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This thesis describes the development of new procedures for the synthesis of homo- and hetero-bimetallic complexes [M-(C≡C-C≡C)-M*] linked by a butadiyndiyl (-C≡C-C≡C-) bridge (where M, M* = Mo(dppe)(η-C7H7) and Fe(dppe)Cp) and also of the diethynyl-anthracenyl bridged complex [{Mo(dppe)(-C7H7)}2(μ-C≡CC14H8C≡C)] in which an anthracenyl group is inserted into the butadiyndiyl bridge. The redox chemistry and mixed valence character of these systems are investigated by a range of synthetic, electrochemical and spectroscopic techniques. Chapter 1: (Introduction) presents a literature review related to all-carbon bridged bimetallics including their synthesis, redox chemistry and mixed valence properties. Chapter 2: describes the synthesis of the key precursor [FeI(dppe)Cp] in multi-gram quantities. A new synthetic protocol has been developed to vinylidene [Fe(C=CH2)(dppe)Cp][PF6] and acetylide [Fe(C≡CH)(dppe)Cp] complexes starting from the iodo precursor [FeI(dppe)Cp] which can be conveniently used in place of the chloro precursor [FeCl(dppe)Cp]. We also identified the carbene species [Fe{C(OMe)Me}(dppe)Cp][PF6], which is formed as a by-product from the reaction of the vinylidene with a molecule of methanol solvent. Chapter 3: describes the synthesis and characterisation of butadiyndiyl bridged [{Fe(dppe)Cp}2(-CCCC)]n+ (n = 0, 1, 2). The electronic structure of [{Fe(dppe)Cp}2(-CCCC)]n+ has been investigated in all thermally accessible oxidation states (0, 1, 2, 3) and compared directly with the closely related ruthenium analogue [{Ru(dppe)Cp}2(-CCCC)]n+. Particular focus was given to the formally 'mixed valent' radical cations [{Fe(dppe)Cp}2(-CCCC)]+ and [{Ru(dppe)Cp}2(-CCCC)]+, where the spectroscopic data highlight significant differences between the iron and ruthenium complexes. Furthermore, in this chapter the heterobimetallic complex [{Fe(dppe)Cp}(µ-C≡C-C≡C){Mo(dppe)(C7H7)}] was successfully synthesised and characterised by microanalysis, IR, mass spectrometry and cyclic voltammetry. Chapter 4: discusses the synthesis of the diethynyl-anthracene bridged complexes [{Mo(dppe)(-C7H7)}2(μ-C≡CC14H8C≡C)]n+ (n = 0, 1, 2). The focus of the investigation is to determine the effect of a diethynyl-benzene vs. diethynyl-anthracene bridge in bimetallics supported by the Mo(dppe)(C7H7) end cap. In the mixed valence (n = +1) state, the odd electron should be more localised on the ligand bridge of the anthracene derivative and this principle was investigated by EPR spectroscopy.

Early-onset Parkinson disease, of which the best studied and most common cause are biallelic mutations in the PRKN gene, is characterized by an age of onset before 40 years. Parkin-deficient patients show slow progression, excellent responsiveness to L-dopa therapy, and are generally spared cognitive decline. At autopsy, PRKN-linked Parkinson disease is further distinguished by relative selectivity in cell loss, namely of dopamine producing neurons in the human brainstem, and the general absence of Lewy body inclusions. Since its discovery two decades ago, the field has focused on the function of parkin as an E3 ubiquitin ligase and its related role in mitophagy. However, its essential, neuroprotective function in ageing human midbrain and the mechanisms by which wildtype parkin preserves dopamine cell health in aged humans are yet to be elucidated. I hypothesized that parkin confers neuroprotection due to a redox function by its many cysteines (7.5%). We first focused on parkin’s biochemistry, reporting a shift from solubility to a nearly insoluble, aggregated state in adult control brain after age 40 years. We detected cysteine-based, post-translational modifications of parkin in response to oxidative stress, and characterized a novel, redox chemistry-based function: Through its own oxidation parkin reduced hydrogen peroxide in vitro. I validated this finding by showing its elevation in parkin-deficient, human brain. Wild-type parkin also participated in dopamine metabolism through the conjugation of reactive radicals at several of its cysteines, which augmented the generation of melanin. (Chapter 2). In addition, we demonstrated parkin’s heretofore unknown, antioxidant function in the cytosol using cellular paradigms and select genetic as well as toxin-based mouse models that featured elevated oxidative stress. Moreover, we uncovered parkin’s contribution to the wider thiol network, namely through an apparent feedback loop with glutathione metabolism (Chapter 3). Lastly, I developed and investigated a bi-genic (prkn-/-//Sod2+/-) model in an attempt to restage the pathogenesis of early-onset parkinsonism in mice; there, we detected a rise in systemic, oxidative stress and in total nitrotyrosination profiles of the brain, but did not observe any dopamine neuron loss in the midbrain. In accordance, the behavioural characterization of these animals did not reveal any motor abnormalities (Chapter 4). Based on this previously unknown function for parkin in redox biology, I envision three future research directions: a) additional studies to delineate the full range of oxidative modifications of parkin versus neutralization of radicals; this, to better define the distinct pathogenesis of parkin-linked Parkinson’s when compared to other forms of the disorder; b) structural studies of its oxidative modifications in vitro and renewed attempts of staging parkin-linked dopamine cell death in vivo; and c) and importantly, the exploration of cause-directed therapy based on parkin’s redox functions to preserve dopamine neurons of the human midbrain throughout adulthood.

<p>The research presented here aims to expand our understanding of the structural factors that contribute to selectivity for iron and to iron complex stability in siderophores, as well as iron transport processes in siderophore systems. This work will also investigate the factors that contribute to therapeutic applications of chelating agents, both for chelation therapy and for antimicrobial agents.</p> <p>The thermodynamics of iron(III) binding of a number of molecules, both natural and synthetic, are determined using pH-dependent spectrophotometric titrations and potentiometric titrations . Three of the synthetic siderophore analogs studied here are a tris-hydroxypyridinone and two bis-hydroxypyridinone ligands. A determination of the solution thermodynamics of the iron(III) complex of a water-soluble analog of Brasilibactin A, a membrane-bound mycobactin-type bacterial siderophore is also presented and related to the role of mycobactins in iron uptake of mycobacteria. The thermodynamics of chelation of iron(III) by a synthetic Trojan Horse antimicrobial agent featuring a 3-hydroxy-4-pyridinone moiety were also determined. In these studies, the thermodynamic stability constants of the iron-chelator complexes are determined through a series of spectrophotometric and potentiometric titrations. Also, the redox chemistry of the iron-chelator complexes are investigated using cyclic voltammetry. The structural features that contribute to complex stability in a series of tripodal tris-hydroxamate siderophores using computational techniques is presented, and it is shown that the position of the arm of an exocyclic siderophore system can contribute to differences in complex stability, as can the orientation of the donor group.</p> <p>Kinetics studies of the iron(III) exchange reactions of some polydentate chelators are presented. The study of the kinetics of some reactions of iron complexes featuring hydroxypyridinone donor-group chelators is performed by spectrophotometric kinetics experiments. A determination of the mechanism of proton-driven complex dissociation of a bishydroxypyridinone siderophore mimic is shown. Also, the mechanism of exchange between desferrioxamine B and an iron(III)-trishydroxypyridinone complex is determined through spectrophotometric monitoring of the reaction. The ability of a bidentate hydroxypyridinone chelator to catalyze the exchange of iron(III) from desferrioxamine B to EDTA is explored and the mechanism is determined.</p> <p>Finally, an investigation into the efficacy of chelation therapy treatments to protect from metal toxicity using the nematode C. elegans as a model organism is presented. The model developed therein can also be used as a model for soil remediation of toxic metals using chelating agents.</p> / Dissertation

Hydrogen sulfide (H2S) is an important biomolecule for its role in mediating redox homeostasis and signaling biological processes. The study of biological sulfide is currently impeded by a lack of tools available that adequately address the questions currently facing the field. The most pressing of these questions are: how does H2S signal biological processes. To produce tools for studying H2S, chemiluminescent scaffolds were designed to study both H2S producing enzymes and directly measure free H2S. Additionally, small molecule organic persulfides were synthesized and characterized in order to study the properties and reactivity of H2S signaling species. By creating methods to directly measure biological H2S and creating model systems to investigate the active signaling species, the biological reactivity of H2S can be better understood. The luminescent methods for detecting H2S were developed in order to avoid photodecomposition inherent with fluorescent methods while still providing a spectroscopic readout for performing measurements in cells. D-cysteine concentrations can be measured using luciferin bioluminescence, and utilized to back out the H2S producing activity of DAO. Free H2S was measured using luminol derived chemiluminescence. The luminol scaffolds were studied in depth to determine what makes an H2S probe selective for H2S in order to inform the design of future H2S probes. Sulfide signaling processes were investigated using organic persulfide model systems. We found that under reducing conditions persulfides liberate free H2S, and that under basic conditions they decompose. The decomposition pathway is governed by substitution at the -carbon, which dictates the steric accessibility of the inner sulfur atom to act as an electrophile. Persulifdes do not react with acids, and are easily tagged by electrophiles to form disulfides. Persulfides are sufficiently reducing to generate NO from nitrite, facilitating cross-talk between multiple signaling species. This cross talk is mediated by formation of perthionitrite, which may function as an independent signaling species.

Important reactions in the environment often occur at the interface between a mineral surface and aqueous phase. Reactions occurring at this interface often control the uptake or release of harmful components resulting in the geochemical cycling of elements in the environment. Additionally, minerals are commonly used in the remediation of contaminated areas, where similar chemistry occurs at their interfaces. Thus, studies of the chemistry of these interfaces are essential to our understanding of complex environments. Many of these processes are controlled by electron transfer reactions between adsorbates and the mineral interface, and it is here where this research presented will concentrate. The studies in this thesis key in on redox chemistry on various environmentally relevant iron minerals, including ferrihydrite, pyrite, and amorphous iron sulfide. A large portion of this body of work is dedicated to the understanding of the surface mediated reaction between chromate (Cr(VI)) and arsenite (As(III)). Both of these species are present in the environment and are detrimental to human health. Using in- and ex-situ experiments we have monitored the coupled redox transformation of Cr(VI) and As(III) to chromite (Cr(III)) and arsenate (As(V)). Quantum mechanical modeling was used to support the experimental studies of this novel redox chemistry. The reaction was monitored in situ, using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), on the surface of the iron oxyhydroxide, ferrihydrite, at various solution pH values by following vibrational modes unique to Cr(VI), As(III), and As(V). At pH &lt; 9 we observed an initial growth of Cr(VI) vibrational modes due to adsorption, followed by the simultaneous decrease in Cr(VI) vibrational modes and increase in As(V) vibrational modes. Ex situ analysis of the reaction products via X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) indicated that there was an increase in the percentage of reaction products as the pH decreased. Quantum mechanical calculations were completed to model the reaction of Cr(VI) and As(III) on the ferrihydrite surface by analyzing differences in geometric and electronic structural changes and thermodynamic preferences. The results indicate that Cr(VI) and As(III) adsorbed physically separated from each other is not only thermodynamically favorable but results in changes in As(III)-Fe and Cr(VI)-Fe atomic distances, towards those characteristic of As(V)-Fe and Cr(III)-Fe. Thus a mechanism where electron transport occurs through bulk states is plausible. Additionally, natural bond order analysis reveals a redistribution of electron density away from the Cr(VI) atomic center upon adsorption, indicating probable changes in Cr(VI) reduction potential. The electrochemical reduction of Cr(VI) on three surfaces, ferrihydrite, titanium dioxide, and aluminum oxides, indicate that Cr(VI) reduction potential is surface dependent, an observation that has significance for redox chemistry in the environment. The interaction of ferric, Fe(III), with iron sulfide surfaces (during and after coal mining activities) contributes to the detrimental environmental problem known as acid mine drainage (AMD). We investigated whether Fe(III) chelating siderophores could be used to suppress the oxidation of iron sulfide surfaces and the resulting AMD chemistry. The exposure of the iron sulfide, pyrite, to the siderophore, desferrioxamine B (DFOB) at initial pH values of 3, 6, and 8 under oxic conditions showed a significant decrease in the rate of dissolution of pyrite: decreases of 43.7%, 37.5% and 78.4%, respectively. An even greater decrease in pyrite oxidation was observed when DFOB was present in anoxic conditions, specifically 56.1%, 74.4% and 91.5%, at pH 3, 6 and 8, respectively. We further compared the rate of dissolution between DFOB and another siderophore, enterobactin, which is a stronger chelator of Fe(III). The presence of enterobactin suppressed pyrite oxidation more than DFOB, consistent with the contention that inhibiting the interaction of Fe(III) with pyrite will decrease the oxidation of the mineral. We also analyzed the exposure of the pyrite surface to DFOB using ATR-FTIR, to determine if any surface chelation occurs. We found that when Fe(III) is present on the pyrite surface, DFOB adsorbs to the surface via hydroxamate groups, similar to the aqueous phase spectra of DFOB-Fe(III) complex. In contrast the spectra do not exhibit hydroxamate vibrational modes when Fe(III) was not initially present on the pyrite surface and in this circumstance the spectra resembled that of aqueous phase unchelated DFOB. Taken together the results showed that siderophore inhibited pyrite oxidation by chelating Fe(III) present on the pyrite surface and in solution. Finally, the reduction of NO(g) to NH3/NH4+ with amorphous iron sulfide (FeS) was studied. The exposure of NO gas to a suspension of FeS solid resulted in the conversion of 2.3% NO(g) to the reaction product ammonia (NH3), which was found to grow over time, while the exposure of NO(g) to water (in the absence of mineral) resulted in no NH3 formation. Additionally, we completed in situ analysis of NO exposure to FeS as a function of water concentration using ATR-FTIR. The exposure of NO to an aqueous paste of FeS or a FeS film (with adsorbed H2O), resulted in the adsorption of NO to the FeS surface and the subsequent production of NH3, as indicated by N-H vibrational modes. In contrast, the removal of all water, via thermal desorption from the film, resulted in the adsorption of NO but did not show vibrational modes consistent with the formation of NH3. We conclude that the presence of H2O, as a source of protons, and a FeS surface, as a source of electrons, results in the transformation of NO to NH3 via a heterogeneous reaction. This result has important implications towards remediation of NOx gases and mechanisms of prebiotic synthesis of NH3. In summary, the research presented expands our understanding of redox reactions at mineral interfaces in the environment. The work herein aims to inform and aid in the development of remediation methods for arsenic and chromium, the formulation of methods to inhibit the production of acid mine drainage, and develop our understanding of toxic NOx gas reduction on surfaces. / Chemistry

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