QZ1 and QZ2: rapid, reversible quinoline-derivatized fluoresceins for sensing biological Zn(II)

Solution and fluorescence properties of symmetric dipicolylamine-containing dichlorofluorescein-based Zn2+ sensors

The mechanism by which dipicolylamine (DPA) chelate-appended fluorophores respond to zinc was investigated by the synthesis and study of five new analogues of the 2',7'-dichlorofluorescein-based Zn(2+) sensor Zinpyr-1 (ZP1). With the use of absorption and emission spectroscopy in combination with potentiometric titrations, a detailed molecular picture has emerged of the Zn(2+) and H(+) binding properties of the ZP1 family of sensors. The two separate N(3)O donor atom sets on ZP1 converge to form binding pockets in which all four heteroatoms participate in coordination to either Zn(2+) or protons. The position of the pyridyl group nitrogen atom, 2-pyridyl or 4-pyridyl, has a large impact on the fluorescence response of the dyes to protons despite relatively small changes in pK(a) values. The fluorescence quenching effects of such multifunctional electron-donating units are often taken as a whole. Despite the structural complexity of ZP1, however, we provide evidence that the pyridyl arms of the DPA appendages participate in the quenching process, in addition to the contribution from the tertiary nitrogen amine atom. Potentiometric titrations reveal ZP1 dissociation constants (K(d)) for Zn(2+) of 0.04 pM and 1.2 nM for binding to the first and second binding pockets of the ligand, respectively, the second of which correlates with the value observed by fluorescence titration. This result demonstrates that both binding pockets of this symmetric, ditopic sensor need to be occupied in order for full fluorescence turn-on to be achieved. These results have significant implications for the design and implementation of fluorescent sensors for studies of mobile zinc ions in biology.

A synthetically simple, click-generated cyclam-based zinc(II) sensor

A cyclam-based macrocyclic sensor has been prepared using synthetically simple "click" chemistry to link a fluorophore to the macrocyclic receptor. This sensor shows high selectivity for Zn(II) over a range of other metals, providing a significant enhancement of fluorescence intensity over a wide pH range. As such, this is the first cyclam-based sensor demonstrated to be selective for Zn(II) and is the first example of a triazole being used as a coordinating ligand on an azamacrocycle. The sensor can access biologically available zinc in mammalian cells, sensing the Zn(II) flux that exists during apoptotic cell death.

Small-molecule fluorescent sensors for investigating zinc metalloneurochemistry

The metalloneurochemistry of Zn(II) is of substantial current interest. Zinc is the second most abundant d-block metal ion in the human brain, and its distribution varies with relatively high concentrations found in the hippocampus. Brain zinc is generally divided into two types, protein-bound and loosely bound, the latter also being termed histochemically observable, chelatable, labile, or mobile zinc. The neurophysiological and neuropathological significance of mobile Zn(II) remains enigmatic. Studies of Zn(II) distribution, translocation, and function in vivo require tools for its detection. Because Zn(II) has a closed-shell d(10) configuration and no convenient spectroscopic signature, fluorescence is a well-suited method for monitoring Zn(II) in biological contexts. This Account summarizes work by our laboratory addressing the design, preparation, characterization, and use of small-molecule fluorescent sensors for imaging Zn(II) in living cells and samples of brain tissue. These sensors provide "turn-on" or ratiometric Zn(II) detection in aqueous solution at neutral pH. By making alterations to the Zn(II)-binding unit and fluorophore platform, we have devised sensors with varied photophysical and metal-binding properties. Several of these probes have been applied to image Zn(II) distribution, uptake, and mobilization in a variety of cell types, including neuronal cultures.

A fluorescent probe for zinc ions based on N-methyltetraphenylporphine with high selectivity

N-methyl-alpha,beta,gamma,delta-tetraphenylporphine (NMTPPH) has been used to detect trace amount of zinc ions in ethanol-water solution by fluorescence spectroscopy. The fluorescent probe undergoes a fluorescent emission intensity enhancement upon binding to zinc ions in EtOH/H(2)O (1:1, v/v) solution. The fluorescence enhancement of NMTPPH is attributed to the 1:1 complex formation between NMTPPH and Zn(II) which has been utilized as the basis for the selective detection of Zn(II). The linear response range covers a concentration range of Zn(II) from 5.0x10(-7) to 1.0x10(-5)mol/L and the detection limit is 1.5x10(-7)mol/L. The fluorescent probe exhibits high selectivity over other common metal ions except for Cu(II).

N2-functionalized blue luminescent guanosines by 2,2'-dipyridylamino and 2-(2'-pyridyl)benzimidazolyl chelate groups and their interactions with Zn(II) ions

The syntheses of new blue luminescent N(2)-modified guanosine derivatives with chromophores p-4,4'-biphenyl-NPh2 (1a), p-4,4'-biphenyl-N(2-py)2 (1b), and p-4,4'-biphenyl-2-(2'-pyridyl)benzimidazolyl (1c), respectively, have been achieved. These new N(2)-guanosines are moderate blue emitters with lambda(max) = 395 nm (1a), 370 nm (1b), and 403 nm (1c) and Phi = 0.13, 0.07, and 0.10 in tetrahydrofuran, respectively. Spectroscopic studies and density-functional theory calculations established that the guanine moiety and the new chromophore in all three molecules are involved in the luminescent process. We have also established that guanosines 1a-1c can interact with metal ions such as Zn(II). The interactions of Zn(II) ions with the three guanosines were examined via absorption, fluorescence, circular dichroism (CD), and NMR spectroscopic analyses. We have found that these guanosines display a distinct fluorescent response toward Zn(II) ions which can be attributed to the presence of the chelate chromophore N(2-py)2 in 1b and 2-py-benzimidazolyl in 1c. For 1a and 1b, the addition of Zn(II) ions causes straight fluorescent quenching while for 1c the addition of Zn(II) ions causes quenching initially, which is followed by a distinct spectral red shift and the intensity enhancement of the new emission peak. NMR and CD studies demonstrated that the Zn(II) ions bind preferentially to the guanine moiety in 1a and 1b but to the 2-(2'-py)benzimidazolyl chelate site in 1c. Moreover, the anion-dependent CD response of 1a-1c toward Zn(II) salts points to the possible involvement of intramolecular hydrogen bonding between the acetate bound to the Zn(II) ion and the hydroxyl groups of the guanosine.

Synthesis of highly selective fluorescent peptide probes for metal ions: tuning selective metal monitoring with secondary structure

Metal selective fluorescent peptide probes (dansyl-Cys-X-Gly-His-X-Gly-Glu-NH2, X = Pro or Gly) were developed by synthesizing peptides containing His, Cys, and Glu residues with Pro-Gly sequence to stabilize a turn structure and Gly-Gly sequence to adopt a random coil. The probe containing two Gly-Gly sequences exhibited marked selectivity only for Cu2+ over 13 metal ions including competitive transition and Group I and II metal ions under physiological buffer condition. In contrast, the probe containing double Pro-Gly sequences showed high selectivity for Zn2+. The peptide probe containing one Pro-Gly sequence exhibited selectivity for Zn2+ and Cu2+. CD spectra indicated that the secondary structure of the probes played an important role in the selective metal monitoring and a pre-organized secondary structure is not required for the selective detection of Cu2+ ion, but is required for the detection of Zn2+. We investigated and characterized the binding affinity, binding stoichiometry, reversibility, and pH sensitivity of the peptide probes.

Design and synthesis of a caged Zn2+ probe, 8-benzenesulfonyloxy-5-N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant 1,4,7,10-tetraazacyclododecane, and its hydrolytic uncaging upon complexation with Zn2+

8-Benzenesulfonyloxy-5- N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant cyclen (BS-caged-L(4), BS = benzenesulfonyl) was designed and synthesized as a "caged" derivative of a previously described Zn(2+) fluorophore, 8-hydroxy-5- N,N-dimethylaminosulfonylquinolin-2-ylmethyl-pendant cyclen (L(4)) (cyclen = 1,4,7,10-tetraazacyclododecane). In the absence of metal ions and in the dark, BS-caged-L(4) (10 microM) showed negligible fluorescence emission at pH 7.4 (10 mM HEPES with I = 0.1 (NaNO3)) and 25 degrees C (excitation at 328 nm). Addition of Zn(2+) induced an increase in the UV/vis absorption of BS-caged-L(4) (10 microM) at 258 nm and a significant increase in fluorescence emission at 512 nm. These responses are results from the formation of Zn(H-1L(4)) by the hydrolysis of the sulfonyl ester at the 8-position of the quinoline unit promoted by the Zn(2+)-bound HO(-). Improvement of cell membrane permeation in comparison with L(4) is also described.

Determination of zinc using carbonic anhydrase-based fluorescence biosensors

This chapter summarizes the use of carbonic anhydrase (CA)-based fluorescent indicators to determine free zinc in solution, in cells, and in subcellular organelles. Expression (both in situ and in vitro) and preparation of CA-based indicators are described, together with techniques of their use, and procedures to minimize contamination. Recipes for zinc buffers are supplied.

Design, synthesis, and evaluation of peptidyl fluorescent probe for Zn2+ in aqueous solution

A new fluorescent peptide probe for the detection of Zn(2+) was synthesized on the basis of zinc binding ligands in zinc enzymes. The peptide that has a unique amino acid sequence displayed a great selectivity for Zn(2+) in the presence of several transition metal ions in aqueous solution. The reversibility, binding stoichiometry, binding affinity, and pH sensitivity of the sensor were studied. Further, on-bead application of the peptide as chemosensors was demonstrated.

"Turn-on" fluorescent sensor for the selective detection of zinc ion by a sterically-encumbered bipyridyl-based receptor

A sterically-encumbered 5,5'-distyryl-2,2'-bipyridyl derivative that enforces a 1:1 metal-to-ligand ratio acts as a selective turn-on sensor for Zn(2+) in THF.

Development of a ratiometric fluorescent zinc ion probe in near-infrared region, based on tricarbocyanine chromophore

Novel ratiometric fluorescent probes for Zn2+ in the near-infrared region, based on a tricarbocyanine chromophore, have been designed, synthesized, and evaluated. Upon addition of Zn2+, a 44 nm red shift of the absorption maximum was observed, which indicates that this probe could work as a ratiometric probe for Zn2+. This change is due to the difference in the electron-donating ability of the amine substituent before and after reaction with Zn2+. This fluorescence modulation of amine-substituted tricarbocyanines should be applicable to dual-wavelength measurement of various biomolecules or enzyme activities.

A water-soluble "switching on" fluorescent chemosensor of selectivity to Cd2+

Compound 1, a new fluorescent chemosensor signaling via significantly enhanced fluorescence when bound with cation analytes, was synthesized and characterized. This fluorescent chemosensor exhibits its selectivity to Cd2+ among a series of cations in HEPES buffer solution. Its in vitro sensitivity to Cd2+ was demonstrated in the HK-2 cell line with use of confocal microscopy. The mechanistic selectivity and sensitivity of compound 1 to Cd2+ was discussed on the basis of fluorescence, 1H NMR, and mass spectroscopic results.

Selective Fluorescence Zinc Ion Sensing and Binding Behavior of 4-Methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol: Biological Application

Zinc ion fluorescence sensing and the binding properties of 4-methyl-2,6-bis(((phenylmethyl)imino)methyl)phenol (HL) have been investigated. It displays high selectivity for Zn2+ and can be used as zinc ion-selective luminescent probe for biological application under physiological conditions. The increase in emission in the presence of Zn2+ is accounted for by the formation of hexanuclear complex [Zn6(L)2(OH)2(CH3COO)8] characterized by X-ray crystallography. An approximately 6-fold Zn2+-selective chelation-enhanced fluorescence response in HEPES buffer (pH 7.4) is attributed due to the strong coordination of Zn(II) that would impose rigidity and hence decrease the nonradiative decay of the excited state. By incubation of cultured living cells (B16F10 mouse melanoma and A375 human melanoma) with HL, intracellular Zn2+ concentration could be monitored.

Midrange affinity fluorescent Zn(II) sensors of the Zinpyr family: syntheses, characterization, and biological imaging applications

The syntheses and photophysical characterization of ZP9, 2-{2-chloro-6-hydroxy-3-oxo-5-[(2-{[pyridin-2-ylmethyl-(1H-pyrrol-2-ylmethyl)amino]methyl}phenylamino)methyl]-3H-xanthen-9-yl}benzoic acid, and ZP10, 2-{2-chloro-6-hydroxy-5-[(2-{[(1-methyl-1H-pyrrol-2-ylmethyl)pyridin-2-ylmethylamino]methyl}phenylamino)methyl]-3-oxo-3H-xanthen-9-yl}benzoic acid, two asymmetrically derivatized fluorescein-based dyes, are described. These sensors each contain an aniline-based ligand moiety functionalized with a pyridyl-amine-pyrrole group and have dissociation constants for Zn(II) in the sub-micromolar (ZP9) and low-micromolar (ZP10) range, which we define as "midrange". They give approximately 12- (ZP9) and approximately 7-fold (ZP10) fluorescence turn-on immediately following Zn(II) addition at neutral pH and exhibit improved selectivity for Zn(II) compared to the di-(2-picolyl)amine-based Zinpyr (ZP) sensors. Confocal microscopy studies indicate that such asymmetrical fluorescein-based probes are cell permeable and Zn(II) responsive in vivo.

Fluorescent molecular sensors and multi-photon microscopy in brain studies

To clarify the brain phenomena, to prove directly the major biochemical pathways in cerebral tissue, and to discover the crucial steps in brain pathology, it is necessary to develop a high speed deep-tissue imaging techniques with high spatial and temporal resolution, and ultra-fast and highly selective molecular sensors, giving a possibility to monitor target molecules directly in their physiological environment. This technical comment accents on the perspectives for development of direct approach for investigation of function/flow coupling phenomenon and zinc transport into the brain, based on the current progress in development of ultra-fast molecular sensors for direct visualization of biochemical mediators and neurotransmitters, and high speed multi-photon deep-tissue imaging.

Turn-on and ratiometric mercury sensing in water with a red-emitting probe

The synthesis, photophysical properties, and Hg(II) binding of a red-emitting sensor for mercuric ion are presented. 2-[11-[(2-[[Bis-(2-ethylsulfanylethyl)amino]methyl]phenylamino)methyl]-3-hydroxy-10-oxo-10H-benzo[c]xanthen-7-yl]benzoic acid (MS5) is based on the seminaphthofluorescein chromophore and employs a thioether-rich metal-binding unit. This sensor affords both turn-on and single-excitation dual-emission ratiometric Hg(II) detection in aqueous solution. The fluorescence response of MS5 is Hg(II)-specific, and the probe is selective for Hg(II) over alkali and alkaline earth metals, most divalent first-row transition metal ions, and the Group 12 congeners Zn(II) and Cd(II). MS5 binds Hg(II) reversibly and can be recycled. The EC50 for 1 microM MS5 is 910 nM, and a lower detection limit of 50 nM is obtained when employing 500 nM probe. X-ray crystallographic studies using a salicylaldehyde-based model of MS5 are also presented. 2-[(2-[[Bis-(2-ethylsulfanylethyl)amine]methyl]phenylamine)methyl]phenol coordinates Hg(II) with two thioether sulfur atoms, two amino nitrogen atoms, and a phenol oxygen atom arranged in a distorted trigonal bipyramidal geometry. Studies of natural water samples spiked with mercuric salts indicate that MS5 can rapidly detect Hg(II) in such complex solutions and demonstrate its potential utility in the field.

Study on the New Fluorescence Enhancement System of Tb –N-(2 - Pyridinyl) Ketoacetamide-Et3N-Zn and its Application

A sensitive fluorescence enhancement system was developed for the determination of zinc (II). The fluorescence intensity of the Tb- N- (2 - Pyridinyl) ketoacetamide (PKA) system was greatly enhanced by the addition of triethylamine (Et(3)N) and zinc nitrate in the methanol solution. The excitation and emission wavelengths were 329 nm and 546 nm, respectively. Under optimal conditions, the fluorescence intensities varied linearly with the concentration of Zn(2+) in the range of 8.0 x 10(-7)-5.0 x 10(-6) M with a detection limit of 9.9 x 10(-8) M. The interferences of some substances were described. This method was applied to the determination of amounts of Zn(2+) in soybean, rice, and wheat, respectively. The results showed that the proposed procedure is a high selective, simple, and rapid method to the determination of Zn(2+) ion. The mechanism of fluorescence enhancement was also studied.

Zinspy sensors with enhanced dynamic range for imaging neuronal cell zinc uptake and mobilization

Thiophene moieties were incorporated into previously described Zinspy (ZS) fluorescent Zn(II) sensor motifs (Nolan, E. M.; Lippard, S. J. Inorg. Chem. 2004, 43, 8310-8317) to provide enhanced fluorescence properties, low-micromolar dissociation constants for Zn(II), and improved Zn(II) selectivity. Halogenation of the xanthenone and benzoate moieties of the fluorescein platform systematically modulates the excitation and emission profiles, pH-dependent fluorescence, Zn(II) affinity, and Zn(II) complexation rates, offering a general strategy for tuning multiple properties of xanthenone-based metal ion sensors. Extensive biological studies in cultured cells and primary neuronal cultures demonstrate 2-{6-hydroxy-3-oxo-4,5-bis[(pyridin-2-ylmethylthiophen-2-ylmethylamino)methyl]-3H-xanthen-9-yl}benzoic acid (ZS5) to be a versatile imaging tool for detecting Zn(II) in vivo. ZS5 localizes to the mitochondria of HeLa cells and allows visualization of glutamate-mediated Zn(II) uptake in dendrites and Zn(II) release resulting from nitrosative stress in neurons.

Highly selective 4-amino-1,8-naphthalimide based fluorescent photoinduced electron transfer (PET) chemosensors for Zn(II) under physiological pH conditions

The design and synthesis of two novel fluorescent sensors based on the photoinduced electron transfer (PET) concept, and , for the detection of zinc under competitive media is described. These sensors are based on the 4-amino-1,8-naphthalimide fluorophore, which has an absorption band centred at 450 nm and emits in the green with lambda(max) approximately 550 nm. By functionalizing the chromophore with a simple benzyl or ethyl-aryl based iminodiacetate receptor at the 4-position, both high selectivity and sensitivity were achieved for the sensing of Zn(II) over other competitive transition and Group I and II metal ions. These sensors were also shown to be pH independent, with a pKa of 2.3 being determined for , which allows these to be used in highly competitive pH media. Upon sensing of Zn(II) the fluorescence emission spectrum is 'switched on' demonstrating the suppression of PET from the receptor to the fluorophore. For , the sensing of Zn(II) was achieved with Kd = 4 nM when measured in pH 7.4 buffered solution, in the presence of 1.1 mM of EGTA.

In Vivo Monitoring of Mercury Ions Using a Rhodamine-Based Molecular Probe

Exposure to mercury causes severe damage to various tissues and organs in humans. Concern over mercury toxicity has encouraged the development of efficient, sensitive, and selective methods for the in vivo detection of mercury. Although a variety of chemosensors have been exploited for this purpose, no in vivo monitoring systems have been described to date. In this report, we describe an irreversible rhodamine chemosensor-based, real-time monitoring system to detect mercury ions in living cells and, in particular, vertebrate organisms. The chemosensor responds rapidly, irreversibly, and stoichiometrically to mercury ions in aqueous media at room temperature. The results of experiments with mammalian cells and zebrafish show that the mercury chemosensor is cell and organism permeable and that it responds selectively to mercury ions over other metal ions. In addition, real-time monitoring of mercury-ion uptake by cells and zebrafish using this chemosensor shows that saturation of mercury-ion uptake occurs within 20-30 min in cells and organisms. Finally, accumulation of mercury ions in zebrafish tissue and organs is readily detected by using this rhodamine-based chemosensor.

Direct Nitric Oxide Detection in Aqueous Solution by Copper(II) Fluorescein Complexes

A series of FL(n) (n = 1-5) ligands, where FL(n) is a fluorescein modified with a functionalized 8-aminoquinoline group as a copper-binding moiety, were synthesized, and the chemical and photophysical properties of the free ligands and their copper complexes were investigated. UV-visible spectroscopy revealed a 1:1 binding stoichiometry for the Cu(II) complexes of FL(1), FL(3), and FL(5) in pH 7.0 buffered aqueous solutions. The reactions of FL(2) or FL(4) with CuCl(2), however, appear to produce a mixture of 1:1 and 1:2 complexes, as suggested by Job's plots. These binding modes were modeled by the synthesis and X-ray crystal structure determination of Cu(II) complexes of 2-[(quinolin-8-ylamino)methyl]phenol (modL), employed as a surrogate of the FL(n) ligand family. Two kinds of crystals, [Cu(modL)(2)](BF(4))(2) and [Cu(2)(modL')(2)(CH(3)OH)](BF(4))(2) (modL' = 2-[(quinolin-8-ylamino)methyl]phenolate), were obtained. The structures suggest that one oxygen and two nitrogen atoms of the FL(n) ligands most likely bind to Cu(II). Introduction of nitric oxide (NO) to pH 7.0 buffered aqueous solutions of Cu(FL(n)) (1 microM CuCl(2) and 1 microM FL(n)) at 37 degrees C induces an increase in fluorescence. The fluorescence response of Cu(FL(n)) to NO is direct and specific, which is a significant improvement over commercially available small molecule-based probes that are capable of detecting NO only indirectly. The NO-triggered fluorescence increase of Cu(FL(5)) occurs by reduction of Cu(II) to Cu(I) with concomitant dissociation of the N-nitrosated fluorophore ligand from copper. Spectroscopic and product analyses of the reaction of the FL(5) copper complex with NO indicated that the N-nitrosated fluorescein ligand (FL(5)-NO) is the species responsible for fluorescence turn-on. Density functional theory (DFT) calculations of FL(5) versus FL(5)-NO reveal how N-nitrosation of the fluorophore ligand brings about the fluorescence increase. The copper-based probes described in the present work form the basis for real-time detection of nitric oxide production in living cells.

Luminescent Properties of Water-Soluble Conjugated Metallopolymers and Their Application to Fluorescent Nitric Oxide Detection

Water-soluble pi-conjugated polymers (CPs) incorporating 5,5'-(2,2'-dipyridyl) (CP1) or 6,6'-(2,2'-dipyridyl) (CP2) units within the pi-conjugated backbone were prepared as scaffolds for macromolecular metal complexation. The response of CP emission to a range of metal ions was investigated in water, 10 mM aqueous sodium dodecyl sulfate, and acetonitrile/water (95:5). Cupric ions are the most efficient quenchers of CP emission, with K(SV) = 1.1 x 10(5) and 5.2 x 10(4) M(-1) in water for CP1a (40% bipyridyl monomer units) and CP1b (20% bipyridyl monomer units), respectively. Quenching is approximately twice as effective in acetonitrile/water (95:5) (K(SV) = 3.1 x 10(5) M(-1) for CP1a and 1.1 x 10(5) M(-1) for CP1b). Partial restoration of emission was observed upon exposure of Cu(II)-CP solutions to excess NO(g) in acetonitrile/water (95:5) or 10 mM SDS(aq).