Rapid synthesis, screening, and identification of xanthone- and xanthene-based fluorophores using click chemistry

Synthesis and spectral properties of 6′-triazolyl-dihydroxanthene-hemicyanine fused near-infrared dyes

Copper(i)-catalyzed azide alkyne cycloaddition (CuAAC) to explore the fluorogenic potential of near-infrared (NIR) dihydroxanthene (DHX) triazole dyes.

Recent advances in organic dyes and fluorophores comprising a 1,2,3-triazole moiety

Since the discovery of the copper catalyzed azide alkyne cycloaddition in the early 2000s, tremendous efforts have been devoted to enlarging the scope of applications of this relatively simple to handle reaction.

Palladium-catalyzed coupling reactions on functionalized 2-trifluoromethyl-4-chromenone scaffolds. Synthesis of highly functionalized trifluoromethyl-heterocycles

The chromenone core is a ubiquitous group in biologically-active natural products and has been extensively used in organic synthesis. Fluorine derived compounds, including those with a trifluoromethyl group (-CF3), have shown enhanced biological activities in numerous pharmaceuticals compared with their non-fluorinated analogs. We have found that 2-trifluoromethyl chromenones can be readily functionalized in the 8- and 7-positions, providing chromenones cores of high structural complexity which are excellent precursors for numerous trifluoromethyl-heterocycles.

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

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

Formation of Phenalenone Skeleton by an Unusual Rearrangement Reaction

The frame rearrangement reaction of dinaphthyl ketones, possessing hydroxy groups at appropriate positions, into phenalenone derivatives under acidic conditions was discovered serendipitously. Although this rearrangement had limited scope, its mechanism was unusual, involving the division of naphthalene rings into one phenalenone ring and one benzene ring. The reaction mechanism was elucidated by direct determination of intermediate structures using ¹H NMR measurements. The generated phenalenones are expected to be key intermediates toward natural products and functional materials.

Click and Fluoresce: A Bioorthogonally Activated Smart Probe for Wash-Free Fluorescent Labeling of Biomolecules

Bioorthogonally activated smart probes greatly facilitate the selective labeling of biomolecules in living system. Herein, we described a novel type of smart probes with tunable reaction rates, high fluorescence turn-on ratio, and easy access. The practicality of such probes was demonstrated by selective labeling of lipid and hCAII in Hela cells.

A cell-permeable and triazole-forming fluorescent probe for glycoconjugate imaging in live cells

A new fluorescence-forming probe, coumOCT, designed by fusing cyclooctyne with a coumarin fluorophore was successfully used for the imaging of azido-glycoconjugates in living HeLa cells.

Structure-Property Relationships in Click-Derived Donor-Triazole-Acceptor Materials

To shed light on intramolecular charge-transfer phenomena in 1,2,3-triazole-linked materials, a series of 1,2,3-triazole-linked push-pull chromophores were prepared and studied experimentally and computationally. Investigated modifications include variation of donor and/or acceptor strength and linker moiety as well as regioisomers. Photophysical characterization of intramolecular charge-transfer features revealed ambipolar behavior of the triazole linker, depending on the substitution position. Furthermore, non-centrosymmetric materials were subjected to second-harmonic generation measurements, which revealed the high nonlinear optical activity of this class of materials.

Synthesis and spectroscopic properties of β-triazoloporphyrin-xanthone dyads

A novel series of β-triazoloporphyrin-xanthone conjugates and xanthone-bridged β-triazoloporphyrin dyads has been synthesized in moderate to good yields through Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of copper(II) 2-azido-5,10,15,20-tetraphenylporphyrin or zinc(II) 2-azidomethyl-5,10,15,20-tetraphenylporphyrin with various alkyne derivatives of xanthones in DMF containing CuSO4 and ascorbic acid at 80 °C. Furthermore, these metalloporphyrins underwent demetalation under acidic conditions to afford the corresponding free-base porphyrins in good to excellent yields. After successful spectroscopic characterization, these porphyrins have been evaluated for their photophysical properties. The preliminary results revealed a bathochromic shift in the UV-vis and fluorescence spectra of these porphyrin-xanthone dyads.

Highly selective fluorescent and colorimetric probe for live-cell monitoring of sulphide based on bioorthogonal reaction

H₂S is the third endogenously generated gaseous signaling compound and has also been known to involve a variety of physiological processes. To better understand its physiological and pathological functions, efficient methods for monitoring of H₂S are desired. Azide fluorogenic probes are popular because they can take place bioorthogonal reactions. In this work, by employing a fluorescein derivative as the fluorophore and an azide group as the recognition unit, we reported a new probe 5-azidofluorescein for H₂S with improved sensitivity and selectivety. The probe shows very low background fluorescence in the absence of H₂S. In the presence of H₂S, however, a significant enhancement for excited fluorescence were observed, resulting in a high sensitivity to H₂S in buffered (10 mmol/L HEPES, pH 7.0) aqueous acetonitrile solution (H₂O/CH₃CN = 1:3, v/v) with a detection limit of 0.035 μmol/L observed, much lower than the previously reported probes. All these features are favorable for direct monitoring of H₂S with satisfactory sensitivity, demonstrating its value of practical application.

Me3N-promoted synthesis of 2,3,4,4a-tetrahydroxanthen-1-one: preparation of thiosemicarbazone derivatives, their solid state self-assembly and antimicrobial properties

Thiosemicarbazones (5a–5j) have been synthesized from 2,3,4,4a-tetrahydroxanthen-1-one, obtained in high yield through Me₃N-promoted domino Baylis–Hillman/oxa-Michael reaction. Their solid-state self-assembly and antimicrobial properties are studied.

6-Arylcoumarins: versatile scaffolds for fluorescent sensors

6-Arylcoumarins are available as versatile scaffolds for various types of fluorescent sensors like those for cation and viscosity.

The development of a nucleus staining fluorescent probe for dynamic mitosis imaging in live cells

The rapid and efficient synthesis of a novel fluorescent xanthone library (AX) and its application for the development of a new nucleus staining fluorescent probe (CDb12) for monitoring real-time mitosis progression in live cells is presented.

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.

Imaging bacterial peptidoglycan with near-infrared fluorogenic azide probes

Fluorescent probes designed for activation by bioorthogonal chemistry have enabled the visualization of biomolecules in living systems. Such activatable probes with near-infrared (NIR) emission would be ideal for in vivo imaging but have proven difficult to engineer. We present the development of NIR fluorogenic azide probes based on the Si-rhodamine scaffold that undergo a fluorescence enhancement of up to 48-fold upon reaction with terminal or strained alkynes. We used the probes for mammalian cell surface imaging and, in conjunction with a new class of cyclooctyne D-amino acids, for visualization of bacterial peptidoglycan without the need to wash away unreacted probe.

Integration of the 1,2,3-triazole "click" motif as a potent signalling element in metal ion responsive fluorescent probes

In a systematic approach we synthesized a new series of fluorescent probes incorporating donor-acceptor (D-A) substituted 1,2,3-triazoles as conjugative π-linkers between the alkali metal ion receptor N-phenylaza-[18]crown-6 and different fluorophoric groups with different electron-acceptor properties (4-naphthalimide, meso-phenyl-BODIPY and 9-anthracene) and investigated their performance in organic and aqueous environments (physiological conditions). In the charge-transfer (CT) type probes 1, 2 and 7, the fluorescence is almost completely quenched by intramolecular CT (ICT) processes involving charge-separated states. In the presence of Na(+) and K(+) ICT is interrupted, which resulted in a lighting-up of the fluorescence in acetonitrile. Among the investigated fluoroionophores, compound 7, which contains a 9-anthracenyl moiety as the electron-accepting fluorophore, is the only probe which retains light-up features in water and works as a highly K(+)/Na(+)-selective probe under simulated physiological conditions. Virtually decoupled BODIPY-based 6 and photoinduced electron transfer (PET) type probes 3-5, where the 10-substituted anthracen-9-yl fluorophores are connected to the 1,2,3-triazole through a methylene spacer, show strong ion-induced fluorescence enhancement in acetonitrile, but not under physiological conditions. Electrochemical studies and theoretical calculations were used to assess and support the underlying mechanisms for the new ICT and PET 1,2,3-triazole fluoroionophores.

Fluorogenic azidofluoresceins for biological imaging

Fluorogenic probes activated by bioorthogonal chemical reactions can enable biomolecule imaging in situations where it is not possible to wash away unbound probe. One challenge for the development of such probes is the a priori identification of structures that will undergo a dramatic fluorescence enhancement by virtue of the chemical transformation. With the aid of density functional theory calculations reported previously by Nagano and co-workers, we identified azidofluorescein derivatives that were predicted to undergo an increase in fluorescence quantum yield upon Cu-catalyzed or Cu-free cycloaddition with linear or cyclic alkynes, respectively. Four derivatives were experimentally verified in model reactions, and one, a 4-azidonaphthylfluorescein analogue, was further shown to label alkyne-functionalized proteins in vitro and glycoproteins on cells with excellent selectivity. The azidofluorescein derivative also enabled cell imaging under no-wash conditions with good signal above background. This work establishes a platform for the rational design of fluorogenic azide probes with spectral properties tailored for biological imaging.

Synthesis of fluorinated benzophenones, xanthones, acridones, and thioxanthones by iterative nucleophilic aromatic substitution

Fluorination of fluorophores can substantially enhance their photostability and improve spectroscopic properties. To facilitate access to fluorinated fluorophores, bis(2,4,5-trifluorophenyl)methanone was synthesized by treatment of 2,4,5-trifluorobenzaldehyde with a Grignard reagent derived from 1-bromo-2,4,5-trifluorobenzene, followed by oxidation of the resulting benzyl alcohol. This hexafluorobenzophenone was subjected to sequential nucleophilic aromatic substitution reactions, first at one or both of the more reactive 4,4'-fluorines, and second by cyclization through substitution of the less reactive 2,2'-fluorines, using a variety of oxygen, nitrogen, and sulfur nucleophiles, including hydroxide, methoxide, amines, and sulfide. This method yields symmetrical and asymmetrical fluorinated benzophenones, xanthones, acridones, and thioxanthones and provides scalable access to known and novel precursors to fluorinated analogues of fluorescein, rhodamine, and other derivatives. Spectroscopic studies revealed that several of these precursors are highly fluorescent, with tunable absorption and emission spectra, depending on the substituents. This approach should allow access to a wide variety of novel fluorinated fluorophores and related compounds.

Highly sensitive detection of saccharides under physiological conditions with click synthesized boronic acid-oligomer fluorophores

Two phenylboronic acid based saccharide sensors bearing conjugated oligomer fluorophores with linear and cruciform π-frameworks were synthesized in a modular approach utilizing a Cu-catalyzed alkyne azide cycloaddition (click) reaction. The cruciform fluorophore showed excellent saccharide sensing function under physiological conditions in the mM range, whereas the linear fluorophore gave very limited sensing functions. The different fluorescent sensing behaviours highlight the important role of oligomer fluorophore in the development of effective saccharide sensors.

A palette of fluorescent probes with varying emission colors for imaging hydrogen peroxide signaling in living cells

We present a new family of fluorescent probes with varying emission colors for selectively imaging hydrogen peroxide (H(2)O(2)) generated at physiological cell signaling levels. This structurally homologous series of fluorescein- and rhodol-based reporters relies on a chemospecific boronate-to-phenol switch to respond to H(2)O(2) over a panel of biologically relevant reactive oxygen species (ROS) with tunable excitation and emission maxima and sensitivity to endogenously produced H(2)O(2) signals, as shown by studies in RAW264.7 macrophages during the phagocytic respiratory burst and A431 cells in response to EGF stimulation. We further demonstrate the utility of these reagents in multicolor imaging experiments by using one of the new H(2)O(2)-specific probes, Peroxy Orange 1 (PO1), in conjunction with the green-fluorescent highly reactive oxygen species (hROS) probe, APF. This dual-probe approach allows for selective discrimination between changes in H(2)O(2) and hypochlorous acid (HOCl) levels in live RAW264.7 macrophages. Moreover, when macrophages labeled with both PO1 and APF were stimulated to induce an immune response, we discovered three distinct types of phagosomes: those that generated mainly hROS, those that produced mainly H(2)O(2), and those that possessed both types of ROS. The ability to monitor multiple ROS fluxes simultaneously using a palette of different colored fluorescent probes opens new opportunities to disentangle the complex contributions of oxidation biology to living systems by molecular imaging.