Design of dual-emission chemosensors for ratiometric detection of ATP derivatives

A change in metal cation switches selectivity of a phospholipid sensor from phosphatidic acid to phosphatidylserine

Phosphatidic acid and phosphatidylserine are anionic phospholipids with emerging signalling roles in cells. Determination of how phosphatidic acid and phosphatidylserine change location and quantity in cells over time requires selective fluorescent sensors that can distinguish these two anionic phospholipids. However, the design of such synthetic sensors that can selectively bind and respond to a single phospholipid within the complex membrane milieu remains challenging. In this work, we present a simple and robust strategy to control the selectivity of synthetic sensors for phosphatidic acid and phosphatidylserine. By changing the coordination metal of a dipicolylamine (DPA) ligand from Zn(II) to Ni(II) on the same synthetic sensor with a peptide backbone, we achieve a complete switch in selectivity from phosphatidic acid to phosphatidylserine in model lipid membranes. Furthermore, this strategy was largely unaffected by the choice and the position of the fluorophores. We envision that this strategy will provide a platform for the rational design of targeted synthetic phospholipid sensors to probe plasma and intracellular membranes.

Azacrown-calixpyrrole isosteres: receptors and sensors for anions

Calix[4]pyrroles (CPs) and polyammonium azacrowns (ACs) are well-known receptors for anions. CPs bind anions by directional hydrogen bonds that do not always work well for aqueous analytes. The positive charge in polyammonium ACs allows for a stronger but non-directional anion-ammonium electrostatic attraction but lack selectivity. Bridging the gap between CPs and ACs could increase affinity and potentially preserve the selectivity of anion binding. We have synthesized a flexible calixpyrrole-azacrown near isosteric receptor and incorporated an environmentally sensitive dansyl fluorophore to enable fluorescence measurements. Anion binding was evaluated using NMR and fluorescence titrations. The isosteric receptor shows a strong affinity for aqueous phosphates and phosphonates (Na⁺ salts) in the order HAsO₄^2- > H₂PO₄^- > H₂P₂O₇^2- > glyphosate^2- > AMP^- > methylphosphonate^- ≫ ADP^2- or ATP^3- but does not bind halides. This is in stark contrast to CP which shows a strong preference for halides over oxyanions. The anion binding by the new receptor was accompanied by analyte-specific changes in fluorescence intensity and spectral width and by a wavelength shift. These parameters were used in qualitative and quantitative sensing of aqueous anions. By applying machine-learning algorithms, such as linear discriminant analysis and support vector machine linear regression, this one sensor can differentiate between 10 different analytes and accurately quantify herbicide glyphosate and methylphosphonate, a product of sarin, soman or cyclosarin hydrolysis. In fact, glyphosate can be quantified even in the presence of competing anions such as orthophosphate (LODs were ≤ 1 μM).

An overview on the development of different optical sensing platforms for adenosine triphosphate (ATP) recognition

Adenosine triphosphate (ATP), one of the biological anions, plays a crucial role in several biological processes including energy transduction, cellular respiration, enzyme catalysis and signaling. ATP is a bioactive phosphate molecule, recognized as an important extracellular signaling agent. Apart from serving as a universal energy currency for various cellular events, ATP is also considered a factor responsible for numerous physiological activities. It regulates cellular metabolism by breaking phosphoanhydride bonds. Several diseases have been reported widely based on the levels and behavior of ATP. The variation of ATP concentration usually causes a foreseeable impact on mitochondrial physiological function. Mitochondrial dysfunction is responsible for the occurrence of many severe diseases such as angiocardiopathy, malignant tumors and Parkinson's disease. Therefore, there is high demand for developing a sensitive, fast-responsive, nontoxic and versatile detection platform for the detection of ATP. To this end, considerable efforts have been employed by several research groups throughout the world to develop specific and sensitive detection platforms to recognize ATP. Although a repertoire of optical chemosensors (both colorimetric and fluorescent) for ATP has been developed, many of them are not arrayed appropriately. Therefore, in this present review, we focused on the design and sensing strategy of some chemosensors including metal-free, metal-based, sequential sensors, aptamer-based sensors, nanoparticle-based sensors etc. for ATP recognition via diverse binding mechanisms.

Solvent-regulated fluorescence off-on signaling of Ni(II) and Zn(II) with the formation of two mononuclear complexes with an ATP detection ability by Zn(II) assembly

The current study shows that Schiff base HL, (Z)-2,4-dibromo-6-(((piperidin-2-ylmethyl)imino)methyl)phenol, can be used successfully as a selective chemosensor for Zn(II) and Ni(II) among several competing cations in purely aqueous and semi-aqueous media. Under UV light in methanol-water (9 : 1) HEPES buffer, the receptor gives its response by changing its color to cyan color in the presence of Zn(II) and to bluish cyan color in the presence of Ni(II). Surprisingly, the chemosensor can only reliably identify Zn(II) in a hundred percent aqueous medium by changing its color to light yellow. UV and fluorescence studies in both aqueous and semi-aqueous media are used to further investigate this Zn(II) and Ni(II) recognition phenomenon. The high values of the host-guest binding constants, obtained by electronic and fluorescence titration, ensure that a strong bond exists between HL and Ni(II)/Zn(II). As anticipated, two highly luminescent mononuclear, crystalline compounds, complexes 1 and 2, have been developed by a separate reaction of HL and Zn(II)/Ni(II), and the high luminous properties are due to the occurrence of Chelation Enhanced Fluorescence (CHEF). According to the single crystal structure, the asymmetric units of both complexes consist of two deprotonated chemosensor units and one Zn(II)/Ni(II), leading to the formation of an octahedral complex. For Ni(II) and Zn(II) sensing, the predicted LOD is in the nanomolar range. Both complexes 1 and 2 are fluorescence active and studies to check their ATP detection ability, but intriguingly, only complex 2 is capable of detecting ATP in a fully aqueous solution. Finally, the live cell imaging study validates the two sensors' biosensing functionality.

Azulene-based fluorescent chemosensor for adenosine diphosphate

AzuFluor® 435-DPA-Zn, an azulene fluorophore bearing two zinc(II)-dipicolylamine receptor motifs, exhibits fluorescence enhancement in the presence of adenosine diphosphate. Selectivity for ADP over ATP, AMP and PPi results from appropriate positioning of the receptor motifs, since an isomeric sensor cannot discriminate between ADP and ATP.

Binding and Sensing Properties of a Hybrid Naphthalimide-Pyrene Aza-Cyclophane towards Nucleotides in an Aqueous Solution

Selective recognition of nucleotides with synthetic receptors is an emerging direction to solve a series of nucleic acid-related challenges in biochemistry. Towards this goal, a new aza-cyclophane with two different dyes, naphthalimide and pyrene, connected through a triamine linker has been synthesized and studied for the ability to bind and detect nucleoside triphosphates in an aqueous solution. The receptor shows Foerster resonance energy transfer (FRET) in fluorescence spectra upon excitation in DMSO, which is diminished dramatically in the presence of water. According to binding studies, the receptor has a preference to bind ATP (adenosine triphosphate) and CTP (cytidine triphosphate) with a “turn-on” fluorescence response. Two separate emission bands of dyes allow one to detect nucleotides in a ratiometric manner in a broad concentration range of 10⁻⁵–10⁻³ M. Spectroscopic measurements and quantum chemical calculations suggest the formation of receptor–nucleotide complexes, which are stabilized by dispersion interactions between a nucleobase and dyes, while hydrogen bonding interactions of nucleobases with the amine linkers are responsible for selectivity.

Binuclear and tetranuclear Zn(ii) complexes with thiosemicarbazones: synthesis, X-ray crystal structures, ATP-sensing, DNA-binding, phosphatase activity and theoretical calculations

Two Zinc(ii) complexes [Zn₄(L¹)₄]·2H₂O (1) and [Zn₂(L²)₂]·2H₂O (2) of pyruvaldehydethiosemicarbazone ligands are reported. The complexes were characterized by elemental analysis, IR, NMR, UV-vis spectroscopy and by single-crystal X-ray crystallography. X-ray crystal structure determinations of the complexes show that though Zn : ligand stoichiometry is 1 : 1 in both the complexes, the molecular unit is tetranuclear for 1 and binuclear for 2. Both the complexes show selective sensing of ATP at pH 7.4 (0.01 M HEPES) in CH₃CN–H₂O (9 : 1) medium in the presence of other anions like AcO⁻, NO₃⁻, F⁻, Cl⁻, H₂PO₄⁻, HPO₄²⁻ and P₂O₇²⁻. The UV-titration experiments of complexes 1 and 2 with ATP results in binding constants of 2.0(±0.07) × 10⁴ M⁻¹ and 7.1(±0.05) × 10³ M⁻¹ respectively. The calculated detection limits of 6.7 μM and 1.7 μM for 1 and 2 respectively suggest that the complexes are sensitive detectors of ATP. High selectivity of the complexes is confirmed by the addition of ATP in presence of an excess of other anions. DFT studies confirm that the ATP complexes are more favorable than those with the other inorganic phosphate anions, in agreement with the experimental results. Phosphatase like activity of both complexes is investigated spectrophotometrically using 4-nitrophenylphosphate (NPP) as a substrate, indicating the complexes possess significant phosphate ester hydrolytic efficiency. The kinetics for the hydrolysis of the substrate NPP was studied by the initial rate method at 25 °C. Michaelis–Menten derived kinetic parameters indicate that rate of hydrolysis of the P–O bond by complex 1 is much greater than that of complex 2, the k_cat values being 212(±5) and 38(±2) h⁻¹ respectively. The DNA binding studies of the complexes were investigated using electronic absorption spectroscopy and fluorescence quenching. The absorption spectral titrations of the complexes with DNA indicate that the CT-DNA binding affinity (K_b) of complex 1 (2.10(±0.07) × 10⁶ M⁻¹) is slightly greater than that of 2 (1.11(±0.04) × 10⁶ M⁻¹). From fluorescence spectra the apparent binding constant (K_app) values were calculated and they are found to be 5.41(±0.01) × 10⁵ M⁻¹ for 1 and 3.93(±0.02) × 10⁵ M⁻¹ for 2. The molecular dynamics simulation demonstrates that the Zn(ii) complex 1 is a good intercalator of DNA.

A binuclear and a tetranuclear zinc(ii) of pyruvaldehyde thiosemicarbazone show selective sensing of ATP at pH 7.4 (0.01 M HEPES) in CH₃CN–H₂O (9 : 1) medium. The DNA binding and phosphatase activities of the complexes are also reported.

A Quinoline-Appended Cyclodextrin Derivative as a Highly Selective Receptor and Colorimetric Probe for Nucleotides

The design and development of specific recognition and sensing systems for biologically important anionic species has received growing attention in recent years, as they play significant roles in biology, pharmacy, and environmental sciences. Herein, a new supramolecular sensing probe L1 was developed for highly selective differentiation of nucleotides. L1 displayed extremely marked absorption and emission differentiation upon binding with nucleotide homologs of AMP, ADP, and ATP, due to the divergent spatial orientations of guests upon binding, which allowed for a naked-eye colorimetric differentiation for nucleotides. A differentiating mechanism was unambiguously rationalized by using various spectroscopic studies and theoretical calculations. Furthermore, we successfully demonstrated that L1 can be applied to the real-time monitoring of the enzyme-catalyzed phosphorylation/dephosphorylation processes and thus demonstrated an unprecedented visualizable strategy for selectively differentiating the structurally similar nucleotides and real-time monitoring of biological processes via fluorescent and colorimetric changes.

Fluorescence Differentiation of ATP-related Multiple Enzymatic Activities in Synovial Fluid as a Marker of Calcium Pyrophosphate Deposition Disease using Kyoto Green

Calcium pyrophosphate deposition disease (CPPD) is a crystal induced inflammation in joints, and causes severe pain in elderly people. The accumulation of pyrophosphate (PPi) in synovial fluid (SF) results from several enzymatic reactions, especially the highly activated e-NPPs, which catalyze the conversion of ATP to PPi. This study demonstrates the detection of relative catalytic activity of 3 enzymes—ecto-nucleotide pyrophosphatase/phosphodiesterases (e-NPPs), tissue nonspecific alkaline phosphatase (TNAP), and ecto-nucleoside triphosphate diphosphohydrolases (e-NTPDases)—using a single molecular sensor called Kyoto Green. Kyoto Green exhibits excellent performance in sensing the catalytic activity of the commercial representatives of the e-NPPs, TNAP, and e-NTPDases, which are ENPP1, PPase, and apyrase, respectively, in both single-enzyme and multi-enzyme assays. Analysis of SF enzymes in 19 SF samples from human and swine revealed moderate activity of e-NPPs, high activity of e-NTPDases, and low activity of TNAP. Our newly developed method for analysis of multiple enzymatic activities using Kyoto Green in biological SF will assist improvement in accuracy of the CPPD prognosis/diagnosis, which will minimize unnecessary medical procedures.

Masking Phosphate with Rare-Earth Elements Enables Selective Detection of Arsenate by Dipycolylamine-ZnII Chemosensor

Functional reassessment of the phosphate-specific chemosensors revealed their potential as arsenate detectors. A series of dipicolylamine (Dpa)-ZnII chemosensors were screened, among which acridine Dpa-ZnII chemosensor showed the highest capability in sensing arsenate. The presence of excess ZnII improved sensitivity and strengthened the binding between acridine Dpa-ZnII complex to arsenate as well as phosphate. However, due to their response to phosphate, these sensors are not suited for arsenate detection when phosphate is also present. This study demonstrated for the first time that rare-earth elements could effectively mask phosphate, allowing the specific fluorescence detection of arsenate in phosphate-arsenate coexisting systems. In addition, detection of arsenate contamination in the real river water samples and soil samples was performed to prove its practical use. This sensor was further employed for the visualization of arsenate and phosphate uptake in vegetables and flowering plants for the first time, as well as in the evaluation of a potent inhibitor of arsenate/phosphate uptake.

Phosphate-sensing with (di-(2-picolyl)amino)quinazolines based on a fluorescence on–off system

Detection and visualization of phosphates such as ATP in living organisms can facilitate the elucidation of various biological events. Although substantial efforts had been made in this area, present methods have disadvantages such as the need for specialized equipment and poor sensitivities. To address these limitations, novel fluorescent probes, (di-(2-picolyl)amino)quinazolines, were developed for application in ATP detection. They selectively recognized copper ions by fluorescence quenching, and their copper complexes displayed fluorescence enhancement in the presence of phosphoric acid derivatives. This fluorescence on–off system enabled highly sensitive fluorescence detection of ATP when combined with a phenyl boronic acid-modified γ-cyclodextrin through a plausible multipoint recognition system.

Supramolecular probe Cu-dpa-QZ2/FPB-γ-CyD recognized ATP with high sensitivity.

Triple Noncovalent-Interaction-Containing Supramolecular Polymer Vesicle Chemosensors with Dynamically Tunable Detection Ranges

Chemosensors (CSs) with dynamically tunable detection ranges have important significance for their expansion in practical applications; however, most CSs possess an unchangeable detection limit. In this work, we report the first example of a supramolecular polymer vesicle (SPV) chemosensor with a dynamically tunable detection range. SPVs containing porphyrin (PP) moieties and β-cyclodextrin (β-CD)/azobenzene (Azo) host-guest interactions were first constructed. The obtained SPVs were used to detect Zn²⁺ with a high selectivity and sensitivity over a wide detection limit range of 8.67×10^-9 to 1.99×10^-11 under UV light irradiation. The corresponding sensing mechanism was attributed to the synergistic effects of the triple noncovalent interactions, including the metal-ligand coordination of PP/Zn²⁺ and the double host-guest interactions of β-CD/Azo and β-CD/PP.

A new water-soluble polythiophene derivative as a probe for real-time monitoring adenosine 5'-triphosphatase activity in lysosome of living cells

Detection of the adenosine 5'-triphosphatase (ATPase) activity in lysosome of living cells is of great importance for clinical diagnosis of many related diseases, including cancer. In this work, a new water-soluble polythiophene derivative named ZnPT bearing both quaternary ammonium salt groups and dipicolylamine-Zn²⁺ (DPA-Zn²⁺) complexes in its side chain, was designed and synthesized for this propose. The probe mainly localized to lysosome with good biocompatibility and membrane penetration. The real-time, continuous, direct, and label-free assays were achieved through a fluorescence "turn-on" mode by taking advantages of the reaction specificity of ATPase with ATP and the high binding selectivity of ZnPT toward ATP substrate over its hydrolysis product (ADP). This well designed strategy should provide a facile and effective way for investigating ATPase-relevant biological processes.

A Toolbox of Chromones and Quinolones for Measuring a Wide Range of ATP Concentrations

A series of 26 3-hydroxychromones, three bis-flavonols and four 3-hydroxyquinolones were studied to evaluate their fluorescence response to interaction with ATP in buffer. The dyes differ by the total charge, the size and number of their aromatic units, as well as the position or electron-donating ability of their substituents. All of them were suggested to form complexes with ATP of 1:1 and 1:2 stoichiometry, which can be evidenced by their bright fluorescence and their 3000-6000 cm^-1 red-shifted excitation band. These fluorescent complexes allow detection of ATP concentrations over 3 orders of magnitude, whereas most other known probes cover no more than two orders. In total, the dyes allow ATP detection from 0.001 to 57 mm. In addition, most of the dye-ATP complexes can be excited in the visible and monitored in the red region of the spectrum. The response amplitude of the described dyes to ATP is as high as for the best known probes. Considering that complexation takes place at neutral pH, the studied dyes form a toolbox of fluorescent probes for intensiometric and ratiometric measurements of ATP concentration in a broad concentration range. Finally, the obtained results stimulate the idea that most of natural 3-hydroxyflavones in living cells may form complexes with ATP.

A metal-free fluorescence turn-on molecular probe for detection of nucleoside triphosphates

We report a fluorescence probe 1, which contains a naphthalimide fluorophore with two symmetric peptidic arms equipped with a tailor-made anion-binding motif, the guanidiniocarbonyl pyrrole moiety, for the detection of nucleoside triphosphates. Upon binding to nucleoside triphosphates, especially ATP, 1 shows significant turn-on fluorescence response. Probe 1 can also be applied for the imaging of ATP in cells.

Fluorescence Sensing of Inorganic Phosphate and Pyrophosphate Using Small Molecular Sensors and Their Applications

The aim of this contribution is to provide an introduction and a brief summary of the principle of fluorescence molecular sensors specific to inorganic phosphate (Pi) and inorganic pyrophosphate (PPi) as well as their applications. In our introduction we describe the impact of both Pi and PPi in the living organism and in the environment, followed by a description of the principle of fluorescence molecular sensors and the sensing mechanism in solution. We then focus on exciting research which has emerged in recent years on the development of fluorescent sensors specific to Pi and PPi, categorized by chemical interactions between the sensor and the target molecule, such as hydrogen bonding, coordination chemistry, displacement assay, aggregation induced emission or quenching, and chemical reactions.

Understanding Stacking Interactions between an Aromatic Ring and Nucleobases in Aqueous Solution: Experimental and Theoretical Study

Stacking interactions between aromatic compounds and nucleobases are crucial in recognition of nucleotides and nucleic acids, but a comprehensive understanding of the strength and selectivity of these interactions in aqueous solution has been elusive. To this end, model complexes have been designed and analyzed by experiment and theory. For the first time, stacking free energies between five nucleobases and anthracene were determined experimentally from thermodynamic double mutant cycles. Three different experimental methods were proposed and evaluated. The dye prefers to bind nucleobases in the order (kcal/mol): G (1.3) > T (0.9) > U (0.8) > C (0.5) > A (0.3). The respective trend of interaction free energies extracted from DFT calculations correlates to that obtained experimentally. Analysis of the data suggests that stacking interactions dominate over hydrophobic effects in an aqueous solution and can be predicted with DFT calculations.

Parallel folding topology-selective label-free detection and monitoring of conformational and topological changes of different G-quadruplex DNAs by emission spectral changes via FRET of mPPE-Ala-Pt(ii) complex ensemble

The formation of supramolecular assemblies between [Pt(bzimpy-Et){C[triple bond, length as m-dash]CC6H4(CH2NMe3-4)}]Cl2 (1) and mPPE-Ala and the FRET properties of the ensemble have been revealed from the UV-vis absorption, steady-state emission and time-resolved emission decay studies. The two-component mPPE-Ala-1 ensemble has been employed in a "proof-of-principle" concept for label-free detection of G-quadruplex DNAs with the intramolecular propeller parallel folding topology, such as c-myc, in aqueous buffer solution. By the modulation of the aggregation/deaggregation of the polymer-metal complex aggregates and hence the FRET from the mPPE-Ala donor to the aggregated 1 as acceptor, the ensemble has been demonstrated for sensitive and selective label-free detection of c-myc via the monitoring of emission spectral changes of the ensemble. Ratiometric emission of the ensemble at 461 and 662 nm has been shown to distinguish the intramolecular propeller parallel G-quadruplex folding topology of c-myc from other G-quadruplex-forming sequences of different folding topologies, owing to the strong and specific interactions between c-myc and 1 as suggested by the UV-vis absorption and UV melting studies. In addition, the formation of high-order intermolecular multimeric G-quadruplexes from c-myc under molecular crowding conditions has been successfully probed by the ratiometric emission of the ensemble. The conformational and topological transition of human telomeric DNA from the mixed-hybrid form to the intramolecular propeller parallel form, as observed from the circular dichroism spectroscopy, has also been monitored by the ratiometric emission of the ensemble. The ability of the ensemble to detect these conformational and topological transitions of G-quadruplex DNAs has been rationalized by the excellent selectivity and sensitivity of the ensemble towards the intramolecular propeller parallel G-quadruplex DNAs and their high-order intermolecular multimers, which are due to the extra stabilization gained from Pt···Pt and π-π interactions in addition to the electrostatic and hydrophobic interactions found in the polymer-metal complex aggregates.

A Cyanuric Acid Platform Based Tripodal Bis-heteroleptic Ru(II) Complex of Click Generated Ligand for Selective Sensing of Phosphates via C-H···Anion Interaction

A new bis-heteroleptic trinuclear Ru(II) complex (1[PF6]6) has been synthesized from electron deficient cyanuric acid platform based copper-catalyzed azide-alkyne cycloaddition, i.e., CuAAC click generated ligand, 1,3,5-tris [(2-aminoethyl-1H-1,2,3-triazol-4-yl)-pyridine]-1,3,5-triazinane-2,4,6-trione (L1). Complex 1[PF6]6 displays weak luminescence (ϕf = 0.002) at room temperature with a short lifetime of ∼5 ns in acetonitrile. It shows selective sensing of hydrogen pyrophosphate (HP2O7(3-)) through 20-fold enhanced emission intensity (ϕf = 0.039) with a 15 nm red shift in emission maxima even in the presence of a large excess of various competitive anions like F(-), Cl(-), AcO(-), BzO(-), NO3(-), HCO3(-), HSO4(-), HO(-), and H2PO4(-) in acetonitrile. Selective change in the decay profile as well as in the lifetime of 1[PF6]6 in the presence of HP2O7(3-) (108 ns) further supports its selectivity toward HP2O7(3-). UV-vis and photoluminescence titration profiles and corresponding Job's plot analyses suggest 1:3 host-guest stoichiometric binding between 1[PF6]6 and HP2O7(3-). High emission enhancement of 1[PF6]6 in the presence of HP2O7(3-) has resulted in the detection limit of the anion being as low as 0.02 μM. However, 1[PF6]6 shows selectivity toward higher analogues of phosphates (e.g., ATP, ADP, and AMP) over HP2O7(3-)/H2PO4(-) in 10% Tris HCl buffer (10 mM)/acetonitrile medium. Downfield shifting of the triazole C-H in a (1)H NMR titration study confirms that the binding of HP2O7(3-)/H2PO4(-) is occurring via C-H···anion interaction. The single crystal X-ray structure of complex 1 having NO3(-) counteranion, 1[NO3]6 shows binding of NO3(-) with complex 1 via C-H···NO3(-) interactions.

Novel method for real-time monitoring of ATP release reveals multiple phases of autocrine purinergic signalling during immune cell activation

AIMS

The activation of immune cells must be tightly regulated to allow an effective immune defence while limiting collateral damage to host tissues. Cellular ATP release and autocrine stimulation of purinergic receptors are recognized as critical regulators of immune cell activation. However, the study of purinergic signalling has been hampered by the short half-life of the released ATP and its breakdown products as well as the lack of real-time imaging methods to study spatiotemporal dynamics of ATP release.

METHODS

To overcome these limitations, we optimized imaging methods that allow monitoring of ATP release with conventional microscopy using the recently developed small molecular ATP probes 1-2Zn(II) and 2-2Zn(II) for imaging of ATP in the extracellular space and release at the surface of living cells.

RESULTS

1-2Zn(II) allowed imaging of <1 μm ATP in the extracellular space, while 2-2Zn(II) provided unprecedented insights into the spatiotemporal dynamics of ATP release from neutrophils and T cells. Stimulation of these cells caused virtually instantaneous ATP release, which was followed by a second phase of ATP release that was localized to the immune synapse of T cells and the leading edge of polarized neutrophils. Imaging these ATP signalling processes along with mitochondrial probes provided evidence for a close spatial relationship between mitochondrial activation and localized ATP release in T cells and neutrophils.

CONCLUSION

We believe that these novel live cell imaging methods can be used to define the roles of purinergic signalling in immune cell activation and in the regulation of other complex physiological processes.

The phenanthroimidazole-based dizinc(ii) complex as a fluorescent probe for the pyrophosphate ion as generated in polymerase chain reactions and pyrosequencing

The phenanthroimidazole-based dizinc(II) complex is an efficient fluorescent probe for the pyrophosphate ion (PPi) in water with a very low detection limit, and also used to detect PPi released from DNA polymerization chain reaction.

The synthesis of UDP-selective fluorescent probe and its imaging application in living cells

A perylene-based probe was developed for uridine diphosphate (UDP) sensing and cell imaging. The probe presented about 4-fold fluorescence enhancement in the presence or absence of 100equiv UDP. The selectivity toward UDP over other phosphor-containing anions was observed. The selective UDP sensing was speculated to be related to the binding affinities of Zn(2+) ions in sensor with the uridine and phosphate moieties of UDP. Furthermore, this probe was also applied to image of UDP in living cells.

A Ga(3+)self-assembled fluorescent probe for ATP imaging in vivo

Adenosine 5'-triphosphate (ATP) is a functional molecule associated with many important biological processes. Fluorescent detection methods for ATP with facile performance and high selectivity are in demand. One of the possible multi-membered arrays assembled between DHBO and Ga(3+) ions was conducted in aqueous solution, which can selectively recognize ATP with fluorescence enhancement from ADP, AMP and other structurally similar nucleoside triphosphates in vitro and in vivo. ATP facilitates the interaction between DHBO and Ga(3+) ions, resulting in the fluorescence increase. The detection limit for ATP was calculated to be 5.49×10(-7)M, which is much lower than that of intracellular concentrations (1-10mM). In addition, DHBO-Ga(3+) can be applied to detect ATP-relevant enzyme activity.

Discrimination of adenine nucleotides and pyrophosphate in water by a zinc complex of an anthracene-based cyclophane

Combining metal-anion coordination and π-π stacking interactions, a zinc complex of a novel anthracene-based cyclophane was designed to recognise adenine nucleoside polyphosphates. This complex was found to show selective fluorescence enhancement for ATP, ADP, AMP and PPi in neutral aqueous solution. Among them, ADP induced the largest fluorescence change to the complex, while ATP showed the strongest binding affinity to the complex. This property was used to sense ATP in the presence of excess amounts of other phosphates such as ADP, AMP, PPi and Pi.

Multidentate europium chelates as luminoionophores for anion recognition: impact of ligand design on sensitivity and selectivity, and applicability to enzymatic assays

The design of photoluminescent molecular probes for the selective recognition of anions is a major challenge for the development of optical chemical sensors. The reversible binding of anions to lanthanide centers is one promising option for the realization of anion sensors, because it leads in some cases to a strong luminescence increase by the replacement of quenching water molecules. Yet, it is an open problem to gain control of the sensitivity and selectivity of the luminescence response. Primarily, the selective detection of (poly)phosphate species such as nucleotides has emerged as a demanding task, because they are involved in many biological processes and enzymatic reactions. We designed a series of pyridyl-based multidentate europium complexes (seven-, six-, and five-dentate) including sensitizing chromophores and studied their luminescence intensity and lifetime responses to different (poly)phosphates (adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), cyclic adenosine monophosphate (cAMP), pyrophosphate, and phosphate anions), and carboxyanions (citrate, malate, oxalacetate, succinate, α-ketoglutarate, pyruvate, oxalate, carbonate). The results reveal that the number of free coordination sites has a significant impact on the sensitivity and selectivity of the response. Because of its reversibility, the lanthanide probes can be applied to monitor the activity of ATP-consuming enzymes such ATPases and apyrases, which is demonstrated by means of the five-dentate complex.

A zinc-dipicolylethylenediamine modified near infrared fluorophore for sensing of ATP

The development of fluorescent probes for sensing of anions in biological environments is still a demanding task. Due to the structural versatility of biological active anions there are many challenges to cope with compared to fluoroionophores for the determination of metal cations. This concerns particularly the design of the recognition element, which has to provide a selective response, preferably unaffected by alterations of pH and ionic strength. Polyphosphate anions such as ATP are interesting targets in bioanalysis because they are involved in many enzymatic reactions and bear versatile biological functions. Zinc dipicolylamine complexes attached to fluorophores have been turned out to be promising candidates for ATP sensing with sufficient sensitivity and selectivity. We now report the first NIR probe that responds to ATP based on a zinc dipicolylethylenediamine receptor. It shows a "turn-on" fluorescence behavior which is selective to other polyphosphate species even at high ionic strength of the sample solution.

Molecule-binding dependent assembly of split aptamer and γ-cyclodextrin: a sensitive excimer signaling approach for aptamer biosensors

A highly sensitive and selective fluorescence aptamer biosensors for the determination of adenosine triphosphate (ATP) was developed. Binding of a target with splitting aptamers labeled with pyrene molecules form stable pyrene dimer in the γ-cyclodextrin (γ-CD) cavity, yielding a strong excimer emission. We have found that inclusion of pyrene dimer in γ-cyclodextrin cavity not only exhibits additive increases in quantum yield and emission lifetime of the excimer, but also facilitates target-induced fusion of the splitting aptamers to form the aptamer/target complex. As proof-of-principle, the approach was applied to fluorescence detection of adenosine triphosphate. With an anti-ATP aptamer, the approach exhibits excimer fluorescence response toward ATP with a maximum signal-to-background ratio of 32.1 and remarkably low detection limit of 80 nM ATP in buffer solution. Moreover, due to the additive fluorescence lifetime of excimer induced by γ-cyclodextrin, time-resolved measurements could be conveniently used to detect as low as 0.5 μM ATP in blood serum quantitatively.

Novel reversible Zn2+-assisted biological phosphate "turn-on" probing through stable aryl-hydrazone salicylaldimine conjugation that attenuates ligand hydrolysis

A novel reversible zinc(II) chemosensing ensemble (2·Zn(2+)) allows for selective "turn-on" fluorescence sensing of ATP and PPi in aqueous media (detection limits: 2.4 and 1.0 μM, respectively) giving selective binding patterns: ATP ∼ PPi > ADP ≫ AMP > monophosphates ≈ remaining ions tested. The conjugated hydrazone [C═N-NH-R] resists hydrolysis considerably, compared to the imine [C═N-CH2-R, pyridin-2-ylmethanamine] functionality, and generalizes to other chemosensing efforts. Prerequisite Zn(2+)·[O(phenol)N(imine)N(pyr)] binding is selective, as determined by UV-vis and NMR spectroscopy; ATP or PPi extracts Zn(2+) to regenerate the ligand-fluorophore conjugate (PPi: turn-on, 512 nm; detection limit, 1.0 μM). Crystallography, 2-D NMR spectroscopy, and DFT determinations (B3LYP/631g*) support the nature of compound 2. 2-Hydrazinyl-pyridine-salicylaldehyde conjugation is unknown, as such; a paucity of chemosensing-Zn(2+) binding reports underscores the novelty of this modifiable dual cation/anion detection platform. A combined theoretical and experimental approach reported here allows us to determine both the potential uniqueness as well as drawbacks of this novel conjugation.

An ATP-selective, lanthanide complex luminescent probe

A luminescent probe based on a europium complex is developed, which effectively distinguishes adenosine-5'-triphosphate (ATP) from adenosine diphosphate (ADP) and adenosine monophosphate (AMP) in pure water at pH 6.8. With a longer lifetime (in ms range), the probe is prospectively applied to biological systems to monitor ATP levels by completely removing the background fluorescence of other molecules.

Highly selective recognition and fluorescence imaging of adenosine polyphosphates in aqueous solution

The design and synthesis of chemosensors for the recognition of a certain nucleoside polyphosphate among various structurally similar nucleoside polyphosphates remain a fundamental challenge. Herein, we report the new fluorescent chemosensor [Zn2L](ClO4)4 (1; L = (3,6,10,13,17,20,24,27-octaaza-1,15(2,6)-dipyridina-8,22(9,10)-dianthracenacyclooctacosaphane), which can selectively recognize adenosine polyphosphates (ATP and ADP) among various nucleoside polyphosphates, with a large fluorescence enhancement (Fmax/F0 = 70 and 80 for ATP and ADP, respectively) and strong binding affinity (K = 3.1 × 10(11) M(-1) for [Zn2HL(H-1ATP)2](-), 2.8 × 10(11) M(-1) for [Zn2L(H-1ATP)2](2-), and 1.5 × 10(13) M(-1) for [Zn2L(H-1ADP)2](2-)) in aqueous solution at physiological pH 7.40. The structure of [Zn2L](P2O7) (2) was investigated, which shows that μ2-pyrophosphate anions alternately link [Zn2L](4+) cations to generate a 1D coordination polymer. The results of (31)P NMR studies and DFT calculations reveal that the two Zn(II) ions in 1 can interact with ATP/ADP anions through coordination interactions between Zn(II) and the polyphosphate groups, and two anthracene moieties in 1 can interact with adenine groups from two ATP or ADP anions through stacking interactions to form a sandwichlike structure. These multiple recognition interactions between 1 and ATP/ADP enhance the affinity and selectivity of 1 toward ATP/ADP. Due to its highly selective and sensitive ability to detect adenosine polyphosphates, 1 was successfully applied to fluorescence imaging for ATP and ADP in living cells, demonstrating the potential utility of 1 as a fluorescent chemosensor for detecting ATP and ADP.

Glutathione-functionalized graphene quantum dots as selective fluorescent probes for phosphate-containing metabolites

Bright blue fluorescent glutathione-functionalized graphene quantum dots (GQDs@GSH) were prepared by a one-step pyrolysis method with a fluorescence quantum yield as high as 33.6%. Futhermore, the obtained GQDs@GSH can be used as a probe to estimate the ATP level in cell lysates and human blood serum.

Reaction-based small-molecule fluorescent probes for chemoselective bioimaging

The dynamic chemical diversity of elements, ions and molecules that form the basis of life offers both a challenge and an opportunity for study. Small-molecule fluorescent probes can make use of selective, bioorthogonal chemistries to report on specific analytes in cells and in more complex biological specimens. These probes offer powerful reagents to interrogate the physiology and pathology of reactive chemical species in their native environments with minimal perturbation to living systems. This Review presents a survey of tools and tactics for using such probes to detect biologically important chemical analytes. We highlight design criteria for effective chemical tools for use in biological applications as well as gaps for future exploration.

Organelle-localizable fluorescent chemosensors for site-specific multicolor imaging of nucleoside polyphosphate dynamics in living cells

ATP and its derivatives (nucleoside polyphosphates (NPPs)) are implicated in many biological events, so their rapid and convenient detection is important. In particular, live cell detection of NPPs at specific local regions of cells could greatly contribute understanding of the complicated roles of NPPs. We report herein the design of two new fluorescent chemosensors that detect the dynamics of NPPs in specific regions of living cells. To achieve imaging of NPPs on plasma membrane surfaces (2-2Zn(II)), a lipid anchor was introduced into xanthene-based Zn(II) complex 1-2Zn(II), which was previously developed as a turn-on type fluorescent chemosensor for NPPs. Meanwhile, for subcellular imaging of ATP in mitochondria, we designed rhodamine-type Zn(II) complex 3-2Zn(II), which possesses a cationic pyronin ring instead of xanthene. Detailed spectroscopic studies revealed that 2-2Zn(II) and 3-2Zn(II) can sense NPPs with a several-fold increase of their fluorescence intensities through a sensing mechanism similar to 1-2Zn(II), involving binding-induced recovery of the conjugated form of the xanthene or pyronin ring. In live cell imaging, 2-2Zn(II) containing a lipid anchor selectively localized on the plasma membrane surface and detected the extracellular release of NPPs during cell necrosis induced by streptolysin O. On the other hand, rhodamine-type complex 3-2Zn(II) spontaneously localized at mitochondria inside cells, and sensed the local increase of ATP concentration during apoptosis. Multicolor images were obtained through simultaneous use of 2-2Zn(II) and 3-2Zn(II), allowing detection of the dynamics of ATP in different cellular compartments at the same time.

Ortho-phenylenediamine-based open and macrocyclic receptors in selective sensing of H2PO4(-), ATP and ADP under different conditions

Ortho-phenylenediamine-based open and macrocyclic receptors have been designed and synthesized. The open receptor 1 and the macrocyclic receptor 2 fluorimetrically distinguish H(2)PO(4)(-) from the other anions examined in CH(3)CN with appreciable binding constant values. As practical applications, they are also sensible to nucleotides in aq. CH(3)CN (1 : 1, v/v). The receptor 1 shows significant emission change upon complexation of ATP and ADP. ADP is selectively distinguished by a ratiometric change in emission. In contrast, the macrocyclic receptor 2, under similar conditions, shows good binding with ATP over the others.

Anion recognition and sensing with Zn(II)-dipicolylamine complexes

This critical review covers the developments in anion recognition and sensing using Zn(II)-dipicolylamine functionalized receptors over the past decade with emphasis on recent rapid advances in the last five years.