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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Investigating the chemistry of binuclear chromium and uranium Pacman complexes

Stevens, Charlotte Jane January 2015 (has links)
Drawing inspiration from nature where enzymes containing multi-metallic active sites are ubiquitous, chemists have designed various ligands to bind more than one metal in precise structural arrangements. In Chapter One, a class of binucleating Schiff base pyrrole (Pacman) macrocycles which are both straightforward to synthesise and can be varied systematically to alter the metal environment and intermetallic separation are introduced, along with the state-of-the-art in this area. Previously reported complexes of these ligands with late transition metals, lanthanides and actinides are also reviewed. The results and discussion chapters of the thesis focus on the isolation and investigation of previously unexplored early transition-metal Pacman complexes and present new advances in low oxidation state uranium Pacman chemistry. In Chapter Two, binuclear chromium(II) complexes of two Schiff base macrocycles, H4LMe and H4LA are described. [Cr2(LMe)] features an ortho-phenyl spacer between the macrocycle donor compartments whereas the Cr(II) ions are separated by a larger anthracenyl spacer in [Cr2(LA)]. Both compounds have been characterised in solution and the solid state. Reactivity studies were carried out for [Cr2(LMe)]. Reactions of [Cr2(LMe)] with isocyanides and triphenylphosphine oxide were investigated leading to the isolation of the contrasting co-ordination compounds [Cr2(OPPh3)2(LMe)] and [Cr2(μ-CNR)(LMe)] (R = xylyl, tBu). Oxidation of [Cr2(LMe)] with I2 yields the Cr(III)/Cr(III) Pacman products [Cr2(μ- I)(I)(THF)(LMe)] and [Cr2(μ-I)(py)2(LMe)][I] when carried out in THF or pyridine, respectively. Cr(III) alkyl compounds are obtained by reaction of [Cr2(μ-I)(I)(THF)(LMe)] with the non-reducing alkyl transfer reagents MgBrEt and ZnEt2. When ZnEt2 in toluene is employed, one zinc cation is incorporated in the molecular cleft, whereas use of MgBrEt in THF yields the simple chromium alkyl complex [{Cr(Et)}2(endo-THF)(LMe)]. One ethyl group may be abstracted from [{Cr(Et)}2(endo-THF)(LMe)] by [CPh3][B(C6F5)4] to form a cationic alkyl complex. The activity of both the neutral and cationic alkyl species towards ethylene was investigated. Conclusions are discussed at the end of the chapter. Previously, investigation of low oxidation state uranium Pacman chemistry has been confined to the smaller macrocycle, H4LMe, and frequently resulted in the formation of insoluble polymeric materials that were intractable and challenging to analyse. In Chapter Three, metallation of the larger macrocycle, H4LA, with UI3 to generate a single soluble species is described, although this product could not be isolated or characterised in the solid state. A new synthesis of [U(BH4)3(THF)2] from UI3 and NaBH4 affords an alternative U(III) precursor to UI3. Metallation of H4LA using a sodium base and U(BH4)3(THF)2 yields the ionic product [Na(THF)4][{U(BH4)}2(μ-BH4)(THF)2(LA)] which was characterised in solution and the solid state. Reaction of this compound with KO(C6H2(tBu)3) forms the ligand substitution product [{U(OAr)}2(endo-BH4K)(THF)2(LA)] which undergoes selective reaction with excess S8 or CS2 to form [{U(OAr)}2(μ-S2)(LA)] and [{U(OAr)}2(μ-S)(LA)] respectively. It was discovered that the [U(BH4)3(THF)2] metallation strategy could be successfully extended to H4LMe to form [Li(THF)4][{U(BH4)}2(μ-BH4)(LMe)]. Protonolysis of the borohydride ligands of the complexes of the two different macrocycles was investigated using the weak acid [HNEt3][BPh4]. NMR spectroscopy indicated that both exo BH4 − groups in both complexes can be successively removed to generate neutral and cationic complexes but these were not isolated. Metallation of H4LA with UCl4 forms the ionic product [Li(THF)4][{U(Cl)}2(μ-Cl)3(LA)]. Various ligand substitution reactions were attempted but the only structurally characterised product was [{U(OtBu)(Cl)}{U(OtBu)(py)}(μ-Cl)(LA)], formed by reaction with KOtBu. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Four.
2

The synthesis and photophysical properties of novel phthalocyanines for potential application in photodynamic therapy

Burton, Anthony January 1999 (has links)
Phthalocyanines are a group of molecules, which have been studied extensively due partly to their use and potential in a wide variety of applications, but also on account of the seemingly endless list of differently substituted moieties, which in principle may be prepared. The history of the compounds in general are discussed briefly in Chapter 1 of this thesis, while Chapter 2 focuses on Photodynamic Therapy, an area in which they have more recently been found to exhibit excellent potential. The synthesiso of novel phthalocyanines for potential application in PDT is reported in Chapter 3. The current research investigated the synthesis of binuclear Pcs from the linking of two preformed phthalocyanine rings by means of an oxalyl bridge functionality and also from the mixed cyclisation of phthalonitriles with bisphthalonitriles, which proved to be extremely problematic. The synthesis of water-soluble phthalocyanine derivatives was investigated as a sideline, which resulted in the synthesis of a novel phthalocyanine monomer substituted with eight triethylene glycol chains. Efforts to synthesise a phthalocyanine linked to a glucose unit were unsuccessful. Chapter 4 describes research into the photophysical properties of a number of mononuclear and binuclear phthalocyanines synthesised both in the current work and also compounds synthesised by co-workers. The electronic absorption and fluorescence spectra of the compounds are investigated, and the absorption spectrum of a purely cofacial dimeric structure is obtained by calculation from the spectrum of one of the binuclear derivatives. Laser Flash Photolysis was performed on a number of the samples, determining triplet lifetimes, triplet quantum yields and singlet oxygen quantum yields. The effect of aggregation on the photoproperties of phthalocyaninesis investigated in Chapter 5. The current work involved low temperature absorption and fluorescence spectroscopy on a number of mononuclear and binuclear phthalocyanine derivatives. The nature of the aggregated structures, which were observed to form at 77K, are then rationalised with reference to previous research into phthalocyanine aggregates and crystal structures.
3

Electronic and magnetic coupling in triangular metal clusters

Sowrey, Frank Edward January 1997 (has links)
No description available.
4

Towards the structural modelling of the active site of ascorbate oxidase

Mackrell, Alexander David January 2001 (has links)
No description available.
5

Toward the Synthesis of Naphthalene-Bridged Bis-Triazole Bimetallic Complexes

Johnson, Sean M. 28 June 2017 (has links)
Bimetallic complexes are known to have unique electronic properties and are used in a variety of organic transformations as catalysts. The use of naphthalene-bridged bis- triazoles (NBT) for bimetallic complexes is unknown. NBTs have the unique property of being fluorescent stemming from a twisted intramolecular charge transfer. With the non- coplanar geometry and the distance between the 1,2,3-triazole rings, we hypothesized that 1,8-bis(4-phenyl-2H-1,2,3-triazol-2-yl)naphthalene (12) would be a suitable ligand to synthesize a bimetallic complex. The synthesis of 12 was optimized for large scale synthesis and was synthesized on a 78 mmol scale in 15% total yield. Metal complexation trials were conducted on 12 and several insoluble solids were observed.
6

Investigation of the mechanism of phosphotriesterase: characterization of the binuclear metal active site by electron paramagnetic resonance spectroscopy

Samples, Cynthia Renee 15 May 2009 (has links)
Phosphotriesterase (PTE) from Pseudomonas diminuta is a zinc metalloenzyme found in soil bacteria capable of organophosphate hydrolysis at rates approaching the diffusion controlled limit. Interest in PTE for degradation of chemical warfare agents and disposal of pesticides supports the need to understand the mechanism by which it performs hydrolysis. For further mechanistic clarity, this work will provide direct confirmation of the solvent bridge identity and the protonated species resulting in loss of catalytic identity. Inhibitor and product binding to the metal center will also be addressed; as well as the evaluation of the catalytic activity of Fe(II)-substituted PTE. This work has determined that the Mn/Mn-PTE electron paramagnetic resonance (EPR) spectrum exhibits exchange coupling that is facilitated through a hydroxide bridge. Protonation of the bridging hydroxide results in the loss of the exchange coupling between the two divalent cations and the loss of catalytic activity. The reversible protonation of the bridging hydroxide has an apparent pKa of 7.3 based upon changes in the EPR spectrum of Mn/Mn-PTE with alterations in pH. The pH-rate profile for the hydrolysis of paraoxon by Mn/Mn-PTE shows the requirement of a single function group that must be unprotonated with a pKa of 7.1. The comparable pKa values are proposed to result from the protonation of the same ionizable species. The effects of inhibitor and product binding on the magnetic properties of the metal center and the hydroxyl bridge are investigated by accessing new EPR spectral features. This work concludes that the binding of inhibitor occurs at the metal center and results in an increase of non-bridged hydroxyl species. These results, in conjunction with kinetic and crystallographic data, suggest that substrate binding via the phosphoryl oxygen at the ?-metal weakens the hydroxyl bridge coordination to the ?-metal. This loss of coordination would increase the nucleophilic character of the bridge, and binding of the substrate to the metal center would result in a stronger nucleophile for hydrolysis. Lastly, Fe(II) binding and activation of apoenzyme is evaluated under anaerobic conditions. This work concludes Fe/Fe-PTE is not catalytically active, but can bind up to 2 equivalent Fe(II) ions per active site.
7

Binuclear and tetranuclear copper(II) complexes containing ligands derived from phenol or hydroquinone

Reed-Mundell, Joseph Jerome January 1990 (has links)
No description available.
8

Kinetics and Mechanism of the Catalysis of the Decomposition of Hydrogen Peroxide by Schiff Base Complexes of Copper(II).

Beng, Timothy Kum 18 December 2004 (has links)
Spectroscopic studies have been used to describe the mechanism of the decomposition of hydrogen peroxide by solutions of a dimeric Cu(II) complex of a dissymetric Schiff base, [CuSALAD]2.H2O, and imidazole or methyl substituted imidazoles, B, which form monomeric CuSALAD.B2 complexes, in aqueous ethanol solvent. Freezing point depression and vapor pressure lowering studies were carried out to confirm the dimeric nature of the [CuSALAD]2.H2O complex that had been previously reported. The stoichiometry of the [CuSALAD]2.H2O-imidazole equilibrium was extensively studied pointing to a 1:4 stoichiometry. The CuSALAD.B2 adducts exhibited certain catalytic properties that mimic those of catalase enzymes. The different imidazoles were buffered to acidic, neutral and basic pH media in order to investigate the pH effects of this reaction. Two charge transfer (CT) bands were observed near 420 and 450 nm upon addition of hydrogen peroxide to CuSALADB2 solutions, and were associated with two proposed intermediates (CuBOOH and CuBOOCu). A mechanism consistent with these results has been developed. First order dependence of the rate on CuSALAD.B2 was observed in the presence of excess CuSALAD.B2 over hydrogen peroxide, whereas second order dependence was observed with the latter in excess. The CuBOOCu intermediate was unstable in the presence of EDTA, and a first order dependence of rate of formation of intermediate on both CuSALAD.B2, and hydrogen peroxide was observed.
9

Structure and Function of Binuclear Metallohydrolases: Enterobacter aerogenes glycerophosphodiesterase and related enzymes

Kieran Hadler Unknown Date (has links)
This thesis is focussed on structural and functional studies of a novel glycerophosphodiesterase (GpdQ) from Enterobacter aerogenes. GpdQ is highly promiscuous and is the first known phosphatase which is capable of degrading all three classes of phosphate esters (mono-, di- and triesters). Remarkably, GpdQ is also able to hydrolyse stable aliphatic phosphate esters and has been shown to degrade the hydrolysis product of the nerve agent VX. For these reasons, GpdQ has been realised to have potential as a powerful bioremediator for the removal of organophosphate pesticides and nerve agents. GpdQ is a binuclear metallohydrolase in which one of the metal ions is very weakly bound. Chapter 1 introduces the catalytic mechanisms of binuclear metallohydrolases by examining two related phosphate ester-degrading enzymes. Since one of the main features of catalysis addressed in this thesis are the differential metal binding affinities of GpdQ, Chapter 1 also canvasses a range of other binuclear metallohydrolases with similar behaviour. Chapter 2 examines the structural and evolutionary relationship between GpdQ and a number of other related enzymes. Using genome database searches, the two most closely related enzymes are identified. In performing these searches, a novel, putative binuclear metallohydrolase from Homo sapiens is also discovered. This enzyme, Hsa_aTRACP, is most closely related to PAPs, however construction of a homology model indicates that the active site tyrosine residue of PAP is replaced by histidine. In this respect, it may represent an evolutionary link to Ser/Thr protein phosphatases and GpdQ. The biology and chemistry of this putative enzyme is discussed. PAPs are the only binuclear enzymes with an established heterovalent active site of the type Fe(III)-M(II) (where M=Fe, Zn or Mn) whereas the majority of enzymes in this family have homovalent metal centres, including GpdQ and Ser/Thr protein. This is brought about due to the nature of the coordination sphere imposed by the enzyme. The activity of GpdQ can be reconstituted in the presence of Co(II), Zn(II), Mn(II) and Cd(II). Chapter 3 examines the kinetic properties of a binuclear homovalent system by studying the kinetic properties of Cd(II)-substituted GpdQ and a corresponding model complex. This comparative study leads to the identification of a terminal hydroxide molecule as the likely reaction-initiating nucleophile in Cd(II)-GpdQ with a pKa of 9.4. In Chapter 4, a detailed study of the structural, kinetic and spectroscopic behaviour of Co(II)-substituted GpdQ is presented. This chapter specifically probes the formation of the binuclear active site, the role of the metal ions in catalysis, the identity of the nucleophile and the potential role of any first or second coordination sphere residues in the regulation of enzyme activity, proton donation and metal ion coordination. Based on these findings, a detailed reaction mechanism is proposed in which the substrate itself promotes the formation of the catalytically competent binuclear centre and phosphorolysis occurs following nucleophilic attack by a terminal hydroxide molecule. A potential role of Asn80 (a ligand of one of the metal ions) in regulating both substrate and metal binding, and the role of the bridging hydroxide molecule in the activation of the terminal nucleophile is proposed. Chapter 5 employs a combination of kinetic and spectroscopic techniques to probe the proposed catalytic mechanism of GpdQ in depth. The formation of the catalytically competent binuclear centre is observed in pre-steady state studies, an integral first step in the catalytic mechanism. The dissociation and rate constants associated with formation of the binuclear centre are quantified. The rate of substrate turnover in GpdQ is relatively modest but is enhanced by a structural rearrangement involving the flexible Asn80 ligand. This structural change fine-tunes the reaction mechanism, leading to optimal reactivity. The steady-state kinetic properties of a series of metal ion derivatives (Co(II), Cd(II) and Mn(II)) of GpdQ and their reactivity towards a number of substrates are also compared. These findings lead to the conclusion that the reaction mechanism of GpdQ is modulated by both substrate and metal ion. In this respect, GpdQ is adaptive to the environmental conditions to which it is exposed by employing a flexible mechanistic strategy to achieve catalysis. Chapter 6 correlates the electronic and geometric structure of the binuclear centre in GpdQ as a means to probe specific aspects of the mechanism. This study uses the wild type enzyme and a site-directed mutant (Asn80Asp) to examine the structure of the metal ions at two stages of catalysis. The role of the bridging hydroxide molecule in nucleophilic activation is specifically addressed by monitoring changes in the electronic exchange interaction and other structural parameters as a result of phosphate binding. Also, the coordination environment of the metal ions in both the free enzyme and the phosphate-bound enzyme of wild type and Asn80Asp GpdQ were assessed against the currently proposed structures. The findings in this chapter corroborate the proposed catalytic mechanism of GpdQ. In summary, this project led to a detailed understanding of the mechanism of GpdQ, and provided insight into how both the metal ion composition and the identity of the substrate may modulate this mechanism. The knowledge gained may lead to the design of catalytically more efficient derivatives (mutants) of GpdQ for application in bioremediation.
10

Quantum Chemical Modeling of Binuclear Zinc Enzymes

Chen, Shilu January 2008 (has links)
In the present thesis, the reaction mechanisms of several di-zinc hydrolases have been explored using quantum chemical modeling of the enzyme active sites. The studied enzymes are phosphotriesterase (PTE), aminopeptidase from Aeromonas proteolytica (AAP), glyoxalase II (GlxII), and alkaline phosphatase (AP). All of them contain a binuclear divalent zinc core in the active site. The density functional theory (DFT) method B3LYP functional was employed in the investigations. The potential energy surfaces (PESs) for various reaction pathways have been mapped and the involved transition states and intermediates have been characterized. The hydrolyses of different types of substrates were examined, including phosphate esters (PTE and AP) and the substrates containing carbonyl group (AAP and GlxII). The roles of zinc ions and individual active-site residues were analyzed and general features of di-zinc enzymes have been characterized. The bridging hydroxide stabilized by two zinc ions has been confirmed to be capable of the nucleophile in the hydrolysis reactions. PTE, AAP, and GlxII all employ the bridging hydroxide as the direct nucleophile. Furthermore, it is shown that either one of or both zinc ions provide the main catalytic power by stabilizing the negative charge developing during the reaction and thereby lowering the barriers. In the cases of GlxII and AP, one of zinc ions also contributes to the catalysis by stabilizing the leaving group. These features perfectly satisfy the two requisites for the hydrolysis, i.e. sufficient nucleophilicity and stabilization of charge. A competing mechanism, in which the bridging hydroxide acts as a base, was shown to have significantly higher barrier in the case of PTE. For phosphate hydrolysis reactions, it is important to characterize the nature of the transition states involved in the reactions. Associative mechanisms were observed for both PTE and AP. The former uses a step-wise associative pathway via a penta-coordinated intermediate, while the latter proceeds through a concerted associative path via penta-coordinated transition states. Finally, with PTE as a test case, systematic evaluation of the computational performance of the quantum chemical modeling approach has been performed. This assessment, coupled with other results of this thesis, provide an effective demonstration of the usefulness and powerfulness of quantum chemical active-site modeling in the exploration of enzyme reaction mechanisms and in the characterization of the transition states involved. / QC 20100715 / Quantum Chemical Modeling of Binuclear Zinc Enzymes

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