Dizinc Complex Research Articles

Phos-tag-based fluorescence resonance energy transfer system for the analysis of protein kinase and phosphatase reactions

We introduce a novel fluorescence resonance energy transfer (FRET) system for the detection of a phosphorylated molecule such as a phosphopeptide using a phosphate-binding tag molecule, Zn(II)-Phos-tag (1,3-bis[bis (pyridin-2-ylmethyl)amino]propan-2-olato dizinc(II) complex) attached with a 7-amino-4-methylcoumarin-3-acetic acid (AMCA). 5-Carboxyfluorescein (FAM)-labeled phosphopeptides and nonphosphopeptides were prepared as the target molecules for the FRET system. A set of FAM (a fluorescent acceptor, emission at 520 nm) and AMCA (a fluorescent donor, excitation at 345 nm) is frequently used for a FRET system. The AMCA-labeled Zn(II)-Phos-tag captured specifically the FAM-labeled phosphopeptide to form a stable 1:1 complex, resulting in efficient FRET. After the FAM-labeled phosphopeptide was dephosphorylated with alkaline phosphatase, the FRET disappeared. Using this FRET system, we demonstrated the detection of the time-dependent reversible phosphorylation of the FAM-labeled substrate peptide. The Phos-tag-based FRET system has the following major advantages: i) The real-time analysis of the reversible phosphorylation reaction is possible without multiple samplings, ii) the analysis requires a simple procedure just using two solutions of AMCA-labeled Phos-tag and a FAM-labeled compound, and iii) the system would be useful for the reliable and comprehensive phosphorylation assays for various phosphopeptides containing phosphoserine, phosphothreonine, or phosphotyrosine, in vitro. Thus, the principle of this system would be applied to high-throughput kinase/phosphatase profiling, measurement of enzyme activity, and determination of an activator or an inhibitor.

Improved Phos-tag SDS-PAGE under neutral pH conditions for advanced profiling of protein phosphorylation

We describe an improved Phos-tag SDS-PAGE (Zn2+-Phos-tag SDS-PAGE) using a dizinc(II) complex of Phos-tag acrylamide in conjunction with a neutral-pH gel system buffered with Bis-Tris hydrochloride (Bis-Tris-HCl) to detect shifts in the mobility of phosphorylated proteins. Our alternative technique (Mn2+-Phos-tag SDS-PAGE) using a polyacrylamide-bound Mn2+-Phos-tag and a conventional Laemmli's buffer system under alkaline pH conditions has limitations for separating certain phosphoproteins. The major improvements and utilities were demonstrated by visualizing novel up-shifted bands of commercially available pepsin, recombinant substrate protein tau treated in vitro with tyrosine kinases, and endogeneous β-catenin in whole cell lysates. Additionally, the Zn2+-Phos-tag SDS-PAGE gels showed better long-term stability than the Mn2+-Phos-tag SDS-PAGE gels. We can thus present a simple, convenient, and more reliable "in-house" gel system for phosphate-affinity SDS-PAGE.

Labile Zinc-Assisted Biological Phosphate Chemosensing and Related Molecular Logic Gating Interpretations

Herein, molecular fluorescence 'OFF-ON' behavior with aqueous addition of biological phosphate and Zn(2+) is studied with Zn(2)(slys)(2)Cl(2) [H(2)slys = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoic acid], a fluorescent water-soluble complex, using various spectroscopic tools (e.g., (31)P NMR, UV-vis, emission, and CD spectroscopy) at the micromolar level. Adduct-dependent fluorescence intensity changes can be interpreted as a two-input (cation/anion) implication molecular logic gating system. A displacement study of PPi from the dizinc complex is also reported. Diphosphate and triphosphate addition/displacements were also studied. (31)P NMR spectroscopy shows gradual NMR peak shifts from bound ADP/GDP to free ADP/GDP with increasing [PPi]. In the emission spectrum, fluorescence quenching is shown: CD signal maxima decrease with addition of PPi. These displacement events are also tested with triphosphates (ATP, GTP), and their binding strength/displacement ability over ADP/GDP is quantified: PPi > ATP ≈ GTP (3.35 ± 0.77 × 10(4) M(-1) for PPi, 7.73 ± 1.79 × 10(3) M(-1) for ATP, 9.21 ± 2.88 × 10(3) M(-1) for GTP over 1·ADP). Many anions and cations were also screened for selectivity. Tubulin polymerization was assayed in the presence of 1 and its copper analogue which reflected a slight inhibition in polymerization.

The new sulphur-containing ligand 4′-(4-methylthiophenyl)-3,2′:6′,3″-terpyridine (L1) and the supramolecular structure of the dinuclear complex [Zn2(μ-L1)(acac)4] (acac=acetylacetonato): The key role of non-covalent S⋯O contacts and C–H⋯S hydrogen bonds

Abstract The new terpyridyl ligand 4′-(4-methylthiophenyl)-3,2′:6′,3″-terpyridine ( L1 ) combines with Zn(acac) 2 to form the dizinc complex [Zn 2 (μ- L1 )(acac) 4 ] ( 1 ) (acac = acetylacetonato). The single crystal structure of 1 shows the existence of two different Zn(acac) 2 centres bridged by the L1 ligand through its two 3-pyridyl-nitrogen atoms. The acac and L1 ligands allow the generation of a series of intra- and intermolecular hydrogen bonds of the C–H⋯N, C–H⋯O and C–H⋯π type. In addition, the presence of the CH 3 –S group in L1 plays a crucial role by providing the non-covalent S⋯O, S⋯H and H(CH 3 )⋯O contacts, which permit to describe fully this unusual supramolecular network. The NMR spectra of 1 in CDCl 3 solution confirm the presence of the two zinc atoms joined by the bridging L1 ligand, but, differently to the solid state, their environments are identical.

DNA Cleavage by the Photocontrolled Cooperation of ZnIICenters in an Azobenzene-Linked Dizinc Complex

We synthesized a new photoactive dinuclear zinc(II) complex by linking two zinc centers with a ligand containing an azobenzene chromophore and investigated the DNA cleavage activities of its trans and cis forms. The trans structure of the dinuclear zinc complex was determined by X-ray crystallography, where each zinc center is situated in an octahedral coordination environment comprised of three nitrogen atoms from the ligand and three oxygen atoms from two nitrate ions. The dinuclear zinc complex containing the azobenzene chromophore was photoisomerizable between the trans and cis forms. The binding affinities of the trans and cis complexes with calf thymus (CT)-DNA were similar. Although the DNA cleavage activity of the trans complex was negligible, the cis complex was able to cleave DNA. We attribute the efficient activity of the cis complex to the cooperation of the two closely located zinc centers and the inactivity of the trans complex to the two metal centers positioned far away from each other. The DNA cleavage activity of the cis complex exhibited a pH-dependent bell-shaped profile, which has been observed in the hydrolytic cleavage of DNA by zinc complexes. The DNA cleavage activity was not inhibited by a major groove binder, methyl green, but decreased significantly by a minor groove binder, 4',6-diamidino-2-phenylindole, indicating that the dinuclear zinc complex binds to the minor groove of DNA. The present work shows the importance of the cooperation of two zinc ions for hydrolytic DNA cleavage, which can be photoregulated by linking the two metal centers with a photoisomerizable spacer, such as an azobenzene chromophore.

Mechanistic Investigation and Reaction Kinetics of the Low-Pressure Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by a Dizinc Complex

The reaction kinetics of the copolymerization of carbon dioxide and cyclohexene oxide to produce poly(cyclohexene carbonate), catalyzed by a dizinc acetate complex, is studied by in situ attenuated total reflectance infrared (ATR-IR) and proton nuclear magnetic resonance ((1)H NMR) spectroscopy. A parameter study, including reactant and catalyst concentration and carbon dioxide pressure, reveals zero reaction order in carbon dioxide concentration, for pressures between 1 and 40 bar and temperatures up to 80 °C, and a first-order dependence on catalyst concentration and concentration of cyclohexene oxide. The activation energies for the formation of poly(cyclohexene carbonate) and the cyclic side product cyclohexene carbonate are calculated, by determining the rate coefficients over a temperature range between 65 and 90 °C and using Arrhenius plots, to be 96.8 ± 1.6 kJ mol(-1) (23.1 kcal mol(-1)) and 137.5 ± 6.4 kJ mol(-1) (32.9 kcal mol(-1)), respectively. Gel permeation chromatography (GPC), (1)H NMR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry are employed to study the poly(cyclohexene carbonate) produced, and reveal bimodal molecular weight distributions, with narrow polydispersity indices (≤1.2). In all cases, two molecular weight distributions are observed, the higher value being approximately double the molecular weight of the lower value; this finding is seemingly independent of copolymerization conversion or reaction parameters. The copolymer characterization data and additional experiments in which chain transfer agents are added to copolymerization experiments indicate that rapid chain transfer reactions occur and allow an explanation for the observed bimodal molecular weight distributions. The spectroscopic and kinetic analyses enable a mechanism to be proposed for both the copolymerization reaction and possible side reactions; a dinuclear copolymerization active site is implicated.

De novo design, synthesis and spectroscopic characterization of chiral benzimidazole-derived amino acid Zn(II) complexes: Development of tryptophan-derived specific hydrolytic DNA artificial nuclease agent

Novel ternary dizinc(II) complexes 1– 3, derived from 1,2-bis(1H-benzimidazol-2-yl)ethane-1,2-diol and l-form of amino acids (viz., tryptophan, leucine and valine) were synthesized and characterized by spectroscopic (IR, 1H NMR, UV–vis, ESI-MS) and other analytical methods. To evaluate the biological preference of chiral drugs for inherently chiral target DNA, interaction of 1– 3 with calf thymus DNA in Tris–HCl buffer was studied by various biophysical techniques which reveal that all these complexes bind to CT DNA non-covalently via electrostatic interaction. The higher K b value of l-tryptophan complex 1 suggested greater DNA binding propensity. Further, to evaluate the mode of action at the molecular level, interaction studies of complexes 1 and 2 with nucleotides (5′-GMP and 5′-TMP) were carried out by UV–vis titrations, 1H and 31P NMR which implicates the preferential selectivity of these complexes to N3 of thymine rather than N7 of guanine. Furthermore, complex 1 exhibits efficient DNA cleavage with supercoiled pBR322. The complex 1 cleaves DNA efficiently involving hydrolytic cleavage pathway. Such chiral synthetic hydrolytic nucleases with asymmetric centers are gaining considerable attention owing to their importance in biotechnology and drug design, in particular to cleave DNA with sequence selectivity different from that of the natural enzymes.

Balance between Fluorescence Enhancement and Association Affinity in Fluorescent Heteroditopic Indicators for Imaging Zinc Ion in Living Cells

A fluorescent heteroditopic indicator for the zinc(II) ion possesses two different zinc(II) binding sites. The sequential coordination of zinc(II) at the two sites can be transmitted into distinct fluorescence changes. In the heteroditopic ligand system that our group developed, the formations of mono- and dizinc(II) complexes along an increasing gradient of zinc(II) concentration lead to fluorescence enhancement and an emission bathochromic shift, respectively. The extents of these two changes determine the sensitivity and, ultimately, the effectiveness of the heteroditopic indicator in quantifying zinc(II) ion over a large concentration range. In this work, a strategy to increase the degree of fluorescence enhancement upon the formation of the monozinc(II) complex of a heteroditopic ligand under simulated physiological conditions is demonstrated. Fluorination of the pyridyl groups in the pentadentate N,N,N'-tris(pyridylmethyl)ethyleneamino group reduces the apparent pK(a) value of the high-affinity site, which increases the degree of fluorescence enhancement as the monozinc(II) complex is forming. However, fluorination impairs the coordination strength of the high-affinity zinc(II) binding site, which in the triply fluorinated ligand reduces the binding strength to the level of the low-affinity 2,2'-bipyridyl. The potential of the reported ligands in imaging zinc(II) ion in living cells was evaluated. The subcellular localization properties of two ligands in five organelles were characterized. Both benefits and deficiencies of these ligands were revealed, which provides directions for the near future in this line of research.

Improvement of Aminopeptidase Activity of Dizinc(II) Complexes by Increasing Substrate Accessibility

A new dizinc(II) complex, [Zn2(bhmp)(MeCO2)2]BPh4 [(bhmp)−: 2,6-bis[bis(2-hydroxyethyl)aminomethyl]-4-methylphenolate anion], performs aminopeptidase activity to hydrolyze L-leucine-p-nitroanilide. As compared with a related dizinc(II) complex [Zn2(bomp)(MeCO2)2]BPh4 [(bomp)−: 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenolate anion], the activity of the present bhmp complex was about 80 times greater than that of the bomp complex. This is mainly because the substrate accessibility was improved by changing the terminal methoxy groups to hydroxyl groups.

Modeling the diiron(II) ferroxidase complex in human H ferritin

Density functional theory calculations, in both the high-spin and broken symmetry approximations, have been conducted on models of the diiron(II) ferroxidase complex of human H ferritin. Initial configurations were chosen from previous experimental and theoretical structures of the dizinc complex. The diiron complexes show no significant deviation in ligand or metal positions from the corresponding dizinc complexes, even maintaining similar structures through an extensive reorganization, and thus the often-made assumption of homology between Fe(II) and Zn(II) is supported. Geometry differences between diiron complexes calculated in the high-spin and broken symmetry approximations are also found to be minor.

A Phos-tag-based fluorescence resonance energy transfer system for the analysis of the kinase reaction of a substrate peptide

We recently reported a fluorescence resonance energy transfer (FRET) system for the analysis of the dephosphorylation of a 5-carboxyfluorescein (FAM)-labeled phosphopeptide by using a Phos-tag (1,3-bis[bis(pyridin-2-ylmethyl)amino]propan-2-olato dizinc(II) complex) derivative attached to a (7-amino-4-methylcoumarin-3-yl)acetic acid (AMCA) moiety. This FRET system is based on the principle that the Phos-tag captures the phosphopeptide in preference to its nonphosphorylated counterpart. Binding of the phosphopeptide to the Phos-tag molecule brings the donor AMCA (λex 345 nm) close to the acceptor FAM (λem 520 nm), resulting in an efficient FRET signal. Here we introduce an application of the FRET system to the reverse reaction, phosphorylation of a FAM-labeled peptide substrate by a kinase, such as epidermal growth factor receptor or c-Src, in the presence of 15 mM magnesium(II) ion, 20 μM ATP, and a hydrophilic AMCA-labeled Phos-tag molecule. Furthermore, the inhibition profiles of Abl and c-Src kinases were determined using specific inhibitors, Glivec and c-Src kinase inhibitor I, respectively. The dose-dependent inhibitions of the kinase reactions were determined from real-time changes in the FRET efficiency, which showed IC50 values of 85 nM of Glivec and 0.13 μM of c-Src kinase inhibitor I.

Conformational Analysis and Computational Modeling of a µ-Phenoxo-bis(µ-carboxylato)dizinc(II) Complex

Conformational analysis was conducted for a dizinc(II) complex cation [Zn2(bomp)(OCOMe)2]+ based on the DFT method, where bomp- represents a dinucleating ligand: 2,6-bis[bis(2-methoxyethyl) aminomethyl]-4-methylphenolate anion. The complex cation is enantiomeric, and when the twisted angle-an angle between a phenolate plane and a plane including two zinc(II) ions and a phenolic oxygen atom-is positive, the most stable conformation of the chelating rings is (+, δ, δ), where the first symbol represents the sign of the twisted angle and the second and third symbols represent the conformations of equatorial and axial chelating rings, respectively. Computational modeling was also conducted to construct the most stable structure of the complex cation without referring to the crystal structure. The model was constructed step by step using molecular mechanics and DFT methods. Ultimately, the most stable structure was successfully obtained.

Improved Phos‐tag SDS‐PAGE under neutral pH conditions for advanced protein phosphorylation profiling

We describe an improved Phos-tag SDS-PAGE (Zn(2+)-Phos-tag SDS-PAGE) using a dizinc(II) complex of Phos-tag acrylamide in conjunction with a Bis-tris-buffered neutral-pH gel system to detect shifts in the mobility of phosphoproteins. An existing technique (Mn(2+)-Phos-tag SDS-PAGE) using a polyacrylamide-bound Mn(2+)-Phos-tag and a conventional Laemmli's buffer system under alkaline pH conditions has limitations for separating certain phosphoproteins. The major improvements were demonstrated by visualizing novel up-shifted bands of commercially available pepsin, recombinant Tau treated in vitro with tyrosine kinases, and endogeneous β-catenin in whole-cell lysates. Additionally, the Zn(2+)-Phos-tag SDS-PAGE gels showed better long-term stability than the Mn(2+)-Phos-tag SDS-PAGE gels. We can therefore provide a simple, convenient, and more reliable homemade gel system for phosphate-affinity SDS-PAGE.

Dibridged Bis(Zn2+–cyclen): A Novel Host Molecule of Malonate Dianion in Aqueous Solution

Abstract A dinuclear zinc(II) complex, m,m-bis(Zn2+–cyclen) was synthesized as a novel host molecule for a malonate dianion. The dizinc(II) complex has two zinc(II)–cyclen moieties (cyclen = 1,4,7,10-tetraazacyclododecane) connected through two m-xylene bridges. The 1:1 association constant of log K (K = {[malonate-bound m,m-bis(Zn2+–cyclen)]/[m,m-bis(Zn2+–cyclen)][malonate]}/M−1) was determined to be 3.6 by potentiometric pH titration at 25 °C with I = 0.10 M (NaNO3) in aqueous solution. The H/D exchange reaction of methylene hydrogen atoms of malonate dianion was accelerated by m,m-bis(Zn2+–cyclen) in D2O. The half-life was determined to be 80 min for a 1:1 mixture (2 mM) of malonate and m,m-bis(Zn2+–cyclen) at 25 °C and pD 7. From an aqueous solution of equimolar malonate and m,m-bis(Zn2+–cyclen) however, a cyclic 2:2 malonate/m,m-bis(Zn2+–cyclen) complex was isolated. The structure was confirmed by X-ray crystallography.

Diphenoxido‐Bridged CoII and ZnII Complexes of Tripodal N2O2 Ligands: Stabilisation of MII‐Coordinated Phenoxyl Radical Species

AbstractThree new tripodal ligands with an N2O2 donor set, namely2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methylphenol (H2L1), 2‐tert‐butyl‐6‐({(2‐hydroxybenzyl)[2‐(2‐pyridyl)ethyl]amino}methyl)‐4‐methoxyphenol (H2L2) and 2‐tert‐butyl‐6‐({[2‐(dimethylamino)ethyl](2‐hydroxybenzyl)amino}methyl)‐4‐methoxyphenol (H2L3) have been synthesised. Treatment of the ligands with Co(CH3CO2)2·4H2O or [Zn(H2O)6][ClO4]2 in the presence of Et3N provides the corresponding CoII and ZnII complexes of composition [MII2(L1)2] [M = Co (1) (single‐crystals are a solvate with the composition [CoII2(L1)2]·2CHCl3, i.e. 1·2CHCl3); M = Zn (2)], [MII2(L2)2] [M = Co (3); Zn (4)] and [CoII2(L3)2] (5). Crystallographic analyses reveal that the complexes have closely similar diphenoxido‐bridged structures. Each metal centre assumes MIIN2O3 coordination. The geometry around each metal ion in 1·2CHCl3 (τ = 0.76), 2 (τ = 0.77), 3 (τ = 0.74), 4 (τ = 0.76) and 5 [one CoII (τ = 0.49) and the other CoII (τ = 0.63)] is intermediate between ideal square‐pyramidal (τ = 0) and trigonal‐bipyramidal (τ = 1). Temperature‐dependent magnetic studies reveal weak intramolecular antiferromagnetic exchange couplings for all the three CoII complexes(–J = 1.84, 1.32 and 5.70 cm–1 for 1, 3 and 5, respectively). Spectroscopic properties of the complexes have also been investigated. Cyclic voltammetric (CV) measurements of 1 and 2 show an irreversible oxidative response at Epa (anodic peak potential) in the range 0.60–0.75 V relative to the SCE (saturated calomel electrode), whereas two successive quasi‐reversible oxidative responses can be observed for 3–5 at E1/2 values in the range 0.40–0.53 V relative to the SCE. Oxidative responses are due to the formation of MII‐coordinated phenoxyl radical species. The metal‐coordinated phenoxyl radical species, generated by two‐electron coulometric oxidation of 3–5, were characterised by CV and by adsorption and EPR spectroscopy. The stability of such species was determined by measuring the decay constant (absorption spectroscopy), which reveals that the phenoxyl radical species of 5 is more stable than that of 3 and 4. EPR spectroscopic studies (120 K) of coulometrically generated two‐electron oxidised species of 4 in CH2Cl2 (containing 0.1 M TBAP) at 298 K reveal a combination of an isotropic S = ${1 \over 2}$ signal at g = 2 [(phenolato)(phenoxyl radical)‐coordinated dizinc(II) complex] and a spin‐triplet resonance (S = 1) that gives rise to the symmetric split‐line pattern [bis(phenoxyl radical)‐cordinated dizinc(II) complex]. To pinpoint the site of oxidation (metal‐ or ligand‐centred) in each case, DFT calculations were performed at the B3LYP level of theory.

Diguanidinium bis-(μ-2-hydroxy-propane-1,2,3-tricarboxyl-ato)bis-[diaqua-zincate(II)] dihydrate.

The asymmetric unit of the title compound, (CH6N3)2[Zn2(C6H5O7)2(H2O)2]·2H2O, contains one-half of a centrosymmetric dizinc(II) complex anion, one guanidinium cation and one water mol­ecule. Each ZnII ion is hexa­coordinated by two citrate anions, one in a bidentate fashion and the second monodentate, and two water mol­ecules in a distorted octa­hedral geometry. Intra­molecular O—H⋯O hydrogen bonds add further stability to the mol­ecular structure. In the crystal structure, mol­ecules are linked into a three-dimensional framework by inter­molecular N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds.

Electronic structural dependence of the photophysical properties of fluorescent heteroditopic ligands – implications in designing molecular fluorescent indicators

Two fluorescent heteroditopic ligands (2a and 2b) for zinc ion were synthesized and studied. The efficiencies of two photophysical processes, intramolecular charge transfer (ICT) and photoinduced electron transfer (PET), determine the magnitudes of emission bathochromic shift and enhancement, respectively, when a heteroditopic ligand forms mono- or dizinc complexes. The electron-rich 2b is characterized by a high degree of ICT in the excited state with little propensity for PET, which is manifested in a large bathochromic shift of emission upon Zn(2+) coordination without enhancement in fluorescence quantum yield. The electron-poor 2a displays the opposite photophysical consequence where Zn(2+) binding results in greatly enhanced emission without significant spectral shift. The electronic structural effects on the relative efficiencies of ICT and PET in 2a and 2b as well as the impact of Zn(2+)-coordination are probed using experimental and computational approaches. This study reveals that the delicate balance between various photophysical pathways (e.g. ICT and PET) engineered in a heteroditopic ligand is sensitively dependent on the electronic structure of the ligand, i.e. whether the fluorophore is electron-rich or poor, whether it possesses a donor-acceptor type of structure, and where the metal binding occurs.

Coorperativity between Metals, Ligands and Solvent: a DFT Study on the Mechanism of a Dizinc Complex-Mediated Phosphodiester Cleavage

Density functional theory (DFT)-Tao-Perdew-Staroverov-Scuseria (TPSS) functional calculations on dizinc complex-mediated phosphodiester cleavage indicate a general base catalytic mechanism. 2-hydroxylpropyl-4-nitrophenyl phosphate (HPNP) favors the bridging of two Zn ions by the formation of two coordination bonds between terminal phosphate oxygens and Zn ions. The Zn-bound hydroxide deprotonates the hydroxyl on the side chain of HPNP and consequently the alkoxide is stabilized by coordination to a Zn ion and a hydrogen-bond to Zn-bound water. A water molecule is tightly bound to two amino protons in the bis (1, 4, 7-triazacyclononane) ligand and this determines the orientation of HPNP during a nucleophilic attack to form a trigonal bipyramidal PO5 intermediate and it also weakens the bond between phosphorus and the phenolate, which makes the leaving of the latter easier. The phenolate formed after the collapse of the five-coordinated phosphorus intermediate easily coordinates to a Zn ion. Surprisingly, the stabilizing solvent effect for the transition state after the formation of the P0, intermediate is much stronger (at least 42 kJ•mol^(-1)) than that of all other species as they have solvation energies that fluctuate around 12.6 kJ•mol^(-1). Thus, the overall free energy barrier for this reaction after reactant-binding and before product release is about 17.0 kJ•mol^(-1), which is too low to be rate-determining. The rate-determining step is very likely part of the release process of the products. Based on various calculations, we discuss possible reasons for the different catalytic efficiencies of the dizinc complex and the enzymes.

Unsymmetrical dizinc complexes as models for the active sites of phosphohydrolases

The unsymmetrical dinucleating ligand 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)aminomethyl)-4-methylphenol (IPCPMP or L) has been synthesized to model the active site environment of dinuclear metallohydrolases. It has been isolated as the hexafluorophosphate salt H(4)IPCPMP(PF(6))(2) x 2 H(2)O (H(4)L), which has been structurally characterized, and has been used to form two different Zn(II) complexes, [{Zn(2)(IPCPMP)(OAc)}(2)][PF(6)](2) (2) and [{Zn(2)(IPCPMP)(Piv)}(2)][PF(6)](2) (3) (OAc = acetate; Piv = pivalate). The crystal structures of and show that they consist of tetranuclear complexes with very similar structures. Infrared spectroscopy and mass spectrometry indicate that the tetranuclear complexes dissociate into dinuclear complexes in solution. Potentiometric studies of the Zn(II):IPCPMP system in aqueous solution reveal that a mononuclear complex is surprisingly stable at low pH, even at a 2:1 Zn(II):L ratio, but a dinuclear complex dominates at high pH and transforms into a dihydroxido complex by a cooperative deprotonation of two, probably terminally coordinated, water molecules. A kinetic investigation indicates that one of these hydroxides is the active nucleophile in the hydrolysis of bis(2,4-dinitrophenyl)phosphate (BDNPP) enhanced by complex 2, and mechanistic proposals are presented for this reaction as well as the previously reported transesterification of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) promoted by Zn(II) complexes of IPCPMP.

Mechanochemical versus sol–gel silica loading of phenolate- and acetate-bridged dizinc complexes: toward instant and inexpensive hybrids for controlled binding and release of Zn2+ in pure water

Herein, we have explored expeditious and lowered expense SiO2-loading in the context of fluorescent “off–on” and “on–off” Zn2+ ion detection/controlled release. A simple racemic ligand (H2rlys) (1) prepared in situ can be mechanically SiO2-loaded so as to induce surface sites that allow for “off–on” aqueous fluorescence (λem,max = ∼450 nm) detection of Zn2+; with Cu2+ as a competing input, “inhibit” logic gate behaviour is present. Oppositely, dimeric, chiral fluorescent zinc complexes (ΦF = ∼0.20) prepared in a “one pot” method, and bearing different Zn⋯Zn internuclear spacings, can be incorporated into silica by two methods. Specifically, the chiral Zn2(slysH)2Cl2 (2) and racemic Zn2(rslysH)2(μ-OAc)2 species (3) [slys or rslys = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoate] were mechanochemically loaded. An adsorption constant was 1.99 × 10−4 (molecules of 2/units SiO2)(min)−1 for times of 5–20 min, as determined by extensive SEM-EDS data. These confirmed the highly selective “on–off” rapid aqueous fluorescent Cu2+ detection (versus, Li+, Na+, Cs+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Ag+, Cd2+, Hg2+, Pb2+) through direct Zn2+ displacement under bulk neat water flow (pH = 7.4) as verified by an authentic sample of [Cu2(slysH)2(NO3)2]–silica (4–silica). With the use of 2·SiO2, Na4EDTA and Cu2+ “inhibit” logic gate behaviour was also found. Sol–gel techniques allowed for loadings of 2 (20–890 mg) up to 1 ∶ 1 (by wt). The preparation of these inorganic hybrids are extremely inexpensive and rapid to prepare from commercial starting materials and can be extended to variegated forms of SiO2; alumina gel was also functionalized with 2. Cu2+ and Zn2+ are metals pertinent to human neurological health promotion and in neurodegenerative diseases.