A Double-Site Chemodosimeter for Selective Fluorescence Detection of a Nerve Agent Mimic.

A novel two-site chemodosimeter (SWJT-4) based on fluorescein skeleton to detect diethyl chlorophosphate (DCP) was designed and synthesized. It is a turn-on fluorescent probe for DCP with good selectivity and obvious color change in aqueous solution. Interestingly, the two oxime groups of SWJT-4 as dual response sites initiated different reactions with DCP to form a cyano group and an isoxazole ring, respectively. The corresponding mechanism was confirmed by 1H NMR, MS and DFT calculation. Moreover, SWJT-4 could be used as a fluorescent test paper to detect DCP vapor.

A photo-responsive chemical modulation of m6A RNA demethylase FTO.

The adenine N6-methylation m6A is a crucial modification that is associated with several biological functions. One of the two m6A demethylases FTO has arisen as an attractive target for the development of novel cancer therapies. Here, we describe a new design, synthesis and evaluation of a photo-responsive and selective inhibitor of FTO.

Phosphonofluoresceins: Synthesis, Spectroscopy, and Applications.

Xanthene fluorophores, like fluorescein, have been versatile molecules across diverse fields of chemistry and life sciences. Despite the ubiquity of 3-carboxy and 3-sulfonofluorescein for the last 150 years, to date, no reports of 3-phosphonofluorescein exist. Here, we report the synthesis, spectroscopic characterization, and applications of 3-phosphonofluoresceins. The absorption and emission of 3-phosphonofluoresceins remain relatively unaltered from the parent 3-carboxyfluorescein. 3-Phosphonofluoresceins show enhanced water solubility compared to 3-carboxyfluorescein and persist in an open, visible light-absorbing state even at low pH and in low dielectric media while 3-carboxyfluoresceins tend to lactonize. In contrast, the spirocyclization tendency of 3-phosphonofluoresceins can be modulated by esterification of the phosphonic acid. The bis-acetoxymethyl ester of 3-phosphonofluorescein readily enters living cells, showing excellent accumulation (>6x) and retention (>11x), resulting in a nearly 70-fold improvement in cellular brightness compared to 3-carboxyfluorescein. In a complementary fashion, the free acid form of 3-phosphonofluorescein does not cross cellular membranes, making it ideally suited for incorporation into a voltage-sensing scaffold. We develop a new synthetic route to functionalized 3-phosphonofluoresceins to enable the synthesis of phosphono-voltage sensitive fluorophores, or phosVF2.1.Cl. Phosphono-VF2.1.Cl shows excellent membrane localization, cellular brightness, and voltage sensitivity (26% ΔF/F per 100 mV), rivaling that of sulfono-based VF dyes. In summary, we develop the first synthesis of 3-phosphonofluoresceins, characterize the spectroscopic properties of this new class of xanthene dyes, and utilize these insights to show the utility of 3-phosphonofluoresceins in intracellular imaging and membrane potential sensing.

A fluorescein-carbazole-based fluorescent probe for imaging of endogenous hypochlorite in living cells and zebrafish.

In this article, a fluorescent probe FCZ with fluorescein-carbazole as the basic skeleton was designed and synthesized. In contrast to the presences of other coexisting anions, active oxygen and organic thiols, the probe could be "turn-on", exhibiting specific fluorescence performance towards hypochlorite (ClO-). Comprehensive analyses by electrospray ionization mass spectrometry (ESI-MS), thin-layer chromatography (TLC), nuclear magnetic resonance (NMR), and density functional theory/time-dependent density functional theory (DFT/TDDFT) revealed that ClO- could react with the imine bond of the probe, forming fluorescein, resulting in a significant increase of fluorescence emission intensity. The probe has a detection limit for ClO- in water of 0.056 µmol/L. In addition, the probe was successfully applied to detect ClO- in water samples, as well as in the imaging of ClO- in RAW264.7 cells and zebrafish.

Nanomolar detection of adenosine triphosphate (ATP) using a nanostructured fluorescent chemosensing ensemble.

We report a novel nanostructured chemosensing ensemble PyNp-C13/UD, obtained by self-assembling uranine dye (UD) and an amphiphilic pyridinium salt PyNp-C13. The ensemble was developed for the fluorescence turn-on sensing of ATP in aqueous solutions and inside living cells. The assembly operates via an indicator displacement assay (IDA) method with an ultra-low detection limit of 6.8 nM.

Fluorescein derived Schiff base as fluorimetric zinc (II) sensor via 'turn on' response and its application in live cell imaging.

A novel Schiff base L composed of fluorescein hydrazine and a phenol functionalized moiety has been designed and prepared via cost-effective condensation reaction. The L is utilized for selective sensing of Zn2+ over other environmental and biological relevant metal ions in aqueous alcoholic solution under physiological pH range. The binding of Zn2+ to the receptor L is found to causes ~23 fold fluorescence enhancement of L. The 1:1 binding mode of the metal complex is established by combined UV-Vis, fluorescence, and HRMS (high-resolution mass spectroscopy) spectroscopic methods. The binding constant (Ka) for complexation and the limit of detection (LOD) of Zn2+ is calculated to be 2.86 × 104 M-1 and 1.59 µM, respectively. Further photophysical investigations including steady-state, time-resolved fluorescence analysis and spectral investigations including NMR (nuclear magnetic resonance), IR (infrared spectroscopy) suggest introduction of CHEF (chelation enhance fluorescence) with the suppression of CN isomerization and PET (photo-induced electron transfer) mechanism for the strong fluorescent response towards Zn2+. Finally, the sensor L is successfully employed to monitor a real-time detection of Zn2+ by means of TLC (thin layer chromatography) based paper strip. The L is used in the cell imaging study using African green monkey kidney cells (Vero cells) for the determination of exogenous Zn2+ by Immunofluorescence Assay (IFA) process.

Synthesis and characterization of a nano fluorescent starch.

A novel nano fluorescent starch, starch-bearing 3-epoxypropoxy fluorescein (ST-EF) was developed by a simple method. First, 3-epoxypropoxy fluorescein (EF) was prepared via a nucleophilic substitution reaction between fluorescein and epichlorohydrin. Then, ST-EF was synthesized via a ring-opening reaction to attach fluorescein to native cassava starch chains. The degree of substitution (DS) of ST-EF was determined by ultraviolet-visible spectrophotometry. The 1H nuclear magnetic resonance (NMR) spectroscopy, elemental analysis, fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscope (TEM), dynamic laser scattering (DLS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to characterize ST-EF. Fluorescent properties of ST-EF in water were studied. The results showed that the nano fluorescent starch shows strong fluorescence as fluorescein, and can be used as a fluorescent polymer in various applications, especially in biomedicine.

Synthesis and Studies of Fluorescein Based Derivatives for their Optical Properties, Urease Inhibition and Molecular Docking.

Herein, we design and synthesized new fluorescein based derivatives by insitu formation of fluorescein ester and further treated with corresponding hydrazide and amine to yield respective compounds i.e. FB1, FB2, FB3 and FB4. The spectral purity and characterization was done by using IR, NMR and Mass spectroscopies. The synthesized derivatives were examined for their photophysical properties by using variety of organic solvents and results were discussed in details. The structural diversity of synthesized compounds motivate us to evaluate these compounds for urease inhibition. The compound FB3 (IC50 = 0.0456 µM) shows 100 fold more active against Jack bean urease than standard drug thiourea (IC50 = 4.7455 µM). Other synthesized compounds showed potent activity. Free radical percentage scavenging assay further supported the capacity of compounds to urease inhibition. While, molecular docking simulations helps to examine the molecular interactions of active compounds FB1, FB2, FB3 and FB4 within the binding site of urease enzyme.

5-Bromo-4',5'-bis(dimethylamino)fluorescein: Synthesis and Photophysical Studies.

In this study, three new fluorescein derivatives-5-bromo-4',5'-dinitrofluorescein (BDNF), 5-bromo-4',5'-diaminofluorescein (BDAF), and 5-bromo-4',5'-bis(dimethylamino)fluorescein (BBDMAF)-were synthesized and their pH-dependent protolytic equilibria were investigated. In particular, BBDMAF exhibited pH-dependent fluorescence, showing strong emission only at pH 3-6. BBDMAF bears a bromine moiety and thus, can be used in various cross-coupling reactions to prepare derivatives and take advantage of its unique emission properties. To confirm this, the Suzuki and Sonogashira reactions of BBDMAF with phenylboronic acid and phenylacetylene, respectively, were performed, and the desired products were successfully obtained.

A fluorescence anisotropy-based assay for determining the activity of tissue transglutaminase.

Tissue transglutaminase (TGase 2) is the most abundantly expressed enzyme of the transglutaminase family and involved in a large variety of pathological processes, such as neurodegenerative diseases, disorders related to autoimmunity and inflammation as well as tumor growth, progression and metastasis. As a result, TGase 2 represents an attractive target for drug discovery and development, which requires assays that allow for the characterization of modulating agents and are appropriate for high-throughput screening. Herein, we report a fluorescence anisotropy-based approach for the determination of TGase 2's transamidase activity, following the time-dependent increase in fluorescence anisotropy due to the enzyme-catalyzed incorporation of fluorescein- and rhodamine B-conjugated cadaverines 1-3 (acyl acceptor substrates) into N,N-dimethylated casein (acyl donor substrate). These cadaverine derivatives 1-3 were obtained by solid-phase synthesis. To allow efficient conjugation of the rhodamine B moiety, different linkers providing secondary amine functions, such as sarcosyl and isonipecotyl, were introduced between the cadaverine and xanthenyl entities in compounds 2 and 3, respectively, with acyl acceptor 3 showing the most optimal substrate properties of the compounds investigated. The assay was validated for the search of both irreversible and reversible TGase 2 inhibitors using the inactivators iodoacetamide and a recently published L-lysine-derived acrylamide and the allosteric binder GTP, respectively. In addition, the fluorescence anisotropy-based method was proven to be suitable for high-throughput screening (Z' factor of 0.86) and represents a non-radioactive and highly sensitive assay for determining the active TGase 2 concentration.

Novel Fluorescein-Based Fluorescent Probe for Detecting H2S and Its Real Applications in Blood Plasma and Biological Imaging.

A broad-spectrum fluorescent probe, which can be applied to monitoring H2S in various biological systems, has been rationally designed and synthesized. This specific probe was applied to localize the endogenous H2S in living Raw264.7 macrophage cells, HepG2 cells, and H9C2 cells. At the same time, the probe has successfully visualized CBS- and CSE-induced endogenous H2S production and monitored CBS and CSE activity in H9C2 cells. This probe could serve as a powerful molecular imaging tool to further explore the physiological function and the molecular mechanisms of endogenous H2S in living animal systems.

Fluorescein dye derivatives and their nanohybrids: Synthesis, characterization and antimicrobial activity.

Fluorescein (resorcinolphthalein) is a synthetic organic photoactive dye compound soluble in water, alcohol and polar solvents. It is widely used as a fluorescent tracer in medicinal and biological applications and tumor infected tissues tracer. In this study, fluorescein (F) was condensed by five coupling agents namely: p,p-phenylene diamine, p-hydroxy aniline, o-hydroxy aniline, p-methoxy aniline and p-methyl aniline in a molar ratio of 2(F):1 (coupling agent). The chemical structures of the synthesized fluorescein derivatives were confirmed using: microelemental analysis, FTIR spectroscopy, 1H-NMR spectroscopy, and mass spectroscopy. The synthesized compounds were loaded on chemically prepared silver nanoparticles via reduction reaction of silver nitrate. The structures and properties of the formed fluorescein derivatives silver nanohybrids were determined using: UV/Vis spectroscopy, TEM images and dynamic light scattering (DLS). The synthesized compounds and their nanohybrids were evaluated for their antimicrobial activities against different bacterial strains and fungi. The results showed that the formed fluorescein derivatives silver nanohybrids are in moderate diameter range, and the loading of the synthesized compounds protect the silver nanoparticles against coagulation. The antimicrobial activity against the studied microorganisms was comparable to the standard used. Moreover, the antimicrobial activity was increased considerably in case of using fluorescein derivatives silver nanohybrids. The antimicrobial activities were correlated to the chemical structures of the compounds, diameter of the formed nanohybrids and to the nature of the tested bacterial strains. The mechanism of the antimicrobial action of the synthesized compounds and their nanohybrids was proposed.

Fluorescein-labeled Bacitracin and Daptomycin Conjugates: Synthesis, Fluorescence Imaging and Evaluation.

BACKGROUND: Previously, glycopeptides antibiotics such as vancomycin, ramoplanin and an antifungal antibiotic nystatin have been studied for their diagnostic and therapeutic potential. OBJECTIVE: To further explore the diagnostic and chemotherapeutic potential of other antibiotics we have now employed daptomycin, a lipopetide antibiotic and bacitracin, a polypeptide antibiotic in uptake and vitality tests on human cell lines. METHOD: Fluorescent conjugates of bacitracin and daptomycin were synthesized using fluorescein isothiocynate (FITC) for confocal laser scanning microscopy (CLSM) and fluorescence activated cell sorting (FACS). The cellular uptake of the synthesized daptomycin and bacitracin conjugates was studied on seven human cell lines, two healthy and five malignant using CLSM and FACS. To examine the cell membrane damage caused by the conjugates FACS experiments were carried out using propidium iodide. RESULTS: The uptake pattern was different for both antibiotics for all the cell lines. The cytoplasmic uptake of daptomycin conjugate was lower than the bacitracin conjugate, resulting in decreased cell membrane damage. CONCLUSION: No preferential uptake into malignant or healthy cells was found for the two different antibiotic conjugates and the uptake patterns were also different between the two antibiotics. However, the lower cytotoxicity and different uptake mechanism makes daptomycin conjugate a prospective candidate for further study as a diagnostic agent for various intracellular infections.

Design and construction of conformational biosensors to monitor ion channel activation: A prototype FlAsH/BRET-approach to Kir3 channels.

Ion channels play a vital role in numerous physiological functions and drugs that target them are actively pursued for development of novel therapeutic agents. Here we report a means for monitoring in real time the conformational changes undergone by channel proteins upon exposure to pharmacological stimuli. The approach relies on tracking structural rearrangements by monitoring changes in bioluminescence energy transfer (BRET). To provide proof of principle we have worked with Kir3 neuronal channels producing 10 different constructs which were combined into 17 donor-acceptor BRET pairs. Among these combinations, pairs bearing the donor Nano-Luc (NLuc) at the C-terminal end of Kir3.2 subunits and the FlAsH acceptor at the N-terminal end (NT) or the interfacial helix (N70) of Kir3.1 subunits were identified as potential tools. These pairs displayed significant changes in energy transfer upon activation with direct channel ligands or via stimulation of G protein-coupled receptors. Conformational changes associated with channel activation followed similar kinetics as channel currents. Dose response curves generated by different agonists in FlAsH-BRET assays displayed similar rank order of potency as those obtained with conventional BRET readouts of G protein activation and ion flux assays. Conformational biosensors as the ones reported herein should prove a valuable complement to other methodologies currently used in channel drug discovery.

Fluorescein Derivatives as Bifunctional Molecules for the Simultaneous Inhibiting and Labeling of FTO Protein.

The FTO protein is unequivocally reported to play a critical role in human obesity and in the regulation of cellular levels of m(6)A modification, which makes FTO a significant and worthy subject of study. Here, we identified that fluorescein derivatives can selectively inhibit FTO demethylation, and the mechanisms behind these activities were elucidated after we determined the X-ray crystal structures of FTO/fluorescein and FTO/5-aminofluorescein. Furthermore, these inhibitors can also be applied to the direct labeling and enrichment of FTO protein combined with photoaffinity labeling assay.

Analysis of chemical equilibrium of silicon-substituted fluorescein and its application to develop a scaffold for red fluorescent probes.

Fluorescein is a representative green fluorophore that has been widely used as a scaffold of practically useful green fluorescent probes. Here, we report synthesis and characterization of a silicon-substituted fluorescein, i.e., 2-COOH TokyoMagenta (2-COOH TM), which is a fluorescein analogue in which the O atom at the 10' position of the xanthene moiety of fluorescein is replaced with a Si atom. This fluorescein analogue forms a spirolactone ring via intramolecular nucleophilic attack of the carboxylic group in a pH-dependent manner. Consequently, 2-COOH TM exhibits characteristic large pH-dependent absorption and fluorescence spectral changes: (1) 2-COOH TM is colorless at acidic pH, whereas fluorescein retains observable absorption and fluorescence even at acidic pH, and the absorption maximum is also shifted; (2) the absorption spectral change occurs above pH 7.0 for 2-COOH TM and below pH 7.0 for fluorescein; (3) 2-COOH TM shows a much sharper pH response than fluorescein because of its pKa inversion, i.e., pKa1 > pKa2. These features are also different from those of a compound without the carboxylic group, 2-Me TokyoMagenta (2-Me TM). Analysis of the chemical equilibrium between pH 3.0 and 11.0 disclosed that 2-COOH TM favors the colorless and nonfluorescent lactone form, compared with fluorescein. Substitution of Cl atoms at the 4' and 5' positions of the xanthene moiety of 2-COOH TM to obtain 2-COOH DCTM shifted the equilibrium so that the new derivative exists predominantly in the strongly fluorescent open form at physiological pH (pH 7.4). To demonstrate the practical utility of 2-COOH DCTM as a novel scaffold for red fluorescent probes, we employed it to develop a probe for ß-galactosidase.

Fluorescence enhancement of a fluorescein derivative upon adsorption on cellulose.

9-[1-(2-Methyl-4-methoxyphenyl)]-6-hydroxy-3H-xanthen-3-one (2-Me-4-OMe TG) is a fluorescein derivative dye whose photophysical properties show a remarkable pH dependence. In aqueous solution the fluorescence quantum yield (Φf) of its anionic species is nearly a hundred times higher than that of its neutral species. Such a large difference in Φf makes 2-Me-4-OMe TG useful as an "on-off" pH indicator. Here we report that adsorption on the surface of microcrystalline cellulose exerts a profound effect upon the photophysical properties of 2-Me-4-OMe TG. On the solid only the dye neutral species is observed and its Φf is 0.31 ± 0.10, which is approximately thirty times higher than the value found for the neutral species in aqueous solution (Φf = 0.01). 2-Me-4-OMe TG and Dabcyl (DB) were co-adsorbed on the surface of microcrystalline cellulose to study the transfer of excitation energy from the former to the latter. In the absence of the dye, the formation of DB aggregates is observed at concentrations greater than 0.34 µmol per gram of cellulose, while in the presence of 2-Me-4-OMe TG the formation of DB aggregates is thoroughly inhibited. The quenching of fluorescence of 2-Me-4-OMe TG by DB reaches efficiencies as high as 90% for the most concentrated samples.

Thermally activated delayed fluorescence of fluorescein derivative for time-resolved and confocal fluorescence imaging.

Compared with fluorescence imaging utilizing fluorophores whose lifetimes are in the order of nanoseconds, time-resolved fluorescence microscopy has more advantages in monitoring target fluorescence. In this work, compound DCF-MPYM, which is based on a fluorescein derivative, showed long-lived luminescence (22.11 µs in deaerated ethanol) and was used in time-resolved fluorescence imaging in living cells. Both nanosecond time-resolved transient difference absorption spectra and time-correlated single-photon counting (TCSPC) were employed to explain the long lifetime of the compound, which is rare in pure organic fluorophores without rare earth metals and heavy atoms. A mechanism of thermally activated delayed fluorescence (TADF) that considers the long wavelength fluorescence, large Stokes shift, and long-lived triplet state of DCF-MPYM was proposed. The energy gap (ΔEST) of DCF-MPYM between the singlet and triplet state was determined to be 28.36 meV by the decay rate of DF as a function of temperature. The ΔE(ST) was small enough to allow efficient intersystem crossing (ISC) and reverse ISC, leading to efficient TADF at room temperature. The straightforward synthesis of DCF-MPYM and wide availability of its starting materials contribute to the excellent potential of the compound to replace luminescent lanthanide complexes in future time-resolved imaging technologies.

Dodecyl and octyl esters of fluorescein as protonophores and uncouplers of oxidative phosphorylation in mitochondria at submicromolar concentrations.

In our search for fluorescent uncouplers of oxidative phosphorylation, three esters of fluorescein, n-butyl-, n-octyl-, and n-dodecyl-oxycarbonyl-fluorescein (C4-FL, C8-FL, C12-FL) were synthesized and characterized. With increasing liposomal lipid content, the long-chain alkyl derivatives of fluorescein (C8-FL, C12-FL and commercially available C18-FL), but not C4-FL and unsubstituted fluorescein, exhibited an increase in fluorescence polarization reflecting the dye binding to liposomes. C12-FL induced proton permeability in lipid membranes, while C4-FL was inactive. In contrast to C4-FL and C18-FL, C12-FL and C8-FL increased the respiration rate and decreased the membrane potential of isolated rat liver mitochondria with half-maximal effective concentrations of 700nM and 300nM, respectively. The effect of Cn-FL on the respiration correlated with that on proton permeability of the inner mitochondrial membrane, as measured by induction of mitochondria swelling in the potassium acetate medium. Binding of C8-FL to mitochondria depended on their energization, which was apparently associated with pH gradient generation across the inner mitochondrial membrane in the presence of a respiratory substrate. In wild-type yeast cells, C12-FL localized predominantly in plasma membrane, whereas in AD1-8 mutants lacking MDR pumps, it stained cytoplasmic organelles with some preference for mitochondria. Fluorescent uncouplers can be useful as a tool for determining their localization in a cell or distribution between different tissues in a living animal by fluorescent microscopy.

Excited-state proton transfer of fluorescein anion as an ionic liquid component.

Fluorescent ionic liquids (FILs) incorporating the fluorescein anion have been prepared by anion exchange of the parent quaternary ammonium chloride (Quat(+)Cl(-)) ionic liquid. By controlling the molar ratio of fluorescein to Quat(+)Cl(-), ionic liquids incorporating different prototropic forms of fluorescein were prepared. The 1:1 molar ratio ionic liquid (FIL1) is essentially composed of monoanionic fluorescein, while dianionic fluorecein is predominant in the FIL with a 1:2 molar ratio (FIL2). The fluorescence excitation spectrum of FIL2 is markedly different from its absorption spectrum. Absorption features the fluorescein dianion, while the excitation spectrum is exclusively due to the monoanion. In FIL1, the absorption and excitation spectra are both characteristic of the monoanion. In both FILs, emission of the dianion is observed upon excitation of the monoanion. This unusual behavior is interpreted in the context of a fast deprotonation of the monoanion in the excited state. The presence of residual water in the ionic liquid is important for the proton transfer process. By lowering the pH of FIL1, the transient proton transfer is inhibited, and the emission of the monoanion could be observed. The FILs have completely different spectroscopic properties from solvated fluorescein in Quat(+)Cl(-), where the prototropic equilibrium is shifted toward the neutral forms.