Bis ( 2,4-dinitrophenyl ) Phosphate Research Articles

Synthesis, characterization, and structures of mononuclear and phenoxo-/acetato-bridged trinuclear zinc(II) complexes from salan ligands: Study of their hydrolytic activity

Synthesis of one mononuclear [ZnL1] (1) and two trinuclear phenoxo-/acetato-bridged zinc(II) complexes [Zn3Cl2(L2)2] (2) and [Zn3(OAc)2(L1)2] (3) containing tetradentate diaminobis(phenolato) “salan” ligands are reported. The ligand precursors and their corresponding zinc complexes are characterized by elemental analysis, NMR, IR, and UV–vis spectroscopy. Molecular structures of complexes 2 and 3 are solved by single-crystal X-ray crystallography, while the fate of the complexes in solution is assessed by ESI-MS spectrometry analysis. All complexes (1–3) were studied for their phosphoesterase activity by using para-nitrophenyl phosphate (PNPP) and bis(2,4–dinitrophenyl) phosphate (BNPP) as substrates. While complex 1 has no activity for these substrates, complex 3 effectively hydrolyses PNPP (1.6 × 10−3 s−1), and complex 2, which is less reactive than 3, displays comparable activities towards both substrates (2.0 × 10−4 s−1 and 5.4 × 10−4 s−1, respectively). The importance of evaluating the speciation of Zn-species present for the understanding of the mechanism of the phosphoesterase reaction is discussed.

Elucidating the Roles of Distinct Chemical Factors in the Hydrolytic Activities of Hetero- and Homonuclear Synthetic Analogues of Binuclear Metalloenzymes

In this study, hydrolytic activities of hetero- and homobinuclear metallovariants of an asymmetric (I) or symmetric (II) ligand with the FeIII-ZnII, ZnII-ZnII, and CuII-CuII cores (i.e., IFZ, IZZ, and ICC or IIFZ, IIZZ, and IICC, respectively) are investigated using DFT calculations through four distinct mechanisms: dissociative (DA), substrate-assisted (SA), water-assisted (WA), and associative (AS). Additionally, the effects of different nucleophiles (μ-OH, terminal-OH, and -O2H3), coordination numbers, para substituents (-CH3, -Cl, and -NO2) of the linker, and an external electric field on the energetics of these reactions are computed. The geometries, spin ground states, and substrate binding modes of the three metal centers for both asymmetric (I) and symmetric (II) ligands differ from each other. There is no Lewis acid activation of the bis(2,4-dinitrophenyl) phosphate (BDNPP) substrate, and hydrolysis is predominantly controlled by the nucleophilicity of the metal-bound hydroxyl ion. The electronic nature of the metal ions determines the activities of their complexes, and the homobinuclear IZZ is found to be the most active complex. However, complexes formed with ligand I are not more active than their ligand-II-containing counterparts for all metal ions. The DFT calculations suggest that the DA pathway is the energetically most feasible among the four pathways. The terminal hydroxyl group is the strongest nucleophile, and the electron-donating -CH3 group is the most suitable para substituent in the linker. Whereas the introduction of an external electric field along the reaction axis lowers the barrier for IFZ, it leaves that unchanged for IZZ and increases that for ICC. These combined results in this study highlight the influence of distinct critical chemical factors such as the electronic nature of the metal ions, the ligand environment, as well as the linker and nucleophile on phosphoester hydrolysis. Insights gained will guide the design of the next generation of versatile metal complexes for a wide range of reactions and applications.

Polynuclear zinc(II) complexes of thiosemicarbazone: Synthesis, X-ray structure and biological evaluation

Two new dimeric Zn(II) ([{ZnL1(DMSO2)}2]·DMSO (1), [{ZnL2Cl}2] (2)) and a novel tetrameric Zn(II) complex ([(Zn2L3)2(μ–OAc)2(μ3–O)2] (3)), where H2L1 = 4–(p–methoxyphenyl) thiosemicarbazone of o–hydroxynapthaldehyde, HL2 = 4–(p–methoxyphenyl)thiosemicarbazone of benzoyl pyridine and H2L3 = 4–(p–chlorophenyl)thiosemicarbazone of o–vanillin are reported. Ligands and their complexes were characterized by spectroscopic and single crystal X–ray diffraction techniques. In addition, the complexes exhibited good binding affinity towards HSA (1012 M−1), which is supported by their ability to quench the tryptophan fluorescence emission spectra of HSA. The complexes were also screened for their DNA binding propensity through UV–vis absorption titration, circular dichroism and fluorescence spectral studies. Results show that they effectively interact with CT–DNA through an intercalative mode of binding, with binding constants ranging from 103 to 104 M−1. Among the three complexes 1 has the highest binding affinity towards CT–DNA. Further, the phosphatase activity was evaluated using bis(2,4–dinitrophenyl)phosphate (BDNPP) as substrate, however, the complexes did not yield any measurable catalytic activity. Nevertheless the complexes showed significant cytotoxic potential against HeLa and HT–29 cancer cell lines that was assessed through MTT assay and DAPI staining. Remarkably, complex 1 showed better activity than cisplatin against HT–29 cell line.

Synthesis, Magnetic Properties, and Catalytic Properties of a Nickel(II)-Dependent Biomimetic of Metallohydrolases.

A dinickel(II) complex of the ligand 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol (HL1) has been prepared and characterized to generate a functional model for nickel(II) phosphoesterase enzymes. The complex, [Ni2(L1)(μ-OAc)(H2O)2](ClO4)2·H2O, was characterized by microanalysis, X-ray crystallography, UV-visible, and IR absorption spectroscopy and solid state magnetic susceptibility measurements. Susceptibility studies show that the complex is antiferromagnetically coupled with the best fit parameters J = −27.4 cm−1, g = 2.29, D = 28.4 cm−1, comparable to corresponding values measured for the analogous dicobalt(II) complex [Co2(L1)(μ-OAc)](ClO4)2·0.5 H2O (J = −14.9 cm−1 and g = 2.16). Catalytic measurements with the diNi(II) complex using the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) demonstrated activity toward hydrolysis of the phosphoester substrate with Km ~10 mM, and kcat ~0.025 s−1. The combination of structural and catalytic studies suggests that the likely mechanism involves a nucleophilic attack on the substrate by a terminal nucleophilic hydroxido moiety.

An Unsymmetric Ligand with a N5O2 Donor Set and Its Corresponding Dizinc Complex: A Structural and Functional Phosphoesterase Model

To mimic the active sites of the hydrolytic enzyme zinc phosphotriesterase, a new dinucleating unsymmetric ligand, PICIMP (2‐{[2‐hydroxy‐5‐methyl‐3‐({[(1‐methyl‐1H‐imidazol‐2‐yl)methyl](pyridin‐2‐ylmethyl)amino}methyl)benzyl][(1‐methyl‐1H‐imidazol‐2‐yl)methyl]amino}acetic acid), has been synthesized and characterized. The hydrolytic efficacy of the complex solution (PICIMP/ZnCl2 = 1:2) has been investigated using bis‐(2,4‐dinitrophenyl)phosphate (BDNPP), a DNA analogue substrate. Speciation studies were undertaken by potentiometric titrations at varying pH for both the ligand and the corresponding dizinc complex to elucidate the formation of the active hydrolysis catalyst; these studies reveal that the dinuclear zinc(II) complexes, [Zn2(PICIMP)]2+ and [Zn2(PICIMP)(OH)]+ predominate in solution above pH 4. The obtained pKa of 7.44 for the deprotonation of water suggests formation of a bridging hydroxide between the two ZnII ions. Kinetic investigations of BDNPP hydrolysis over the pH range 5.5–10.5 have been performed. The cumulative results indicate the hydroxo‐bridged dinuclear ZnII complex [Zn2(PICIMP)(µ‐OH)]+ as the effective catalyst. Density functional theory calculations were performed to investigate the detailed reaction mechanism. The calculations suggest that the bridging hydroxide becomes terminally coordinated to one of the zinc ions before performing the nucleophilic attack in the reaction.

Phosphate ester cleavage by a positively charged porous silica adorned with lanthanum (III)

Phosphate esters and analogs thereof are well known in chemistry and biology by their formidable stability. We used a porous silica material (PSiM) functionalized with amino groups (8.4%) and lanthanum ions (1.5%) to catalyze the cleavage of bis(2,4-dinitrophenyl)phosphate (BDNPP). The functionalized PSiM attracted the substrate from solution and accelerated its hydrolysis by 38-fold in relation to the uncatalyzed reaction at pH 8.14. After an induction period associated to hydration of the catalyst during the first use, the catalytic activity increased in the first reuse to about 600-fold. Supramolecular effects were proposed to drive the catalytic process. Kinetic studies in different pH's showed that the lanthanum ion and amino groups acted mutually in the BDNPP hydrolysis and a reaction mechanism that took this into account was proposed.

Second-Sphere Effects in Dinuclear FeIIIZnII Hydrolase Biomimetics: Tuning Binding and Reactivity Properties

Herein, we report the synthesis and characterization of two dinuclear FeIIIZnII complexes [FeIIIZnIILP1] (1) and [FeIIIZnIILP2] (2), in which LP1 and LP2 are conjugated systems containing one and two pyrene groups, respectively, connected via the diamine -HN(CH2)4NH- spacer to the well-known N5O2-donor H2L ligand (H2L = 2-bis{[(2-pyridylmethyl)aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl}-4-methylphenol). The complex [FeIIIZnIIL1] (3), in which H2L was modified to H2L1, with a carbonyl group attached to the terminal phenol group, was included in this study for comparison purposes.1 Both complexes 1 and 2 were satisfactorily characterized in the solid state and in solution. Extended X-ray absorption fine structure data for 1 and 3 in an acetonitrile solution show that the multiply bridged structure seen in the solid state of 3 is retained in solution. Potentiometric and UV-vis titration of 1 and 2 show that electrostatic interaction between the protonated amino groups and coordinated water molecules significantly decreases the pKa of the iron(III)-bound water compared to those of 3. On the other hand, catalytic activity studies using 1 and 2 in the hydrolysis of the activated substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) resulted in a significant increase in the association of the substrate (Kass ≅ 1/KM) compared to that of 3 because of electrostatic and hydrophobic interactions between BDNPP and the side-chain diaminopyrene of the ligands H2LP1 and H2LP2. In addition, the introduction of the pyrene motifs in 1 and 2 enhanced their activity toward DNA and as effective antitumor drugs, although the biochemical mechanism of the latter effect is currently under investigation. These complexes represent interesting examples of how to promote an increase in the activity of traditional artificial metal nucleases by introducing second-coordination-sphere effects.

DNA binding, cleavage and cytotoxicity of a novel dimetallic Fe(III) triaza-cyclononane complex

A novel bimetallic Fe(III) complex with the bis(triaza-cyclononane) ligand 2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)-4-methylphenol (bcmp) is reported. [Fe2{bcmp(-H)}(μ-OH)Cl2]Cl2 (2) contains two octahedral Fe(III) centers bound to the two triaza-cyclononane rings of bcmp. The coordination sphere is completed by one chlorine, one bridging phenolate oxygen and one bridging hydroxide group. The complex has been characterized by elemental analysis, Mössbauer spectroscopy, UV–Vis spectroscopy, pH potentiometric titration, ESI mass spectrometry and cyclic voltammetry. The complex hydrolyzes the DNA model bis(2,4-dinitrophenyl) phosphate (BDNPP) with a maximum activity a pH 7. Michaelis–Menten behavior is observed with kcat=3.56×10−4s−1 and Km=0.56mM (pH 7.0, 40°C). The interaction of 2 with CT DNA was studied by electronic absorption spectroscopy and gel electrophoresis. Notably, the complex relaxes supercoiled pUC19 DNA into the nicked form at low micromolar concentration (10μM) in the presence of an external reducing agent (ascorbic acid). Finally, the in vitro antiproliferative activity of 2 was assessed on a panel of human cancer cell lines and results revealed that the complex exhibited a significant cytotoxic effects in particular versus colon LoVo cancer cells, wih IC50 value 2.5 times lower than that shown by the reference metallodrug cisplatin (3.54 versus 8.53μM).

Investigation of the identity of the nucleophile initiating the hydrolysis of phosphate esters catalyzed by dinuclear mimics of metallohydrolases

di-Zinc(II) complexes of the ligands 2,6-bis((bis(2-methoxyethyl)amino)methyl)-4-methylphenol (HL1), 2,6-bis(bis(hydroxyethyl)aminomethyl)-4-methylphenol (HL2) and 2,6-bis((hydroxyethyl)(methoxyethyl)-aminomethyl)-4-methylphenol (HL3) have been prepared and characterized. The three ligands differ in their donor types, having ether donors (HL1), alkoxido donors (HL2) and both ether and alkoxido donors (HL3). These differences allowed an investigation into the role of the potential nucleophiles in the hydrolysis reaction with the phosphodiester substrate bis(2,4-dinitrophenyl)phosphate (BDNPP). In addition, the di-Mg(II) complex of ligand HL2 was prepared in order to examine the potential for Mg(II) to replace Zn(II) in these biomimetic systems. Kinetically relevant pKa values for the three di-Zn(II) complexes were determined to be 7.14 and 9.21 for [Zn2(L1)(CH3COO)2](PF6), 7.90 and 10.21 for [Zn2(L2)(CH3COO)2](BPh4) and 8.43 and 10.69 for [Zn2(L3)(CH3COO)2](BPh4). At the respective pH optima the relevant catalytic parameters are kcat=5.44(0.11)×10−5s−1 (Km=5.13(0.92) mM), 2.60(0.87)×10−4s−1 (Km=5.49(1.51) mM) and 1.53(0.27)×10−4s−1 (Km=2.14(0.50) mM) for [Zn2(L1)(CH3COO)2](PF6), [Zn2(L2)(CH3COO)2](BPh4) or [Zn2(L3)(CH3COO)2](BPh4), respectively. The di-Mg(II) complex was found to be unreactive in the hydrolysis reaction with BDNPP under the conditions employed. Computational methods using the [Zn2(L2)(CH3COO)2](BPh4) complex were used to discriminate between different possible mechanistic pathways. The DFT calculations indicate that an alkoxido-mediated pathway in the complexes formed with ligands L2 or L3 is unlikely, because it induces significant distortion of the Zn2(L) unit; a direct attack by a coordinated hydroxide is preferred in each of the three systems studied here. The calculations also revealed the important role of ligand structural rigidity.

Theoretical Study of Phosphodiester Hydrolysis and Transesterification Catalyzed by an Unsymmetric Biomimetic Dizinc Complex.

Density functional theory calculations have been used to investigate the reaction mechanisms of phosphodiester hydrolysis and transesterification catalyzed by a dinuclear zinc complex of the 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)amino-methyl)-4-methylphenol (IPCPMP) ligand, mimicking the active site of zinc phosphotriesterase. The substrates bis(2,4)-dinitrophenyl phosphate (BDNPP) and 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) were employed as analogues of DNA and RNA, respectively. A number of different mechanistic proposals were considered, with the active catalyst harboring either one or two hydroxide ions. It is concluded that for both reactions the catalyst has only one hydroxide bound, as this option yields lower overall energy barriers. For BDNPP hydrolysis, it is suggested that the hydroxide acts as the nucleophile in the reaction, attacking the phosphorus center of the substrate. For HPNP transesterification, on the other hand, the hydroxide is proposed to act as a Brønsted base, deprotonating the alcohol moiety of the substrate, which in turn performs the nucleophilic attack. The calculated overall barriers are in good agreement with measured rates. Both reactions are found to proceed by essentially concerted associative mechanisms, and it is demonstrated that two consecutive catalytic cycles need to be considered in order to determine the rate-determining free energy barrier.

A Heterodinuclear FeIIIZnII Complex as a Mimic for Purple Acid Phosphatase with Site‐Specific ZnII Binding

AbstractPurple acid phosphatases (PAPs) are the only dinuclear metallohydrolases for which the necessity for a heterovalent active site (FeIII–MII; M = Fe, Zn or Mn) for catalysis has been established. A major goal for the synthesis of PAP biomimetics is to design a ligand in which the two coordination sites exhibit discrimination between the trivalent and divalent metal ions. With this goal in mind the ligand 2‐{[bis(2‐methoxyethyl)amino]methyl}‐6‐{[(2‐hydroxybenzyl)(2‐pyridylmethyl)amino]methyl}‐4‐methylphenol (BMMHPH2), with two distinct coordination sites, N2O2 (α) and NO3 (β), has been prepared. Although not exactly mimicking the active site of PAP, the ligand facilitated the formation of the complex [FeIIIZnII(BMMHP)(CH3COO)2](BPh4), which exhibited regioselectivity in the two metal binding sites. The phosphoesterase‐like activity of the complex in 50:50 acetonitrile/water was investigated by using the substrate bis(2,4‐dinitrophenyl) phosphate (BDNPP) yielding kinetically relevant pKa values of 6.89, 7.37 and 9.00, a KM of 10.8 ± 2.1 mM and a kcat of 3.20 ± 0.38 × 10–3 s–1 (at pH = 7.5). Attempts to prepare a diiron analogue resulted in a centrosymmetric dimer, [FeIII2(BMMHPH)2(μ‐OH)2](BPh4)2, with one six‐coordinate FeIII atom in each of the α‐sites, connected by two μ‐hydroxido groups. In this Fe(μ‐OH)2Fe diamond core the FeIII ions are weakly antiferromagnetically coupled, with J = –1.76 ± 0.03 cm–1. The β‐sites were vacant. Attempts to replace the ZnII ion with MgII resulted in the formation of a centrosymmetric trimer, i.e. [FeIII2MgII(BMMHPH)2(CH3COO)2(CH3O)2](BPh4)2.

Experimental and theoretical studies of hydrolysis of nerve agent sarin by binuclear zinc biomimetic catalysts

A complex (ZnL1) of 2,2-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis ((pyridin-2-ylmethyl)azanediyl)diethanol (this ligand is named by L1) functionalized with two Zn(II) centers, has been previously suggested to be a structural model for binuclear zinc phosphotriesterases (PTEs) and proven to be an effective catalyst for the hydrolysis of bis(2,4-dinitrophenyl)phosphate (BDNPP). In this paper, ZnL1 was further found to have a high catalytic activity for the hydrolysis of isopropyl methylphosphonofluoridate (sarin, GB) with kcat/Km=0.051s−1M−1 at 303K, examined by on-site NMR analysis. The subsequent density functional theory (DFT) calculations indicate that a terminal alkoxide (Ot) bound by Zn may work as a general base to activate a water molecule and then a hydroxide derived from the latter performs the initial nucleophilic attack on the phosphor in GB. Inspired by this mechanism, a new biomimetic catalyst was designed and synthesized by replacing the two pyridines of ZnL1 by hydroxyls, i.e. a complex of two Zn(II) with 2,6-bis((bis(2-hydroxyethyl)amino)methyl)-4-methylphenol (the ligand is named by L2). This replacement was expected to increase the Ot basicity, thereby facilitating the nucleophilic attack and the overall hydrolysis of GB. It was shown that ZnL2 had a very high catalytic efficiency for the hydrolysis of GB, with kcat/Km=0.11s−1M−1 at 303K and about 90% conversion in 30min. The following DFT calculations proposed a detailed reaction mechanism of ZnL2 and gave an energy barrier (5.8kcalM−1) very close to the experimental activation energy (5.6kcalM−1). In this study, a mechanism-inspired design strategy has been demonstrated to be successful in developing biomimetic catalyst.

DNA damage and induction of apoptosis in pancreatic cancer cells by a new dinuclear bis(triazacyclonane) copper complex

The dinuclear copper(II) complex [Cu2{bcmp(-H)}(μ-OH)](NO3)2·H2O (1, bcmp=2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)-4-methylphenol) has been synthesized and characterized by electrospray ionization mass spectrometry, potentiometric titration and cyclovoltammetry. The X-ray structure of the analogous perchlorate salt [Cu2{bcmp(-H)}(μ-OH)](ClO4)2·2.5H2O (2) was determined. Cytotoxicity studies showed very promising activity of 1 against various pancreatic tumor cell lines with IC50 values comparable or even lower than those of cisplatin. The Cu complex displayed low toxicity against a human non-tumor cell line (HEK 293) demonstrating selectivity for cancer cells. 1 converts supercoiled pUC19 plasmid DNA into the nicked form at micromolar concentrations in the absence of added reductants. A detailed kinetic study on the hydrolysis of the DNA model bis(2,4-dinitrophenyl) phosphate (BDNPP) has been performed. 1 hydrolyses BDNPP with a second order rate constant of 0.047Ms−1 at pH8 and 40°C. Finally, single cell electrophoresis (comet assay) and fluorescence microscopy analysis showed that 1 interacts with cellular DNA and induces apoptotic cell death of Capan-1 pancreatic cancer cells. Western blotting analysis indicated that the Cu complex activates the p53 dependent pathway of apoptosis.

Phosphate Diester Cleavage, DNA Interaction and Cytotoxic Activity of a Bimetallic Bis(1,4,7‐triazacyclononane) Zinc Complex

AbstractThe dinuclear zinc complex [Zn2{bcmp(‐H)}(μ‐Cl)](ClO4)2·H2O {bcmp = 2,6‐bis(1,4,7‐triazacyclonon‐1‐ylmethyl)‐4‐methylphenol} has been synthesized and structurally characterized. The DNA binding affinity was assessed by ethidium bromide fluorescence quenching experiments. The complex relaxes supercoiled pUC19 DNA into the nicked form at low micromolar concentration. Mechanistic studies were carried out using the DNA and RNA models bis(2,4‐dinitrophenyl) phosphate (BDNPP) and 2‐hydroxypropyl p‐nitrophenyl phosphate (HPNP). A detailed kinetic analysis suggested that the bridging OH group of the solution species [Zn2{bcmp(‐H)}(μ‐OH)]2+ acts as the nucleophile in the hydrolysis of BDNPP, while in the case of HPNP, the bridging OH group acts as a general base and seems to shift to a terminal position upon substrate coordination. Finally, the cytotoxicity profile of the dinuclear zinc(II) complex was assessed. The complex showed promising in vitro antitumour activity against pancreatic and lung cancers cell lines.

O‐Nucleophilicity of Hydroxamate Ions for Cleavage of Carboxylate and Phosphate Esters in Cationic Micelles

ABSTRACTThe nucleophilic reactivities of hydroxamate (HA−) ions of the structure RCONHO− [R = CH3 (acetohydroxamate, AHA−), C6H5 (benzohydroxamate, BHA−), 2‐OHC6H4 (salicylhydroxamate, SHA−), and 4‐CH3OC6H4 (4‐methoxbenzohydroxamate, MBHA−)] for the hydrolysis of p‐nitrophenyl benzoate (PNPB), tris(3‐nitrophenyl) phosphate (TRIS), and bis(2,4‐dinitrophenyl) phosphate (BDNPP) have been examined kinetically. Over the pH range of 6.7–11.4, the α‐nucleophile (HA−) accelerates deacylation of PNPB and dephosphorylation of TRIS (in cetyltrimethylammonium bromide (CTAB) micelle, 2.0 × 10−3 M). The salicylhydroxamate ion encountered effective catalysis than AHA−, BHA−, and MBHA− ions. The monoanionic SHA− and dianionic SA2− forms of salicylhydroxamic acid are the reactive species. The hydroxamic acid concentration–dependent critical micelle concentration (cmc) and fractional ionization constant (α) and of CTAB provide qualitative information for the micellar incorporation of the hydroxamate ion. The ab initio calculations performed on the hydroxamate ions at restricted Hartree–Fock using the 6‐311G (d,p) basis set revealed the O‐nucleophilicity of hydroxamate ions toward C=O and P=O centers. On the basis of ab initio calculation, it has been concluded that hydroxamic acids can exist into E‐amide and Z‐amide forms. The large stable amide or imide anions of hydroxamate are strong nucleophilic for the esterolytic cleavage of carboxylate and phosphate esters.

Catalytic hydrolysis of phosphodiesters by nucleophilic ions in gemini micellar media

The catalytic hydrolysis of bis(4-nitrophenyl)phosphate (BNPP) and bis(2,4-dinitrophenyl)phosphate (BDNPP) catalyzed by α-nucleophiles in gemini micellar media was investigated at 27 °C. The cationic gemini surfactants, i.e., alkanediyl-α-ω-bis(hydroxyethylmethylhexadecylammonium bromide) (16-s-16 MEA 2Br−, where s = 4 and 6) were used. Nucleophilic reactivity of α-nucleophiles such as hydroperoxide (HOO−), acetohydroxamate (AHA−), and butane 2,3-dione monoximate ions (BDMO−) were compared. The kinetic rate data were treated by applying the pseudophase model. The cationic gemini surfactants show unusual rate acceleration toward the cleavage of phosphodiesters with nucleophiles. These studies reveal that the hydroperoxide ion shows the highest catalytic activity reported so far with an unprecedented acceleration rate, 107 times faster than that of the uncatalyzed reaction. The possible mechanism for the BNPP and BDNPP cleavage promoted by α-nucleophiles is proposed on the basis of kinetic analysis. Copyright © 2014 John Wiley & Sons, Ltd.

A Heterobimetallic FeIIIMnII Complex of an Unsymmetrical Dinucleating Ligand: A Structural and Functional Model Complex for the Active Site of Purple Acid Phosphatase of Sweet Potato

AbstractThe heterodinuclear mixed‐valence complex [FeMn(ICIMP)(OAc)2Cl] (1) {H2ICIMP = 2‐(N‐carboxylmethyl)‐[N‐(N‐methylimidazolyl‐2‐methyl)aminomethyl]‐[6‐(N‐isopropylmethyl)‐[N‐(N‐methylimidazolyl‐2‐methyl)]aminomethyl‐4‐methylphenol], an unsymmetrical N4O2 donor ligand} has been synthesized and fully characterized by several spectroscopic techniques as well as by X‐ray crystallography. The crystal structure of the complex reveals that both metal centers in 1 are six‐coordinate with the chloride ion occupying the sixth coordination site of the MnII ion. The phenoxide moiety of the ICIMP ligand and both acetate ligands bridge the two metal ions of the complex. Mössbauer spectroscopy shows that the iron ion in 1 is high‐spin FeIII. Two quasi‐reversible redox reactions for the complex, attributed to the FeIIIMnII/FeIIMnII (at –0.67 V versus Fc/Fc+) and FeIIIMnII/FeIIIMnIII (at 0.84 V), were observed by means of cyclic voltammetry. Complex 1, with an FeIII–MnII distance of 3.58 Å, may serve as a model for the mixed‐valence oxidation state of purple acid phosphatase from sweet potato. The capability of the complex to effect organophosphate hydrolysis (phosphatase activity) has been investigated at different pH levels (5.5–11) by using bis(2,4‐dinitrophenyl)phosphate (BDNPP) as the substrate. Density functional theory calculations indicate that the substrate coordinates to the MnII ion. In the transition state, a hydroxide ion that bridges the two metal ions becomes terminally coordinated to the FeIII ion and acts as a nucleophile, attacking the phosphorus center of BDNPP with the concomitant dissociation of the leaving group.

A dinuclear zinc(II) complex of a new unsymmetric ligand with an N5O2 donor set; A structural and functional model for the active site of zinc phosphoesterases

The dinuclear complex [Zn2(DPCPMP)(pivalate)](ClO4), where DPCPMP is the new unsymmetrical ligand [2-(N-(3-((bis((pyridin-2-yl)methyl)amino)methyl)-2-hydroxy-5-methylbenzyl)-N-((pyridin-2-yl)methyl)amino)acetic acid], has been synthesized and characterized. The complex is a functional model for zinc phosphoesterases with dinuclear active sites. The hydrolytic efficacy of the complex has been investigated using bis-(2,4-dinitrophenyl)phosphate (BDNPP), a DNA analog, as substrate. Speciation studies using potentiometric titrations have been performed for both the ligand and the corresponding dizinc complex to elucidate the formation of the active hydrolysis catalyst; they reveals that the dinuclear zinc(II) complexes, [Zn2(DPCPMP)]2+ and [Zn2(DPCPMP)(OH)]+ predominate the solution above pH4. The relatively high pKa of 8.38 for water deprotonation suggests that a terminal hydroxide complex is formed. Kinetic investigations of BDNPP hydrolysis over the pH range 5.5–11.0 and with varying metal to ligand ratio (metal salt:ligand=0.5:1 to 3:1) have been performed. Variable temperature studies gave the activation parameters ΔH‡=95.6kJmol−1, ΔS‡=−44.8Jmol−1K−1, and ΔG‡=108.0kJmol−1. The cumulative results indicate the hydroxido-bridged dinuclear Zn(II) complex [Zn2(DPCPMP)(μ-OH)]+ as the effective catalyst. The mechanism of hydrolysis has been probed by computational modeling using density functional theory (DFT). Calculations show that the reaction goes through one concerted step (SN2 type) in which the bridging hydroxide in the transition state becomes terminal and performs a nucleophilic attack on the BDNPP phosphorus; the leaving group dissociates simultaneously in an overall inner sphere type activation. The calculated free energy barrier is in good agreement with the experimentally determined activation parameters.

Cadmium(II) Complexes: Mimics of Organophosphate Pesticide Degrading Enzymes and Metallo-β-lactamases

Cadmium(II) complexes of ethyl 4-hydroxy-3,5-bis(((2-hydroxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO(2)EtH(3)L1) and ethyl 4-hydroxy-3,5-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)benzoate (CO(2)EtHL2) are described. The two ligands possess an ethyl ester (CO(2)Et-) at the position para to the phenolic -OH; CO(2)EtHL2, with methyl ether donors in contrast to potentially nucleophilic alkoxide donors in CO(2)EtH(3)L1, offers a direct comparison of potential ligand-centered nucleophiles. The complex with CO(2)EtH(3)L1 was characterized using (1)H and (13)C NMR spectroscopy, mass spectrometry and microanalysis; X-ray crystallography defined a tetranuclear structure [Cd(4)(CO(2)EtH(2)L1)(2)(CH(3)COO)(3.75)Cl(0.25)(H(2)O)(2)](PF(6))(2). Functional studies of the cadmium(II) complexes were undertaken with the substrates bis(2,4-dinitrophenyl)phosphate (BDNPP), and nitrocefin to assess their phosphatase and β-lactamase activities, respectively. The complexes with CO(2)EtH(3)L1 and CO(2)EtHL2 are competent phosphoesterase mimics with K(M) = 9.4 ± 2.1 mM and 10.1 ± 3.4 mM, k(cat) = 9.4 ± 0.2 × 10(-3) s(-1) and 9.7 ± 2.7 × 10(-3) s(-1), respectively. Use of a solvent mixture containing H(2)(18)O/H(2)(16)O in the reaction with BDNPP showed that for the complex with CO(2)EtH(3)L1 the (18)O label was incorporated in the reaction product suggesting that the nucleophile involved is a Cd-OH moiety and not a metal bound alkoxide; for CO(2)EtHL2 the presence of the methyl-ether dictates that the active nucleophile must also be a hydroxide. The cadmium(II) complex with CO(2)EtH(3)L1 was furthermore found to be a competent β-lactamase mimic with k(cat) = 1.39 × 10(-2) ± 3 × 10(-3) s(-1), K(M) = 0.11 ± 0.03 mM, and pK(a) = 7.9 ± 0.1. Mass spectral evidence suggested that the active nucleophile in this reaction is the alkoxide; lack of β-lactamase activity of the complex with CO(2)EtHL2 supports this assignment. Similar to enzyme-catalyzed reactions, a blue reaction intermediate in the β-lactamase reaction of the CO(2)EtH(3)L1 complex was also identified. It is proposed that the Cd(II) complexes of CO(2)EtH(3)L1 and CO(2)EtHL2 react identically as phosphatases, with a terminal hydroxide as the nucleophile; the former exhibits β-lactamase activity with the alkoxide as a nucleophile, while the latter, without a potentially nucleophilic alkoxide, is inactive.

Ligand modifications modulate the mechanism of binuclear phosphatase biomimetics

Complexation of dimethyl-6,6′-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis((2-hydroxyethyl)azanediyl)bis(methylene)dipicolinate (Me2H3L4) and 2,2′-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis(((6-methylpyridin-2-yl)methyl)azanediyl)diethanol (H3L5) with Zn(II) afforded the complexes [Zn2(H2L4)(H2O)2](ClO4) and [Zn2(H2L5)(CH3CO2)(H2O)](PF6)2·2H2O, which were characterized by 1H and 13C NMR spectroscopy, mass spectrometry, microanalysis, and the former by X-ray crystallography. Functional studies of the zinc complexes with the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) showed the complexes to be competent catalysts with kcat=3.52±0.03×10−4 and 1.27±0.04×10−3s−1 (Km=6.7±0.9; 13.8±1.5mM), with catalytically relevant pKas of 9.4 and 6.6, respectively. The pKa values are discussed with respect to the potential nucleophilic species and the effect of the donor environment.