First artificial receptors and chemosensors toward phosphorylated peptide in aqueous solution

Indium(III)-Induced Fluorescent Excimer Formation and Extinction in Calix[4]arene−Fluoroionophores

New fluorogenic or/and chromogenic calix[4]arenes 1-3 with two facing amide groups linked to fluorescent pyrene units are synthesized. Orientations of the pyrene units are remote from each other in 1 and face-to-face pi-stacked in 2, which produces different photophysical properties. In the excited state, the two pyrene units of 2 form a strong intramolecular excimer displaying an emission at 472 nm with a relatively weak monomer emission at 395 nm. In contrast, 1 exhibits only a monomer emission at 398 nm because intramolecular hydrogen bonding between the phenolic OH oxygens and the amide hydrogens prevents pi-stacking of the two pyrene groups. Fluorescence changes upon addition of various metal ions show that 1 has a remarkably high selectivity for In(3+) over the other metal ions tested. Compound 1 forms 2:1 (metal:ligand), as well as 1:1 complexes, with In(3+), with fluorescence varying uniquely with the complex stoichiometry. Compound 3, which possess two pyrene units and two chromogenic azo groups, shows almost the same binding behavior toward metal ions as does 1, together with additional bathochromic shifts of the absorption maximum. Compared with 1, compound 3 emits a considerably weaker fluorescence, which is attributed to electron transfer from the pyrene units to the nitro groups of the phenylazo moieties.

Cooperation between Artificial Receptors and Supramolecular Hydrogels for Sensing and Discriminating Phosphate Derivatives

This study has successfully demonstrated that the cooperative action of artificial receptors with semi-wet supramolecular hydrogels may produce a unique and efficient molecular recognition device not only for the simple sensing of phosphate derivatives, but also for discriminating among phosphate derivatives. We directly observed by confocal laser scanning microscopy that fluorescent artificial receptors can dynamically change the location between the aqueous cavity and the hydrophobic fibers upon guest-binding under semi-wet conditions provided by the supramolecular hydrogel. On the basis of such a guest-dependent dynamic redistribution of the receptor molecules, a sophisticated means for molecular recognition of phosphate derivatives can be rationally designed in the hydrogel matrix. That is, the elaborate utilization of the hydrophobic fibrous domains, as well as the water-rich hydrophilic cavities, enables us to establish three distinct signal transduction modes for phosphate sensing: the use of (i) a photoinduced electron transfer type of chemosensor, (ii) an environmentally sensitive probe, and (iii) an artificial receptor displaying a fluorescence resonance energy transfer type of fluorescent signal change. Thus, one can selectively sense and discriminate the various phosphate derivatives, such as phosphate, phospho-tyrosine, phenyl phosphate, and adenosine triphosphate, using a fluorescence wavelength shift and a seesaw type of ratiometric fluorescence change, as well as a simple fluorescence intensity change. It is also shown that an array of the miniaturized hydrogel is promising for the rapid and high-throughput sensing of these phosphate derivatives.

Detection and Quantification of On-Chip Phosphorylated Peptides by Surface Plasmon Resonance Imaging Techniques Using a Phosphate Capture Molecule

We describe herein a detection and quantification system for on-chip phosphorylation of peptides by surface plasmon resonance (SPR) imaging techniques using a newly synthesized phosphate capture molecule (i.e., biotinylated zinc(II) complex). The biotinylated compound is a dinuclear zinc(II) complex that is suitable for accessing phosphate anions as a bridging ligand on the two zinc(II) ions. The compound was exposed on the peptide array and detected with streptavidin (SA) via a biotin-SA interaction by SPR imaging. In the conventional method using antibody, both anti-phosphoserine and anti-phosphotyrosine antibodies were required for phosphoserine and phosphotyrosine detection, respectively. Detection of the phosphate group by the zinc(II) complex, however, was independent of the phosphorylated amino acid residues. The calibration curve for the phosphorylation ratios was established with a calibration chip, on which phosphoserine-containing peptide probes were immobilized. The peptide probes, which were phosphorylated on the surface by protein kinase A, were detected and quantified by SPR imaging using the zinc(II) complex, SA, and anti-SA antibody. The reaction rate and the kinetics of on-chip phosphorylation were also evaluated with the peptide array. The phosphorylation ratio was saturated at approximately 20% in 2 h in this study.

Synthesis, Dual Fluorescence, and Fluoroionophoric Behavior of Dipyridylaminomethylstilbenes

The synthesis, dual fluorescence, and fluoroionophoric behavior of two donor-sigma spacer-acceptor (D-s-A) compounds, trans-4-(N,N-bis(2-pyridyl)amino)methylstilbene (1H) and trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-cyanostilbene (1CN), are reported and compared to that of trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-(N,N-dimethylamino)stilbene (1DPA). To gain insights into the dual fluorescence properties for 1H and 1CN in polar but not in nonpolar solvents, model compounds resulting from a replacement of the stilbene group by alkyl (2R) or xylyl (2X) groups or from a replacement of the dipyridylamino (dpa) group by dianisoleamino (3AA), diethylamino (3EE), methylanilino (3MP), or diphenylamino (3PP) groups also have been investigated. In addition to 1H and 1CN, all four compounds of 3 display dual fluorescence. The locally excited (LE) fluorescence mainly results from the stilbene group and the ICT fluorescence from the through-bond interactions between the amino donor and the stilbene acceptors. In the presence of transition metal ions such as Zn(II), Ni(II), Cu(II), and Cd(II), the ICT processes are switched from dpa (D) --> stilbene (A) in 1H and 1CN to stilbene (D) --> dpa/metal ion (A) in their complexes. Whereas the ICT states for the complexes are generally nonfluorescent, an exception was found for the case of 1H/Zn(II). As a result, substituent-dependent fluoroionophoric behavior has been demonstrated by 1H, 1CN, and 1DPA in response to Zn(II).

Anion-Mediated Phase Transfer of Zinc(II)-Coordinated Tyrosine Derivatives

[reaction: see text] Tyrosine-derived Zn(2+) coordination complexes and their fluorescent NBD conjugates are synthesized in a short, high-yielding procedure. The Zn(2+) complexes are highly water soluble, but in the presence of sodium laurate they readily transfer into an octanol layer. Furthermore, the NBD-labeled bis-Zn(2+) complex can partition into vesicle membranes containing anionic phospholipids.

Bifunctional Fluorescent Calix[4]arene Chemosensor for Both a Cation and an Anion

[reaction: see text] A new fluorogenic cone calix[4]triazacrown-5 (1) bearing two pyrene amide groups and its structural analogue 2 have been prepared. Excited at 343 nm, 1 and 2 reveal excimer emissions at 448 and 472 nm, respectively. When heavy metal ions such as Pb(2+) and Co(2+) are bound to 1, the fluorescence intensities of both monomer and excimer are quenched whereas H bonding-assisted F(-) binding to 1 gives rise to a quenched monomer emission with little excimer emission change.

Photophysics and Biological Applications of the Environment-Sensitive Fluorophore 6-N,N-Dimethylamino-2,3-naphthalimide

We have synthesized a new environment-sensitive fluorophore, 6-N,N-dimethylamino-2,3-naphthalimide (6DMN). This chromophore exhibits valuable fluorescent properties as a biological probe with emission in the 500-600 nm range and a marked response to changes in the environment polarity. The 6DMN fluorescence is red-shifted in polar protic environments, with the maximum emission intensity shifting more than 100 nm from 491 nm in toluene to 592 nm in water. Additionally, the fluorescence quantum yield decreases more than 100-fold from chloroform (Phi = 0.225) to water (Phi = 0.002). The scope and applications of the 6DMN probe are expanded with the synthesis of an Fmoc-protected amino acid derivative (5), which contains the fluorophore. This unnatural amino acid has been introduced into several peptides, demonstrating that it can be manipulated under standard solid-phase peptide synthesis conditions. Peptides incorporating the new residue can be implemented for monitoring protein-protein interactions as exemplified in studies with Src homology 2 (SH2) phosphotyrosine binding domains. The designed peptides exhibit a significant increase in the quantum yield of the long wavelength fluorescence emission band (596 nm) upon binding to selected SH2 domains (e.g., Crk SH2, Abl SH2, and PI3K SH2). The peptides can be used as ratiometric sensors, since the short wavelength band (460 nm) was found almost invariable throughout the titrations.

A fluorescent pyrophosphate sensor via excimer formation in water

A new fluorescent sensor based on a pyrene/Zn(II)-dpa (dpa = bis(2-pyridylmethyl)amine) conjugate displays excimer emission selective for pyrophosphate over other anions.

An Excimer-Based, Binuclear, On−Off Switchable Calix[4]crown Chemosensor

A new fluorescent chemosensor with two different types of cation binding sites on the lower rims of a 1,3-alternate calix[4]arene (1) is synthesized. Two pyrene moieties linked to a cation recognition unit composed of two amide groups form a strong excimer in solution. For 1, the excimer fluorescence is quenched by Pb2+, but revived by addition of K+ to the Pb2+ ligand complex. Thus, metal ion exchange produces an on-off switchable, fluorescent chemosensor. Computational results show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the two pyrene moieties interact under UV irradiation of 1 and its K+ complex, while such HOMO-LUMO interactions are absent in the Pb2+ complex.

Molecular Recognition in a Supramolecular Hydrogel to Afford a Semi-Wet Sensor Chip

This communication describes a new molecular recognition chip using a semi-wet microenvironment provided by a self-assembled hydrogel. On the basis of the evidence that the molecular recognition capability of artificial chemosensors are practically retained even in the hydrogel compared to those in aqueous solution, we miniaturized the functionalized hydrogel to produce an unprecedented molecular recognition chip. We believe that the present noncovalent immobilization method is generally applicable to many chemosensors, which leads to a unique semi-wet sensor chip suitable to convenient and high-throughput assay to plural analytes.

Two-Photon Sensor for Metal Ions Derived from Azacrown Ether

A two-photon sensor for the metal ions derived from azacrown ether as the receptor is reported. The sensor emits strong two-photon fluorescence when excited by 800 nm laser photons. Moreover, the binding constants measured by the one- and two-photon fluorescence are similar. This result may be useful for the design of efficient two-photon fluorescence probes for biological substrates.

Recent advances in enzyme assays

Enzyme assays for high-throughput screening and enzyme engineering, which are often based on derivatives of coumarin, nitrophenol, fluorescein, nitrobenzofurazane or rhodamine dyes, can be divided into two categories: those that depend on labelled substrates, and those that depend on sensing the reactions of unmodified substrates. Labelled substrates include, for example, fluorogenic and chromogenic substrates that generate a reporter molecule by beta-elimination, fluorescence resonance energy transfer (FRET) substrates and isotopic labels for enantioselectivity screening. By contrast, endpoint sensing can be done using amine reagents, fluorescent affinity labels for phosphorylated proteins, or synthetic multifunctional pores. Sensing assays can also be done in real time by using, for example, aldehyde trapping to follow vinyl ester acylation in organic solvent or calcein-copper fluorescence for sensing amino acids. The current trend is to assemble many such assays in parallel for enzyme profiling and enzyme fingerprinting.

Unique Hydrogen Bonds between 9-Anthracenyl Hydrogen and Anions

Unique hydrogen bonds of the 9-H of anthracene moieties in hosts 1 and 2 with fluoride and pyrophosphate ions were observed on the basis of the (1)H NMR experiments. Furthermore, hosts 1 and 2 act as a colorimetric sensor and a fluorescent chemosensor for the recognition of fluoride ion, respectively.

Zn(II)-Induced Ground-State π-Deconjugation and Excited-State Electron Transfer in N,N-Bis(2-pyridyl)amino-Substituted Arenes

The synthesis and X-ray crystal structures of two N,N-bis(2-pyridyl)amino (dpa)-substituted aromatic systems (Ar-dpa) 1 (Ar = 4,4'-disubstituted trans-stilbene) and 2 (Ar = 1,4-disubstituted benzene) and their ZnCl(2) complexes (1/ZnCl(2) and 2/ZnCl(2)) are reported. The fluoroionophoric behavior of 1-2 in response to Zn(II) in acetonitrile also has been investigated. In addition, compound 3DPA has been prepared and served as a pi-deconjugated model for 1DPA. The observed crystal structures for 1/ZnCl(2) and 2/ZnCl(2) could be divided into two distinct types, the planar and the twisted forms, depending on the aryl-dpa (C(ph)-NC(3)) dihedral angle. The twisted form is more favorable for these complexes unless the arene has a strong "push-pull" character. Nonetheless, the degree of pi-conjugation between the N-pyridyl and the N-aryl group is reduced in both complex forms when compared with the free ligands. Such a Zn(II)-induced pi-deconjugation not only directly affects the internal charge transfer (ICT) fluorescence of the dpa-substituted stilbenes but also facilitates the occurrence of photoinduced electron transfer (PET) from the stilbene donor to the dpa/Zn(II) acceptor. The PET process is particularly important in accounting for the observed Zn(II)-induced fluorescence quenching for 1DPA as well as 3DPA.

Molecular Recognition and Fluorescence Sensing of Monophosphorylated Peptides in Aqueous Solution by Bis(zinc(II)−dipicolylamine)-Based Artificial Receptors

The phosphorylation of proteins represents a ubiquitous mechanism for the cellular signal control of many different processes, and thus selective recognition and sensing of phosphorylated peptides and proteins in aqueous solution should be regarded as important targets in the research field of molecular recognition. We now describe the design of fluorescent chemosensors bearing two zinc ions coordinated to distinct dipicolylamine (Dpa) sites. Fluorescence titration experiments show the selective and strong binding toward phosphate derivatives in aqueous solution. On the basis of (1)H NMR and (31)P NMR studies, and the single-crystal X-ray structural analysis, it is clear that two Zn(Dpa) units of the binuclear receptors cooperatively act to bind a phosphate site of these derivatives. Good agreement of the binding affinity estimated by isothermal titration calorimetry with fluorescence titration measurements revealed that these two receptors can fluorometrically sense several phosphorylated peptides that have consensus sequences modified with natural kinases. These chemosensors display the following significant features: (i) clear distinction between phosphorylated and nonphosphorylated peptides, (ii) sequence-dependent recognition, and (iii) strong binding to a negatively charged phosphorylated peptide, all of which can be mainly ascribed to coordination chemistry and electrostatic interactions between the receptors and the corresponding peptides. Detailed titration experiments clarified that the phosphate anion-assisted coordination of the second Zn(II) to the binuclear receptors is crucial for the fluorescence intensification upon binding to the phosphorylated derivatives. In addition, it is demonstrated that the binuclear receptors can be useful for the convenient fluorescent detection of a natural phosphatase (PTP1B) catalyzed dephosphorylation.

Chemoselective signalling of selected phospho-anions using lanthanide luminescence

Selectivity in the binding of phosphorylated tyrosine residues to aqua-lanthanide complexes is signalled by changes in spectral form by luminescence emission and 1H NMR spectroscopy.

The affinity of phosphates to zinc(ii) complexes can be increased with hydrogen bond donors

Amino H-bond donors adjacent to a zinc(II) centre increase the affinity of phosphates to the zinc(II) centre.

A Lanthanide Hybrid Cluster as a Selective Optical Chemosensor for Phosphate-containing Anions in Aqueous Solution

An europium-chelating [2Fe-2S] cluster was used to assemble an optical molecular chemosensor highly selective for phosphate-containing anions. Phosphate, pyrophosphate, AMP, ADP, ATP, DNA and RNA were well distinguished by UV/V is absorption or fluorescence studies.

Recognition and fluorescence sensing of specific amino acid residue on protein surface using designed molecules

Many biological processes are mediated by surface recognition between proteins. Small molecules that recognize and bind a specific region of a protein surface may be promising agents for disrupting certain protein-protein surface interactions, which consequently leads to regulation of cellar functions. This article describes our recent efforts toward the development of the designed small molecules, which can recognize histidine or phosphorylated amino acid residues on peptide surfaces in a sequence-selective manner. These results demonstrate that cooperative metal-ligand interaction is powerful for tight and selective binding to the specific amino acid residues of proteins in aqueous medium.

Highly Effective Fluorescent Sensor for H2PO4-

A new anthracene dimer connected by two imidazolium moieties has been systematically designed and synthesized as a fluorescent chemosensor for selective binding of H(2)PO(4)(-) over other anions, which have been examined using fluorescence and (1)H NMR and rationalized with ab initio study.

Versatile Fluorescence Probes of Protein Kinase Activity

We introduce a versatile fluorescent peptide reporter of protein kinase activity. The probe can be modified to target a desired kinase by changing the kinase recognition motif in the peptide sequence. The reporter motif contains the Sox amino acid, which generates a fluorescence signal when bound to Mg2+ present in the reaction mixture. The phosphorylated peptide exhibits a much greater affinity for Mg2+ than its unphosphorylated analogue and, thus, a greater fluorescence intensity. Product formation during phosphorylation by the kinase is easily followed by the increase in fluorescence intensity over time. These probes exhibit a 3-5-fold increase in fluorescence intensity upon phosphorylation, the magnitude of which depends on the substrate. Peptides containing the reporter functionality are phosphorylated on serine by Protein Kinase C and cAMP-dependent protein kinase and are shown to be good substrates for these enzymes. The principle of this design extends to peptides phosphorylated on threonine and tyrosine.

Cross-linking strategy for molecular recognition and fluorescent sensing of a multi-phosphorylated peptide in aqueous solution

In the research field of molecular recognition, selective recognition and sensing of phosphorylated protein surfaces is strongly desirable both for elucidation of protein-protein recognition at the molecular level and for regulation of signal transduction through protein surfaces. Here we describe a new strategy for molecular recognition of a multi-phosphorylated peptide using intrapeptide cross-linking on the basis of coordination chemistry. The present artificial receptor can selectively bind to doubly phosphorylated peptide through multiple-point interactions and fluorescently sense the binding event with an association constant of more than 106 M-1 in neutral aqueous solution.

A trinuclear heterobimetallic Ru(II)/Pt(II) complex as a chemodosimeter selective for sulfhydryl-containing amino acids and peptides

A trinuclear heterobimetallic Ru(II)/Pt(II) complex, cis-{Ru(phen)2[CN-Pt(DMSO)Cl2]2} (phen = 1,10-phenanthroline), is able to function as a "switch-on" luminescent chemodosimeter for sulfhydryl-containing amino acids and peptides via specific binding of the amino acids/peptides with the Pt(II) centers and the subsequent cleavage of the Ru(II)-Pt(II) cyano-bridge.

Fluorescence and NMR binding studies of N-aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives

N-Aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives perform as fluorescent photoinduced electron-transfer (PET) sensors with very selective response toward Ca(2+) versus Mg(2+), Na(+), and K(+). The fluorescence intensity was increased by a factor of up to 170 in the presence of Ca(ClO(4))(2). (1)H NMR studies show that metal cations affect these molecules very differently: Ca(2+) has a global effect on each molecule, while Mg(2+) affects part of each molecule, and K(+) and Na(+) affect each molecule moderately, which is very consistent with the fluorescence response.

Hydrogen-bond forming ionophore for highly efficient transport of phosphate anions across the nitrobenzene-water interface

Thiourea-based hydrogen-bond forming ionophore 2, alpha,alpha'-bis(N'-p-nitrophenylthioureylene)-m-xylene, is synthesized and investigated by using ion transfer polarography for the facilitated transfers of H2PO4-, HPO42- and Cl- across the nitrobenzene-water interface. Bis-thiourea 2 has a significant ability to assist H2PO4- transfer across the interface whereas its counterpart, N-(p-nitrophenyl)-N'-propylthiourea (ionophore 3), cannot facilitate the transfer of this hydrophilic anion. The H2PO4- transfer assisted by 2 is based on the formation of a 2:1 complex between H2PO4- and ionophore, and the transfer reaction is more stable by over -12 kJ mol(-1) than the case of 3. The stabilization of the H2PO4- transfer for 2 is even stronger by -11 kJ mol(-1) than that for bis-thiourea 1, 2,7-di-t-butyl-4,5-bis(N'-butylthioureylene)-9,9-dimethylxanthene, which forms a 1:1 complex through the formation of four hydrogen bonds. Bis-thiourea 2 is also able to facilitate transfers of HPO42- and Cl- by the formation of 1:1 complex. As compared to bis-thiourea 1, HPO42- transfer by 2 is significantly stabilized by -27 to -31 kJ mol(-1) while the stabilization of the Cl- transfer is relatively moderate (-6.1 kJ mol(-1)). These binding properties of bis-thiourea 2 are discussed for the design of phosphate-selective ionophores for use in two-phase distribution systems such as ion-selective electrodes.