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Charlotte K Williams - One of the best experts on this subject based on the ideXlab platform.

  • pentablock copolymer from tetracomponent monomer mixture using a switchable Dizinc catalyst
    Macromolecules, 2018
    Co-Authors: Thomas T D Chen, Charlotte K Williams
    Abstract:

    Well-defined pentablock copolymers are prepared using a single catalyst, in one pot, from four different monomers: anhydride, epoxide, lactone, and CO2. The Dizinc catalyst bridges three distinct polymerization cycles and performs a double switch in polymerization mechanism to produce pentablock copolymers. The new materials are hydroxyl-telechelic and are efficiently postfunctionalized to introduce polar and nonpolar side-chains.

  • Dizinc lactide polymerization catalysts hyperactivity by control of ligand conformation and metallic cooperativity
    Angewandte Chemie, 2016
    Co-Authors: Arnaud Thevenon, Charles Romain, Michael S Bennington, Andrew J P White, Hannah J Davidson, Sally Brooker, Charlotte K Williams
    Abstract:

    Understanding how to moderate and improve catalytic activity is critical to improving degradable polymer production. Here, di- and monozinc catalysts, coordinated by bis(imino)diphenylamido ligands, show remarkable activities and allow determination of the factors controlling performance. In most cases, the Dizinc catalysts significantly out-perform the monozinc analogs. Further, for the best Dizinc catalyst, the ligand conformation controls activity: the catalyst with “folded” ligand conformation shows turnover frequency (TOF) values up to 60 000 h−1 (0.1 mol % loading, 298 K, [LA]=1 m), whilst that with a “planar” conformation is much slower, under similar conditions (TOF=30 h−1). Dizinc catalysts also perform very well under immortal conditions, showing improved control, and are able to tolerate loadings as low as 0.002 mol % whilst conserving high activity (TOF=12 500 h−1).

  • macrocyclic Dizinc ii alkyl and alkoxide complexes reversible co2 uptake and polymerization catalysis testing
    Inorganic Chemistry, 2015
    Co-Authors: Charles Romain, Michael S Bennington, Andrew J P White, Charlotte K Williams, Sally Brooker
    Abstract:

    The synthesis of three new Dizinc(II) complexes bearing a macrocyclic [2 + 2] Schiff base ligand is reported. The bis(anilido)tetraimine macrocycle reacts with diethylzinc to form a bis(ethyl)Dizinc(II) complex, [LEtZn2Et2] (1). The reaction of complex 1 with isopropyl alcohol is reported, forming a bis(isopropyl alkoxide)Dizinc complex, [LEtZn2(iPrO)2] (2). Furthermore, complex 1, with 2 equiv of alcohol, is applied as an initiator for racemic lactide ring-opening polymerization. It shows moderately high activity, resulting in a pseudo-first-order rate coefficient of 9.8 × 10–3 min–1, with [LA] = 1 M and [initiator] = 5 mM at 25 °C and in a tetrahydrofuran solvent. Polymerization occurs with good control, as evidenced by the linear fit to a plot of molecular weight versus conversion, the narrow dispersities, and the limited transesterification. The same initiating system is inactive for the ring-opening copolymerization of carbon dioxide (CO2) and cyclohexene oxide at 80 °C and 1 bar of CO2 pressure. How...

  • selective polymerization catalysis controlling the metal chain end group to prepare block copolyesters
    Journal of the American Chemical Society, 2015
    Co-Authors: Yunqing Zhu, Charles Romain, Charlotte K Williams
    Abstract:

    Selective catalysis is used to prepare block copolyesters by combining ring-opening polymerization of lactones and ring-opening copolymerization of epoxides/anhydrides. By using a Dizinc complex with mixtures of up to three different monomers and controlling the chemistry of the Zn–O(polymer chain) it is possible to select for a particular polymerization route and thereby control the composition of block copolyesters.

  • influences of a Dizinc catalyst and bifunctional chain transfer agents on the polymer architecture in the ring opening polymerization of e caprolactone
    Macromolecules, 2015
    Co-Authors: Charles Romain, Valentin Poirier, Charlotte K Williams
    Abstract:

    The polymerization of e-caprolactone is reported using various bifunctional chain transfer agents and a Dizinc catalyst. Conventionally, it is assumed that using a bifunctional chain transfer agent (CTA), polymerization will be initiated from both functional groups; however, in this study this assumption is not always substantiated. The different architectures and microstructures of poly(e-caprolactone) samples (PCL) are compared using a series of bifunctional and monofunctional alcohols as the chain transfer agents, including trans-1,2-cyclohexanediol (CHD), ethylene glycol (EG), 1,2-propanediol (PD), poly(ethylene glycol) (PEG), 2-methyl-1,3-propanediol (MPD), 1-hexanol, 2-hexanol, and 2-methyl-2-pentanol. A mixture of two architectures is observed when diols containing secondary hydroxyls are used, such as cyclohexanediol or propanediol; there are chains that are both chain-extended and chain-terminated by the diol. These findings indicate that not all secondary hydroxyl groups initiate polymerization....

Franc Meyer - One of the best experts on this subject based on the ideXlab platform.

  • highly active and readily accessible proline based Dizinc catalyst for co2 epoxide copolymerization
    Chemistry: A European Journal, 2017
    Co-Authors: Mike Schutze, Sebastian Dechert, Franc Meyer
    Abstract:

    In the pursuit of CO2-based materials, the development of efficient catalysts for the alternating copolymerization of CO2 and epoxides to give polycarbonates is receiving particular attention. Desirable attributes for such catalysts are high copolymerization activity at low CO2 pressure, as well as chemo- and stereocontrol over the formed polymer. Here we report a novel chiral zinc catalyst that can be isolated in 97 % yield from commercial sources, and that produces polycarbonates selectively from neat cyclohexene oxide under 1 bar of CO2 pressure at temperatures above 50 °C. At 80 °C reaction temperature, TONs of 1684 and initial TOFs up to 149 h-1 were measured, producing an isotactic-enriched polycarbonate with a probability Pm of 65 % for the formation of a meso diad. Insight into the dinuclear nature of the active species and the copolymerization progress has been gained from structural and spectroscopic studies.

  • binding of β lactam antibiotics to a bioinspired Dizinc complex reminiscent of the active site of metallo β lactamases
    Inorganic Chemistry, 2012
    Co-Authors: Simone Wockel, Joanna Galezowska, Sebastian Dechert, Franc Meyer
    Abstract:

    Metallo-β-lactamases (mβls) cause bacterial resistance toward a broad spectrum of β-lactam antibiotics by catalyzing the hydrolytic cleavage of the four-membered β-lactam ring, thus inactivating the drug. Minutiae of the mechanism of these enzymes are still not well understood, and reports about binding studies of the substrates to the enzymes as well as to synthetic model systems are rare. Here we report a new pyrazolate-based bioinspired Dizinc complex (1) reminiscent of the active site of binuclear mβls. Since 1 does not mediate hydrolytic degradation of β-lactams, the binding of a series of common β-lactam antibiotics (benzylpenicillin, cephalotin, 6-aminopenicillanic acid, ampicillin) as well as the inhibitor sulbactam and the simplest β-lactam, 2-azetidinone, to the Dizinc core of 1 could now be studied in detail by NMR and IR spectroscopy as well as mass spectrometry. X-ray crystallographic information was obtained for 1 and its complexes with 2-azetidinone (2) and sulbactam (3); the latter represe...

  • unsymmetrical Dizinc complexes as models for the active sites of phosphohydrolases
    Dalton Transactions, 2010
    Co-Authors: Martin Jarenmark, Franc Meyer, Matti Haukka, Simone Wockel, Etelka Farkas, Edit Csapo, Jyoti Singh, Ebbe Nordlander
    Abstract:

    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 H4IPCPMP(PF6)2·2H2O (H4L), which has been structurally characterized, and has been used to form two different Zn(II) complexes, [{Zn2(IPCPMP)(OAc)}2][PF6]2 (2) and [{Zn2(IPCPMP)(Piv)}2][PF6]2 (3) (OAc = acetate; Piv = pivalate). The crystal structures of 2 and 3 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.

  • Efficient catalytic phosphate ester cleavage by binuclear zinc(ll) pyrazolate complexes as functional models of metallophosphatases
    Inorganic Chemistry, 2009
    Co-Authors: Larysa V. Penkova, Anna Maciag, Elena V. Rybak-akimova, Turganbay S. Iskenderov, Franc Meyer, Vladimirovich Aleksandr Pavlenko, Henryk Kozlowski, Matti Haukka, Igor O. Fritsky
    Abstract:

    A series of Dizinc(II) complexes based on the pyrazolate ligands 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (H(3)L(1)), (1E,1'E)-1,1'-(4-methyl-1H-pyrazole-3,5-diyl)diethanone dihydrazone (HL(2)), (E,E)-(4-methyl-1H-pyrazole-3,5-diyl)bis(methylmethanone) dioxime (H(3)L(3)), (E,E)-(4-phenyl-1H-pyrazole-3,5-diyl)bis(phenylmethanone) dioxime (H(3)L(4)), and 1H-pyrazole-3,5-dicarboxylic acid (H(3)L(5)) have been synthesized and investigated as functional models of phosphoesterases, focusing on correlations between the hydrolytic activity and molecular parameters of the bimetallic core. Speciation of the various Dizinc complexes in solution has been determined potentiometrically, and the structures in the solid state have been established by X-ray crystallography. The hydrolysis of two phosphoesters, an RNA model 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) and the pesticide paraoxon-ethyl (POE), promoted by the dinuclear phosphoesterase model complexes has been investigated in DMSO/buffered water (1:1) at 50 degrees C as a function of complex concentration, substrate concentration, and pH. Drastic differences in the hydrolytic activities of [Zn(2)(HL(1))(2)](0), [Zn(2)(L(2))(2)](2+), [Zn(2)(H(2)L(3))(2)](2+), and [Zn(2)(HL(5))(2)](2-) are observed and can be attributed to molecular peculiarities. Pyrazolate-bridged dinuclear zinc(II) complexes seem to provide a sufficient number of coordination sites for both activating the substrate and generating the nucleophile, where the phosphate esters are preferentially bound in a bidentate bridging fashion (in the case of HPNP) and in a monodentate fashion (in the case of POE).

  • structural flexibility of carboxylate bridging exemplified by a series of μ acetato Dizinc complexes
    Zeitschrift für anorganische und allgemeine Chemie, 2007
    Co-Authors: Walter Maringgele, Sebastian Dechert, Franc Meyer
    Abstract:

    A series of closely related Dizinc acetato complexes [LZn2(OAc)](ClO4)2 has been prepared from an assortment of known (HL1 – HL3) and new (HL4 – HL6) binucleating compartmental pyrazolate-based ligands that provide two tripodal tetradentate {N4}-type binding pockets but differ by the type of N-donor within the chelate side arms. Six complexes 1a, 1b, 2 – 5 have been characterized by X-ray crystallography. In all cases the two zinc ions are five-coordinate, are nested in the adjacent ligand compartments as anticipated, and are spanned by a μ1,3-bridging acetate that is hosted within the bimetallic pocket. Significant flexibility of the μ1,3-acetato ligand is inferred from the structural data, resulting in dislocations of the metal ions out of the pyrazolate plane as well as severe twisting and tilting of the acetate bridge with respect to the bimetallic scaffold. These different orientations of the bridging acetate cannot be distinguished by their IR spectra, in contrast to the different acetate binding modes known from the dynamic behaviour termed “carboxylate shift”. However, the high flexibility of the μ1,3-acetate even without changes in its coordination mode may well be a prerequisite for reactivity patterns of acetate-bridged bimetallic arrangements.

Andre B Charette - One of the best experts on this subject based on the ideXlab platform.

Rhett C. Smith - One of the best experts on this subject based on the ideXlab platform.

  • Dizinc Phosphohydrolase Model Built on a m‐Terphenyl Scaffold and Its Use in Indicator Displacement Assays for Pyrophosphate Under Physiological Conditions (Eur. J. Org. Chem. 3/2009)
    European Journal of Organic Chemistry, 2009
    Co-Authors: Michael K. Coggins, Austa M. Parker, Anshuman Mangalum, Gabriela A. Galdamez, Rhett C. Smith
    Abstract:

    The cover picture shows a Dizinc phosphohydrolase model built on a m-terphenyl scaffold that binds pyrophosphate with concomitant displacement of complexometric indicators; this leads to dramatic color changes that can be detected visually and quantified spectroscopically. Details are discussed in the article by R. C. Smith et al. on p. 343 ff.

  • Dizinc phosphohydrolase model built on a m terphenyl scaffold and its use in indicator displacement assays for pyrophosphate under physiological conditions
    European Journal of Organic Chemistry, 2009
    Co-Authors: Michael K. Coggins, Austa M. Parker, Anshuman Mangalum, Gabriela A. Galdamez, Rhett C. Smith
    Abstract:

    A dinucleating ligand {1,3-bis[2-(di-2-picolylaminomethyl)phenyl]benzene, L2} built on a m-terphenyl scaffold was prepared. The dissociation constants for the Dizinc complex of L2 (Zn2L2) binding to phosphate, pyrophosphate and commercially available complexometric indicators were determined under physiological pH [10 mMN-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (HEPES) buffer at pH 7.4]. Colorimetric and fluorescence-based indicator displacement assays with selectivity for pyrophosphate over other anions were achieved with Zn2L2 as the receptor component. These assays show good response characteristics for quantification of pyrophosphate concentrations as low as 2.5 × 10–6M, suggesting their utility for measuring pyrophosphate levels in synovial fluid. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

  • Dizinc enzyme model complexometric indicator pairs in indicator displacement assays for inorganic phosphates under physiological conditions
    Inorganic Chemistry, 2007
    Co-Authors: Brad P Morgan, Susan He, Rhett C. Smith
    Abstract:

    A Dizinc phosphohydrolase enzyme model complex employing the dinucleating ligand 2,6-bis-[(bis-pyridin-2-ylmethyl-amino)methyl]-4-methylphenol (L1) was tested for binding to a series of 11 commercially available complexometric indicators in aqueous N-2-hydroxyethylpiperazine-N‘-2-ethanesulfonic acid (HEPES) buffer at pH 7.4, with the aim of determining the applicability of these indicators in indicator displacement assays (IDAs) under physiological conditions. Dissociation constants (Kd) were determined for 11 indicator−Zn2L1 complexes, spanning 2 orders of magnitude from 2.8 × 10-4 M (alizarin red S) to 2.7 × 10-6 M (bromo pyrogallol red). Phosphate and pyrophosphate were tested for their ability to displace bound indicator and produce a detectable colorimetric response. Three indicators (bromo pyrogallol red, mordant blue 9, and zincon) complex to Zn2L1 to form an indicator displacement assay selective for pyrophosphate over phosphate. Because selection of an indicator/analyte pair having appropriate re...

  • Dizinc enzyme model/complexometric indicator pairs in indicator displacement assays for inorganic phosphates under physiological conditions
    Inorganic Chemistry, 2007
    Co-Authors: Brad P Morgan, Susan He, Rhett C. Smith
    Abstract:

    A Dizinc phosphohydrolase enzyme model complex employing the dinucleating ligand 2,6-bis-[(bis-pyridin-2-ylmethyl-amino)methyl]-4-methylphenol (L1) was tested for binding to a series of 11 commercially available complexometric indicators in aqueous N-2-hydroxyethylpiperazine-N‘-2-ethanesulfonic acid (HEPES) buffer at pH 7.4, with the aim of determining the applicability of these indicators in indicator displacement assays (IDAs) under physiological conditions. Dissociation constants (Kd) were determined for 11 indicator−Zn2L1 complexes, spanning 2 orders of magnitude from 2.8 × 10-4 M (alizarin red S) to 2.7 × 10-6 M (bromo pyrogallol red). Phosphate and pyrophosphate were tested for their ability to displace bound indicator and produce a detectable colorimetric response. Three indicators (bromo pyrogallol red, mordant blue 9, and zincon) complex to Zn2L1 to form an indicator displacement assay selective for pyrophosphate over phosphate. Because selection of an indicator/analyte pair having appropriate re...

Dingguo Xu - One of the best experts on this subject based on the ideXlab platform.

  • antibiotic deactivation by a Dizinc β lactamase mechanistic insights from qm mm and dft studies
    Journal of the American Chemical Society, 2007
    Co-Authors: Dingguo Xu
    Abstract:

    Hybrid quantum mechanical/molecular mechanical (QM/MM) methods and density functional theory (DFT) were used to investigate the initial ring-opening step in the hydrolysis of moxalactam catalyzed by the Dizinc L1 β-lactamase from Stenotrophomonas maltophilia. Anchored at the enzyme active site via direct metal binding as suggested by a recent X-ray structure of an enzyme−product complex (Spencer, J.; et al. J. Am. Chem. Soc. 2005, 127, 14439), the substrate is well aligned with the nucleophilic hydroxide that bridges the two zinc ions. Both QM/MM and DFT results indicate that the addition of the hydroxide nucleophile to the carbonyl carbon in the substrate lactam ring leads to a metastable intermediate via a dominant nucleophilic addition barrier. The potential of mean force obtained by SCC-DFTB/MM simulations and corrected by DFT/MM calculations yields a reaction free energy barrier of 23.5 kcal/mol, in reasonable agreement with the experimental value of 18.5 kcal/mol derived from kcat of 0.15 s-1. It is...

  • antibiotic binding to Dizinc β lactamase l1 from stenotrophomonas maltophilia scc dftb charmm and dft studies
    Journal of Physical Chemistry A, 2007
    Co-Authors: Dingguo Xu
    Abstract:

    A Dizinc β-lactamase (L1 from Stenotrophomonas maltophilia) complexed with an antibiotic compound (moxalactam) has been studied using a hybrid quantum mechanical/molecular mechanical (QM/MM) approach. The QM region is described by the self-consistent charge-density functional tight binding (SCC-DFTB) model while the MM by CHARMM. The Michaelis complex, which is constructed from a recent X-ray structure of the L1 enzyme with the hydrolyzed moxalactam, is simulated by molecular dynamics. The simulation yields valuable insights into substrate-enzyme interaction, whose implications in the enzyme catalysis are discussed. Finally, the QM/MM results are compared with a high-level density functional theory study of a truncated active-site model and the agreement provides strong support for the SCC-DFTB treatment of the QM region.

  • Antibiotic Binding to Dizinc β-Lactamase L1 from Stenotrophomonas maltophilia: SCC-DFTB/CHARMM and DFT Studies†
    Journal of Physical Chemistry A, 2007
    Co-Authors: Dingguo Xu
    Abstract:

    A Dizinc β-lactamase (L1 from Stenotrophomonas maltophilia) complexed with an antibiotic compound (moxalactam) has been studied using a hybrid quantum mechanical/molecular mechanical (QM/MM) approach. The QM region is described by the self-consistent charge-density functional tight binding (SCC-DFTB) model while the MM by CHARMM. The Michaelis complex, which is constructed from a recent X-ray structure of the L1 enzyme with the hydrolyzed moxalactam, is simulated by molecular dynamics. The simulation yields valuable insights into substrate-enzyme interaction, whose implications in the enzyme catalysis are discussed. Finally, the QM/MM results are compared with a high-level density functional theory study of a truncated active-site model and the agreement provides strong support for the SCC-DFTB treatment of the QM region.