1,3-Di-amidoquinoline conjugate of calix[4]arene (L) as a ratiometric and colorimetric sensor for Zn2+: spectroscopy, microscopy and computational studies

Carboxamidoquinoline appended calix[4]arene-1,3-di-conjugate (L) has been synthesized and characterized and its single crystal XRD structure has been established. L has been shown to act as selective ratiometric turn-on fluorescence sensor for Zn(2+) up to a lowest concentration of 183 ± 18 ppb (2.82 μM) with a nine-fold enhancement by exhibiting blue-green emission. The coordination features of the species of recognition have been computationally evaluated by DFT methods and found to have distorted tetrahedral Zn(2+) center in an N(4) core. The spherical nano-structural features observed for L in TEM are being transformed into the Koosh nano-flower like structure when complexed with Zn(2+) and hence these two can be easily differentiated. Even the features observed in AFM can distinctly differentiate L from its Zn(2+) complex.     

Formation and stability of zinc(II) and cadmium(II) citrate complexes in aqueous solution at various temperatures

The citrate complexes of Zn(2+) and Cd(2+) have been investigated by pH titration at I = 0.1 M (KNO(3)) and 10, 25, 35 and 45 degrees . The species found were [Zn(cit)](-), [Zn(cit)H], [Zn(cit)(2)](4-) and [Zn(2)(cit)(2)H(-)(2)](4-), [Cd(cit)](-), [Cd(cit)H], [Cd(cit)(2)](4-) and [Cd(cit)H(-1)](2-). From the dependence of the formation constants on temperature, DeltaH degrees and DeltaS degrees values were calculated. Speciation in the Zn(2+)]- and Cd(2+)-citrate systems is discussed with particular attention to formation of polynuclear species. Some comparisons with literature data are made.     

Bis[1,1'-N,N'-(2-picolyl)aminomethyl]ferrocene as a redox sensor for transition metal ions

The compound bis[1,1'-N,N'-(2-picolyl)aminomethyl]ferrocene, L(1), was synthesized. The protonation constants of this ligand and the stability constants of its complexes with Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+) were determined in aqueous solution by potentiometric methods at 25 degrees C and at ionic strength 0.10 mol dm(-3) in KNO(3). The compound L(1) forms only 1:1 (M:L) complexes with Pb(2+) and Cd(2+) while with Ni(2+) and Cu(2+) species of 2 [ratio] 1 ratio were also found. The complexing behaviour of L(1) is regulated by the constraint imposed by the ferrocene in its backbone, leading to lower values of stability constants for complexes of the divalent first row transition metals when compared with related ligands. However, the differences in stability are smaller for the larger metal ions. The structure of the copper complex with L(1) was determined by single-crystal X-ray diffraction and shows that a species of 2:2 ratio is formed. The two copper centres display distorted octahedral geometries and are linked through the two L(1) bridges at a long distance of 8.781(10) Angstrom. The electrochemical behaviour of L(1) was studied in the presence of Ni(2+), Cu(2+), Zn(2+), Cd(2+) and Pb(2+), showing that upon complexation the ferrocene-ferrocenium half-wave potential shifts anodically in relation to that of the free ligand. The maximum electrochemical shift ([capital Delta]E(1/2)) of 268 mV was found in the presence of Pb(2+), followed by Cu(2+)(218 mV), Ni(2+)(152 mV), Zn(2+)(111 mV) and Cd(2+)(110 mV). Moreover, L(1) is able to electrochemically and selectively sense Cu(2+) in the presence of a large excess of the other transition metal cations studied.     

Allosteric deprogramming of a trinuclear heterometallic helicate

Two multidentate ditopic ligands L1 and L2 which contain both N-donor and crown ether units have been synthesised. The potentially octadentate ligand L1 forms a trinuclear heterometallic double helicate with Cu(I) and Zn(II) ([Zn2Cu(L1)2](5+)), whereas L2 forms a tetranuclear heterometallic double helicate with the same metal ions ([Zn2Cu2(L2)2](6+)). Both species have been characterised by (1)H NMR, ESI-MS and single crystal X-ray crystallography. Reaction of [Zn2Cu2(L2)2](6+) with Ba(2+) results in the coordination of the crown ether units giving the simple barium coordinated species [Zn2Cu2(L2)2Ba2](10+). However, reaction of [Zn2Cu(L1)2](5+) with Ba(2+) deprograms the ligand and results in the formation of a mixture of species.     

Reactions of the fluorescent sensor, Zinquin, with the zinc-proteome: adduct formation and ligand substitution

Zinquin (ZQ) is a commonly used sensor for cellular Zn(2+) status. It has been assumed that it measures accessible Zn(2+) concentrations in the nanomolar range. Instead, this report shows a consistent pattern across seven mammalian cell and tissue types that ZQ reacts with micromolar concentrations of Zn(2+) bound as Zn-proteins. The predominant class of products were ZQ-Zn-protein adducts that were characterized in vivo and in vitro by a fluorescence emission spectrum centered at about 470 nm, by their migration over Sephadex G-75 as protein not low molecular weight species, by the exclusion of reaction with lipid vesicles, and by their large aggregate concentration. In addition, variable, minor formation of Zn(ZQ)(2) with a fluorescence band at about 490 nm was observed in vivo in each case. Because incubation of isolated Zn-proteome with ZQ also generated similar amounts of Zn(ZQ)(2), it was concluded that this species had formed through direct ligand substitution in which ZQ had successfully competed for protein-bound Zn(2+). Parallel studies with the model Zn-proteins, alcohol dehydrogenase (ADH), and alkaline phosphatase (AP) revealed a similar picture of reactivity: ZQ(ACID) (Zinquin acid, (2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate)) able to bind to one Zn(2+) and extract the other in Zn(2)-ADH, whereas it removed one Zn(2+) from Zn(2)-AP and did not bind to the other. Zinquin ethyl ester (ethyl(2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate); ZQ(EE)) bound to both proteins without sequestering Zn(2+) from either one. In contrast to a closely related sensor, 6-methoxy-8-p-toluenesulfonamido-quinoline (TSQ), neither ZQ(ACID) nor ZQ(EE) associated with Zn-carbonic anhydrase. A survey of reactivity of these sensors with partially fractionated Zn-proteome confirmed that ZQ and TSQ bind to distinct, overlapping subsets of the Zn-proteome.     

Zn(2+) Responsive Bimodal Magnetic Resonance Imaging and Fluorescent Imaging Probe Based on a Gadolinium(III) Complex

A Zn(2+)-responsive bimodal magnetic resonance imaging (MRI) and luminescence imaging probe GdL was synthesized. The relaxivity and luminescence properties were examined. In the presence of 0.5 equiv of Zn(2+), the longitudinal relaxivity is increased from 3.8 mM(-1) s(-1) to 5.9 mM(-1) s(-1) at 23 MHz and 25 °C with 55% enhancement, whereas the fluorescence exhibits a 7-fold increase. The Zn(2+) responsive imaging probe shows favorable selectivity and tolerance over a variety of biologically relevant anions and metal ions in physiological pH range for both relaxivity and luminescence. In vitro phantom images and confocal fluorescence images in living cells show that the bimodal Zn(2+) probe can effectively enhance T(1)-weighted imaging contrast and luminescence imaging effect through Zn(2+) coordination with excellent cellmembrane permeability and biocompatibility. Spectral and electrospray ionization mass spectrometry (ESI-MS) studies indicate that two different Zn(2+)-bound species, (GdL)(2)Zn and GdLZn, are formed when 0.5 and 1 equiv of Zn(2+) are bound to GdL complex, respectively. Crystal structural determination and dysprosium-induced (17)O NMR shift (DIS) experiment demonstrate that the increased molecular weight and the improved molecular rigidity upon complexation of Zn(2+) with GdL is the primary factor for relaxivity enhancement. Significant enhancement of the luminescence is due to a heavy atom effect and much increased molecular rigidity upon Zn(2+) binding to 8-sulfonamidoquinoline chromophore.     

(Zn, Mg)2GeO4:Mn2+ submicrorods as promising green phosphors for field emission displays: hydrothermal synthesis and luminescence properties

(Zn(1-x-y)Mg(y))(2)GeO(4): xMn(2+) (y = 0-0.30; x = 0-0.035) phosphors with uniform submicrorod morphology were synthesized through a facile hydrothermal process. X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence (PL), and cathodoluminescence (CL) spectroscopy were utilized to characterize the samples. SEM and TEM images indicate that Zn(2)GeO(4):Mn(2+) samples consist of submicrorods with lengths around 1-2 μm and diameters around 200-250 nm, respectively. The possible formation mechanism for Zn(2)GeO(4) submicrorods has been presented. PL and CL spectroscopic characterizations show that pure Zn(2)GeO(4) sample shows a blue emission due to defects, while Zn(2)GeO(4):Mn(2+) phosphors exhibit a green emission corresponding to the characteristic transition of Mn(2+) ((4)T(1)→(6)A(1)) under the excitation of UV and low-voltage electron beam. Compared with Zn(2)GeO(4):Mn(2+) sample prepared by solid-state reaction, Zn(2)GeO(4):Mn(2+) phosphors obtained by hydrothermal process followed by high temperature annealing show better luminescence properties. In addition, codoping Mg(2+) ions into the lattice to substitute for Zn(2+) ions can enhance both the PL and CL intensity of Zn(2)GeO(4):Mn(2+) phosphors. Furthermore, Zn(2)GeO(4):Mn(2+) phosphors exhibit more saturated green emission than the commercial FEDs phosphor ZnO:Zn, and it is expected that these phosphors are promising for application in field-emission displays.     

Highly mixed phases in ball-milled Cu/ZnO catalysts: an EXAFS and XANES study

New highly mixed phases have been identified in Cu/ZnO systems by EXAFS and XANES at both the Cu and Zn K-edge. The phases were generated by ball-milling Cu(2)O/ZnO mixtures under three different atmospheres of synthetic air (SA), SA + CO(2) and CO(2). The system milled in CO(2) shows disproportionation of Cu(2)O into Cu(0), Cu(1+) (cuprite Cu(2)O-type phase) and Cu(2+) (tenorite CuO-type phase), while most of the Zn(2+) is transformed into a nanocrystalline/amorphous ZnO-type zincite that forms a superficial mixture of oxide and carbonate phases. When synthetic air is added to the CO(2) atmosphere, ball milling results in the oxidation of nearly half the Cu(1+) into Cu(2+) with no Cu metal formed. The copper phase in this material is almost entirely amorphous. In SA, a significant amount of Cu(2+)- and Zn(2+)-based phases appears to react to form a nanocrystalline/amorphous Cu(1-x)Zn(x)O (x approximately 0.3) solid solution. This distorted rock saltlike solid solution, in which Zn and Cu feature different octahedral environments, was never reported before. It is thought to be formed by incorporation of Zn(2+) in the Cu fcc sublattice of the cuprite Cu(2)O matrix and the concomitant oxidation of Cu(1+) into Cu(2+). The formation of such a highly mixed Cu(1-x)Zn(x)O phase indicates strong Cu/Zn interaction in the Cu/ZnO system, which also suggests the presence of highly mixed phases in conventionally prepared activated catalysts.     

Catalysis of the methanolysis of activated amides by divalent and trivalent metal ions. The effect of Zn(2+), Co(2+), and La(3+) on the methanolysis of acetylimidazole and its (NH(3))(5)Co(III) complex.

The metal ions Zn(2+), Co(2+), and La(3+) strongly catalyze the methanolysis of the activated amides acetylimidazole (1) and its ligand-exchange-inert Co(III) complex, (NH(3))(5)Co(III)-AcIm (2). Studies of the kinetics of methanolysis are performed with pH measurement and control, and the metal ions are soluble in the medium throughout the pH regions where ionization of the M(x+)(CH(3)OH)(y) occurs. Zn(2+) and Co(2+) act as Lewis acids toward 1, catalyzing attack of external methoxide on a 1:M(2+) complex at values only 100-fold lower than the diffusion limit, the k(OR) values being 5.6 x 10(7) M(-1) s(-1) and 2.5 x 10(7) M(-1) s(-1), while that for CH(3)O(-) attack on 2 is 4.69 x 10(7) M(-1) s(-1). Since neither Zn(2+) nor Co(2+) promotes the methanolysis of 2, these metals appear to be acting through transient binding to the distal N of 1, which activates the C=O of the complex to external CH(3)O(-) attack. La(3+) catalyzes the methanolysis of both 1 and 2, which occurs by a mechanism that is fundamentally different from that exhibited by Zn(2+) and Co(2+) in that the active species appears to be a bis-methoxy-bridged dimer (La(3+))(2)(CH(3)O(-))(2)(CH(3)OH)(x)() that interacts directly with the C=O unit of the substrate.     

Study of pH-dependent zinc(II)-carboxamide interactions by zinc(II)-carboxamide-appended cyclen complexes (cyclen = 1,4,7,10-tetraazacyclododecane)

To elucidate intrinsic recognition of carboxamides by zinc(II) in carbonic anhydrase (CA) (as inhibitors) and carboxypeptidase A (CPA) (as substrates), a new series of Zn(2+)-carboxamide-appended cyclen complexes have been synthesized and characterized (cyclen = 1,4,7,10-tetraazacyclododecane). Two types of Zn(2+)-carboxamide interactions have been found. In the first case represented by a zinc(II) complex of carbamoylmethyl-1,4,7,10-tetraazacyclododecane (L(1)), the amide oxygen binds to zinc(II) at slightly acidic pH (to form ZnL(1)), and the deprotonated amide N(-) binds to zinc(II) at alkaline pH (to form ZnH(-1)L(1)) with pK(a) = 8.59 at 25 degrees C and I = 0.1 (NaNO(3)), as determined by potentiometric pH titrations, infrared spectral changes, and (13)C and (1)H NMR titrations. The X-ray crystal structure of ZnH(-1)L(3) (where L(3) = N-(4-nitrophenyl)carbamoylmethyl cyclen, pK(a) = 7.01 for ZnL(3) <==> ZnH(-1)L(3)) proved that the zinc(II) binds to the amidate N(-) (Zn-N(-) distance of 1.974(3) A) along with the four nitrogen atoms of cyclen (average Zn-N distance 2.136 A). Crystal data: monoclinic, space group P2(1)/n (No. 14) with a = 10.838(1) A, b = 17.210(2) A, c = 12.113(2) A, b = 107.38(1) degrees, V = 2156.2(5) A(3), Z = 4, R = 0.042, and R(w) = 0.038. These model studies provide the first chemical support that carboxamides are CA(-) inhibitors by occupying the active Zn(2+) site both in acidic and alkaline pH to prevent the occurrence of the catalytically active Zn(2+)-OH(-) species. In the second case represented by a zinc(II) complex of 1-(N-acetyl)aminoethylcyclen, ZnL(6), the pendant amide oxygen had little interaction with zinc(II) at acidic pH. At alkaline pH, the monodeprotonation yielded a zinc(II)-bound hydroxide species ZnL(6)(OH(-)) (pK(a) = 7.64) with the amide pendant remaining intact. The ZnL(6)(OH(-)) species showed the same nucleophilic activity as Zn(2+)-cyclen-OH(-). The second case may mimic the Zn(2+)-OH(-) mechanism of CPA, where the nucleophilic Zn(2+)-OH(-) species does not act as a base to deprotonate a proximate amide.     

Guanidine is a Zn(2+)-binding ligand at neutral pH in aqueous solution

We have found the first well-characterized coordination of guanidine with Zn(2+) in a 1:1 complex (ZnL(1)) with cyclen (= 1,4,7,10-tetraazacyclododecane) functionalized with guanidinylethyl group (L(1) = (2-guanidinyl)ethyl-cyclen). The X-ray structure analysis of the 1:1 complex crystallized at pH 7.5 revealed an apical coordination of the pendant guanidinyl group to Zn(2+) ion in ZnL(1). By potentiometrtic pH titration, initial formation of a 1:1 Zn(L(1).H(+)) complex was indicated, where only the cyclen N's bind to Zn(2+) with the complexation constant, log K(s) (K(s) = [Zn(L(1).H(+))]/[Zn(2+)][L(1).H(+)] (M(-1))), being 12.4 +/- 0.1. Facile deprotonation of the guanidinium pendant in the Zn(L(1).H(+)) occurred with a pK(a) value of 5.9 +/- 0.1 at 25 degrees C with I = 0.1 (NaNO(3)) to yield the guanidine-coordinating complex ZnL(1). 4-Nitrophenyl phosphate dianion (NPP(2-)) interacted with ZnL(1) through a new Zn(2+)-phosphate coordination, as indicated by (31)P NMR titration and potentiometric pH titration. An apparent complexation constant for this new species, log K(app)(Zn(L(1).H(+))-NPP), was 4.0 +/- 0.1, which is larger than the log K(app)(ZnL(2)-NPP) value of 3.1 for the 1:1 complex of Zn(2+)-cyclen (ZnL(2)) with NPP at the common pH 5.6. The interaction of ZnL(1) with a phosphate dianion was proven by the X-ray crystal structure analysis of the 1:1 ZnL(1)-PP(2-) complex (PP(2-) is a dianion of phenyl phosphate) obtained from an aqueous solution at pH 6.5. At higher pH, the pendant guanidinium cation is deprotonated to displace the phosphate to yield the Zn(2+)-guanidine bond.     

ZP4, an improved neuronal Zn2+ sensor of the Zinpyr family

A second-generation fluorescent sensor for Zn(2+) from the Zinpyr family, ZP4, has been synthesized and characterized. ZP4 (Zinpyr-4, 9-(o-carboxyphenyl)-2-chloro-5-[2-(bis(2-pyridylmethyl)aminomethyl)-N-methylaniline]-6-hydroxy-3-xanthanone) is prepared via a convergent synthetic strategy developed from previous studies with these compounds. ZP4, like its predecessors, has excitation and emission wavelengths in the visible range ( approximately 500 nm), a dissociation constant (K(d)) for Zn(2+) of less than 1 nM and a high quantum yields (Phi = approximately 0.4), making it well suited for biological applications. A 5-fold fluorescent enhancement is observed under simulated physiological conditions corresponding to the binding of the Zn(2+) cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The metal-binding stereochemistry of ZP4 was evaluated through the synthesis and X-ray structural characterization of [M(BPAMP)(H(2)O)(n)](+) complexes, where BPAMP is [2-(bis(2-pyridylmethyl)aminomethyl)-N-methylaniline]-phenol and M = Mn(2+), Zn(2+) (n = 1) or Cu(2+) (n = 0).     

Structural and electronic characterization of the complexes obtained by the interaction between bare and hydrated first-row transition-metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)) and glycine

The complexes formed by the simplest amino acid, glycine, with different bare and hydrated metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+)) were studied in the gas phase and in solvent in order to give better insight into the field of the metal ion-biological ligand interactions. The effects of the size and charge of each cation on the organization of the surrounding water molecules were analyzed. Results in the gas phase showed that the zwitterion of glycine is the form present in the most stable complexes of all ions and that it usually gives rise to an eta(2)O,O coordination type. After the addition of solvation sphere, a resulting octahedral arrangement was found around Ni(2+), Co(2+), and Fe(2+), ions in their high-spin states, whereas the bipyramidal-trigonal (Mn(2+) and Zn(2+)) or square-pyramidal (Cu(2+)) geometries were observed for the other metal species, according to glycine behaves as bi- or monodentate ligand. Despite the fact that the zwitterionic structure is in the ground conformation in solution, its complexes in water are less stable than those obtained from the canonical form. Binding energy values decrease in the order Cu(2+) > Ni(2+) > Zn(2+) approximately Co(2+) > Fe(2+) > Mn(2+) and Cu(2+) > Ni(2+) > Mn(2+) approximately Zn(2+) > Fe(2+) > Co(2+) for M(2+)-Gly and Gly-M(2+) (H(2)O)(n) complexes, respectively. The nature of the metal ion-ligand bonds was examined by using natural bond order and charge decomposition analyses.     

Magnetic and high rate adsorption properties of porous Mn(1-x)Zn(x)Fe2O4 (0 ≤ x ≤ 0.8) adsorbents

Porous spinel ferrites Mn(1-x)Zn(x)Fe(2)O(4) (0 ≤ x ≤ 0.8) are synthesized by a simple sol-gel method with egg white. All samples exhibit porous morphologies and large BET surface area (S(BET)). The substitution of Zn(2+) affects the magnetic properties of ferrites and the adsorption properties of methylene blue (MB) on ferrites, obviously. The saturation magnetization (Ms) of Mn(1-x)Zn(x)Fe(2)O(4) increases before x=0.4, and decreases with further increase of Zn(2+) substitution. This can be ascribed to the changes of the cationic distribution and the variation of spin arrangement in A-site and B-site of spinel structure. All samples show high adsorption capacity and the removal efficiencies of MB reach up to >90% within 3 h. The Zn(2+) substitution accelerates the adsorption rate and capacity of MB on Mn(1-x)Zn(x)Fe(2)O(4). The quickest adsorption occurred at x=0.2 and the largest adsorption capacity occurred at x=0.8.     

Zinc thiolate reactivity toward nitrogen oxides: insights into the interaction of Zn2+ with S-nitrosothiols and implications for nitric oxide synthase

Zinc thiolate complexes containing N(2)S tridentate ligands were prepared to investigate their reactivity toward reactive nitrogen species, chemistry proposed to occur at the zinc tetracysteine thiolate site of nitric oxide synthase (NOS). The complexes are unreactive toward nitric oxide (NO) in the absence of dioxygen, strongly indicating that NO cannot be the species directly responsible for S-nitrosothiol formation and loss of Zn(2+) at the NOS dimer interface in vivo. S-Nitrosothiol formation does occur upon exposure of zinc thiolate solutions to NO in the presence of air, however, or to NO(2) or NOBF(4), indicating that these reactive nitrogen/oxygen species are capable of liberating zinc from the enzyme, possibly through generation of the S-nitrosothiol. Interaction between simple Zn(2+) salts and preformed S-nitrosothiols leads to decomposition of the -SNO moiety, resulting in release of gaseous NO and N(2)O. The potential biological relevance of this chemistry is discussed.     

Reaction of metal-binding ligands with the zinc proteome: zinc sensors and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine

The commonly used Zn(2+) sensors 6-methoxy-8-p-toluenesulfonamidoquinoline (TSQ) and Zinquin have been shown to image zinc proteins as a result of the formation of sensor-zinc-protein ternary adducts not Zn(TSQ)(2) or Zn(Zinquin)(2) complexes. The powerful, cell-permeant chelating agent N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) is also used in conjunction with these and other Zn(2+) sensors to validate that the observed fluorescence enhancement seen with the sensors depends on intracellular interaction with Zn(2+). We demonstrated that the kinetics of the reaction of TPEN with cells pretreated with TSQ or Zinquin was not consistent with its reaction with Zn(TSQ)(2) or Zn(Zinquin)(2). Instead, TPEN and other chelating agents extract between 25 and 35% of the Zn(2+) bound to the proteome, including zinc(2+) from zinc metallothionein, and thereby quench some, but not all, of the sensor-zinc-protein fluorescence. Another mechanism in which TPEN exchanges with TSQ or Zinquin to form TPEN-zinc-protein adducts found support in the reactions of TPEN with Zinquin-zinc-alcohol dehydrogenase. TPEN also removed one of the two Zn(2+) ions per monomer from zinc-alcohol dehydrogenase and zinc-alkaline phosphatase, consistent with its ligand substitution reactivity with the zinc proteome.     

Histidine-rich peptide: evidence for a single zinc-binding site on H5WYG peptide that promotes membrane fusion at neutral pH

The histidine-rich peptide H5WYG (GLFHAIAHFIHGGWHGLIHGWYG) was found to induce membrane fusion at physiologic pH in the presence of zinc chloride. In this study, we examined the ion selectivity of the interaction of Zn(2+) with H5WYG. This investigation was conducted by using adsorption at air/water interface and mass spectrometry. We found that a peptide-metal complex is formed with Zn(2+) ions. Electrospray ionisation-mass spectrometry (ESI-MS) reveals that the [H5WYG + Zn + 2H](4+), [H5WYG + Zn + H](3+) and [H5WYG + Zn](2+) ions, appearing by increasing the amount of Zn(2+) equivalent, correspond to a monomolecular H5WYG - Zn(2+) complex. Tandem mass spectrometry (MS/MS) provides evidence for the binding of the single Zn(2+) ion to the H(11) and H(19) and probably H(15) residues.     

The inorganic perspective of nerve growth factor: interactions of Cu2+ and Zn2+ with the N-terminus fragment of nerve growth factor encompassing the recognition domain of the TrkA receptor

There is a significant overlap between brain areas with Zn(2+) and Cu(2+) pathological dys-homeostasis and those in which the nerve growth factor (NGF) performs its biological role. The protein NGF is necessary for the development and maintenance of the sympathetic and sensory nervous systems. Its flexible N-terminal region has been shown to be a critical domain for TrkA receptor binding and activation. Computational analyses show that Zn(2+) and Cu(2+) form pentacoordinate complexes involving both the His4 and His8 residues of the N-terminal domain of one monomeric unit and the His84 and Asp105 residues of the other monomeric unit of the NGF active dimer. To date, neither experimental data on the coordination features have been reported, nor has one of the hypotheses according to which Zn(2+) and Cu(2+) may have different binding environments or the Ser1 α-amino group could be involved in coordination been supported. The peptide fragment, encompassing the 1-14 sequence of the human NGF amino-terminal domain (NGF(1-14)), blocked at the C terminus, was synthesised and its Cu(2+) and Zn(2+) complexes characterized by means of potentiometric and spectroscopic (UV/Vis, CD, NMR, and EPR) techniques. The N-terminus-acetylated form of NGF(1-14) was also investigated to evaluate the involvement of the Ser1 α-amino group in metal-ion coordination. Our results demonstrate that the amino group is the first anchoring site for Cu(2+) and is involved in Zn(2+) coordination at physiological pH. Finally, a synergic proliferative activity of both NGF(1-14) and the whole protein on SHSY5Y neuroblastoma cell line was found after treatment in the presence of Cu(2+). This effect was not observed after treatment with the N-acetylated peptide fragment, demonstrating a functional involvement of the N-terminal amino group in metal binding and peptide activity.     

Impact of metal binding on the antitumor activity and cellular imaging of a metal chelator cationic imidazopyridine derivative

A new water soluble cationic imidazopyridine species, viz. (1E)-1-((pyridin-2-yl)methyleneamino)-3-(3-(pyridin-2-yl)imidazo[1,5-a]pyridin-2(3H)-yl)propan-2-ol (1), as a metal chelator is prepared as its PF(6) salt and characterized. Compound 1 shows fluorescence at 438 nm on excitation at 342 nm in Tris-HCl buffer giving a fluorescence quantum yield (φ) of 0.105 and a life-time of 5.4 ns. Compound 1, as an avid DNA minor groove binder, shows pUC19 DNA cleavage activity in UV-A light of 365 nm forming singlet oxygen species in a type-II pathway. The photonuclease potential of 1 gets enhanced in the presence of Fe(2+), Cu(2+) or Zn(2+). Compound 1 itself displays anticancer activity in HeLa, HepG2 and Jurkat cells with an enhancement on addition of the metal ions. Photodynamic effect of 1 at 365 nm also gets enhanced in the presence of Fe(2+) and Zn(2+). Fluorescence-based cell cycle analysis shows a significant dead cell population in the sub-G1 phase of the cell cycle suggesting apoptosis via ROS generation. A significant change in the nuclear morphology is observed from Hoechst 33258 and an acridine orange/ethidium bromide (AO/EB) dual nuclear staining suggesting apoptosis in cells when treated with 1 alone or in the presence of the metal ions. Apoptosis is found to be caspase-dependent. Fluorescence imaging to monitor the distribution of 1 in cells shows that 1 in the presence of metal ions accumulates predominantly in the cytoplasm. Enhanced uptake of 1 into the cells within 12 h is observed in the presence of Fe(2+) and Zn(2+).     

A minimal core based fluorophore for selective detection of Zn(II) ions in aqueous solution and living cells

A minimal core based fluorophore was introduced as a selectively fluorescent "turn on" sensor for Zn(2+) ions in aqueous solution. Addition of Zn(2+) ions to the fluorophore generates a significant emission through a 1:1 ligand-to-metal complex. The fluorescence titration experiment of the minimal core based fluorophore with various metal ions shows that the pyromellitic diimide derivative also has the advantage of a high selectivity to Zn(2+) ions over other metals such as Ni(2+), or Co(2+), Cu(2+), Fe(3+), Fe(2+). More than 8 fold increase in the intensity of fluorescence was observed for the Zn(2+)-bound fluorophore compared to Zn-free fluorophore. Due to its small molecular size, the fluorophore was cell-permeable and successfully applied to the detection of Zn(2+) in living cells. With its relatively high sensitivity to Zn(2+) in living cells, the synthesized new fluorophore will be very useful in the studies on various biological functions of Zn(2+).