and (64) Cu have already proven capable of producing images with high tumor contrast and low levels of uptake in background, nontarget organs. Ultimately, the characteristics of inverse electron-demand Diels-Alder click chemistry make it almost uniquely well-suited for radiochemistry, and although the field is young, this ligation has the potential to make a tremendous impact on the synthesis, development, and study of novel radiopharmaceuticals.

Bioorthogonal chemistry: implications for pretargeted nuclear (PET/SPECT) imaging and therapy

Due to their rapid and highly selective nature, bioorthogonal chemistry reactions are attracting a significant amount of recent interest in the radiopharmaceutical community. Over the last few years, reactions of this type have found tremendous utility in the construction of new radiopharmaceuticals and as a method of bioconjugation. Furthermore, reports are beginning to emerge in which these reactions are also being applied in vivo to facilitate a novel pretargeting strategy for the imaging and therapy of cancer. The successful implementation of such an approach could lead to dramatic improvements in image quality, therapeutic index, and reduced radiation dose to non-target organs and tissues. This review will focus on the potential of various bioorthogonal chemistry reactions to be used successfully in such an approach.

Design of spiro[2.3]hex-1-ene, a genetically encodable double-strained alkene for superfast photoclick chemistry

Reactive yet stable alkene reporters offer a facile route to studying fast biological processes via the cycloaddition-based bioorthogonal reactions. Here, we report the design and synthesis of a strained spirocyclic alkene, spiro[2.3]hex-1-ene (Sph), for an accelerated photoclick chemistry, and its site-specific introduction into proteins via amber codon suppression using the wild-type pyrrolysyl-tRNA synthetase/tRNA_CUA pair. Because of its high ring strain and reduced steric hindrance, Sph exhibited fast reaction kinetics (k₂ up to 34 000 M^–1 s^–1) in the photoclick chemistry and afforded rapid (<10 s) bioorthogonal protein labeling.

Synthesis and reactivity comparisons of 1-methyl-3-substituted cyclopropene mini-tags for tetrazine bioorthogonal reactions

Substituted cyclopropenes have recently attracted attention as stable "mini-tags" that are highly reactive dienophiles with the bioorthogonal tetrazine functional group. Despite this interest, the synthesis of stable cyclopropenes is not trivial and their reactivity patterns are poorly understood. Here, the synthesis and comparison of the reactivity of a series of 1-methyl-3-substituted cyclopropenes with different functional handles is described. The rates at which the various substituted cyclopropenes undergo Diels-Alder cycloadditions with 1,2,4,5-tetrazines were measured. Depending on the substituents, the rates of cycloadditions vary by over two orders of magnitude. The substituents also have a dramatic effect on aqueous stability. An outcome of these studies is the discovery of a novel 3-amidomethyl substituted methylcyclopropene tag that reacts twice as fast as the fastest previously disclosed 1-methyl-3-substituted cyclopropene while retaining excellent aqueous stability. Furthermore, this new cyclopropene is better suited for bioconjugation applications and this is demonstrated through using DNA templated tetrazine ligations. The effect of tetrazine structure on cyclopropene reaction rate was also studied. Surprisingly, 3-amidomethyl substituted methylcyclopropene reacts faster than trans-cyclooctenol with a sterically hindered and extremely stable tert-butyl substituted tetrazine. Density functional theory calculations and the distortion/interaction analysis of activation energies provide insights into the origins of these reactivity differences and a guide to the development of future tetrazine coupling partners. The newly disclosed cyclopropenes have kinetic and stability advantages compared to previously reported dienophiles and will be highly useful for applications in organic synthesis, bioorthogonal reactions, and materials science.

The nitrilimine-alkene cycloaddition is an ultra rapid click reaction

The transient formation of nitrilimine in aqueous conditions is greatly influenced by pH and chloride. In basic conditions (pH 10) with no chloride, a diarylnitrilimine precursor readily ionizes to form diarylnitrilimine that reacts almost instantly with an acrylamide-containing protein and fluorescently labels it.

Conformationally Strained trans-Cyclooctene with Improved Stability and Excellent Reactivity in Tetrazine Ligation

Computation has guided the design of conformationally-strained dioxolane-fused trans-cyclooctene (d-TCO) derivatives that display excellent reactivity in the tetrazine ligation. A water soluble derivative of 3,6-dipyridyl-s-tetrazine reacts with d-TCO with a second order rate k₂ 366,000 (+/- 15,000) M^-1s^-1 at 25 °C in pure water. Furthermore, d-TCO derivatives can be prepared easily, are accessed through diastereoselective synthesis, and are typically crystalline bench-stable solids that are stable in aqueous solution, blood serum, or in the presence of thiols in buffered solution. GFP with a genetically encoded tetrazine-containing amino acid was site-specifically labelled in vivo by a d-TCO derivative. The fastest bioorthogonal reaction reported to date [k₂ 3,300,000 (+/- 40,000) M^-1s^-1 in H₂O at 25 °C] is described herein with a cyclopropane-fused trans-cyclooctene. d-TCO derivatives display rates within an order of magnitude of these fastest trans-cyclooctene reagents, and also display enhanced stability and aqueous solubility.

Copper-assisted click reactions for activity-based proteomics: fine-tuned ligands and refined conditions extend the scope of application

Copper-catalysed alkyne-azide 1,3-dipolar cycloaddition (CuAAC) is the predominantly used bioconjugation method in the field of activity-based protein profiling (ABPP). Several limitations, however, including conversion efficiency, protein denaturation and buffer compatibility, restrict the scope of established procedures. We introduce an ABPP customised click methodology based on refined CuAAC conditions together with new accelerating copper ligands. A screen of several triazole compounds revealed the cationic quaternary {3-[4-({bis[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl]amino}methyl)-1H-1,2,3-triazol-1-yl]propyl}trimethylammonium trifluoroacetate (TABTA) to be a superior ligand. TABTA exhibited excellent in vitro conjugation kinetics and optimal ABPP labelling activity while almost exclusively preserving the native protein fold. The application of this CuAAC-promoting system is amenable to existing protocols with minimal perturbations and is even compatible with previously unusable buffer systems such as Tris⋅HCl.

trans-Cyclooctene--a stable, voracious dienophile for bioorthogonal labeling

Discussed herein is the development and advancement of trans-cyclooctene as a tool for facilitating bioorthogonal labeling through reactions with s-tetrazines. While a number of strained alkenes have been shown to combine with tetrazines for applications in bioorthogonal labeling, trans-cyclooctene enables fastest reactivity at low concentration with rate constants in excess of k2=10(6) M(-1) s(-1). In the present article, we describe advances in computation and synthesis that have enabled applications in chemical biology and nuclear medicine.

Expanding room for tetrazine ligations in the in vivo chemistry toolbox

There is tremendous interest in developing and refining methods to predictably perform chemical reactions within the framework of living systems. Here we review recent advances in applying tetrazine cycloadditions to live cell and in vivo chemistry. We highlight new syntheses of the tetrazine and dienophile precursors useful for in vivo studies. We briefly overview the use of this reaction in combination with unnatural amino acid technology and discuss applications involving the imaging of glycans on live cells. An emerging area is the use of tetrazine ligations for the development of in vivo imaging probes such as those used for positron emission tomography. We summarize recent applications involving tetrazine cycloadditions performed in live mice for pretargeted imaging of cancer cell biomarkers.

Orthogonality in organic, polymer, and supramolecular chemistry: from Merrifield to click chemistry

The concept of orthogonality has been applied to many areas of chemistry, ranging from wave functions to chromatography. But it was Barany and Merrifield's orthogonal protecting group strategy that paved the way for solid phase peptide syntheses, other important classes of biomaterials such as oligosaccharides and oligonucleotides, and ultimately to a term in widespread usage that is focused on chemical reactivity and binding selectivity. The orthogonal protection strategy has been extended to the development of orthogonal activation, and recently the click reaction, for streamlining organic synthesis. The click reaction and its variants are considered orthogonal as the components react together in high yield and in the presence of many other functional groups. Likewise, supramolecular building blocks can also be orthogonal, thereby enabling programmed self-assembly, a superb strategy to create complex architectures. Overall, orthogonal reactions and supramolecular interactions have dramatically improved the syntheses, the preparation of functional materials, and the self-assembly of nanoscale structures.