Browsing by Subject "nonheme"

Now showing 1 - 4 of 4
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    Characterization and Reactivity of Synthetic Nonheme Oxoiron(IV) Complexes
    (2015-05) Bigelow, Jennifer
    Nonheme iron enzymes are prevalent throughout nature and utilize oxygen as the oxidant. While some intermediates are proposed, such as iron(IV)-oxo and iron(III)-peroxo species, the nature of the reactivity of these species has not yet been fully explored. Nonheme synthetic iron model complexes allow for easy modification to probe the reactivity of such species, and allow for characterization for later comparison with enzymes. This dissertation explores the reactivity of iron(IV)-oxo species supported by a tetramethycyclam framework. Interesting, in contrast to what had been reported previously, the electron-donating properties of the axial ligand do not correspond to the hydrogen atom transfer (HAT) reactivity, while a consistent trend for oxygen atom transfer (OAT) is observed. Ligand tethering is found to have a large impact on the enthalpy of activation. It is proposed that the iron(IV)-oxo moiety rises out of the plane to react, which forces tethered ligands to weaken the axial bond. The activation of oxygen by synthetic iron complexes, in the presence of either a hydrogen atom donor or an acid and a proton source, has been proposed to mimic enzymatic activity. However, the reexamination of mechanisms previously reported to follow an enzyme-mimicking pathway are instead due to peroxyl radicals. This highlights the importance of testing such mechanisms, as autooxidation is a common problem with many compounds in the presence of dioxygen. Finally, species such as iron(IV)-oxo and iron(III)-peroxo complexes, as well as related complexes, are characterized by resonance Raman spectroscopy. Many of these complexes have a ligand with a carboxylate moiety, as seen in nonheme enzymes. Characterization of these complexes show similarities between iron(IV)-oxo and iron(III)-peroxo and chromium(IV)-peroxo species reported previously, having similar vibration values, while major differences exist in vibrations between previously reported iodosylarene-iron(III) complexes and new iodosylarene-iron(III) complexes.
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    Spectroscopic and Structural Analysis of Oxygen-Activating Nonheme Diiron Enzymes and Related Synthetic Models
    (2017-05) Jasniewski, Andrew
    The general mechanism of O2 activation by nonheme diiron enzymes begins when the diferrous iron cluster binds dioxygen. The diiron cluster is oxidized to a peroxo-diferric intermediate that in some cases reacts directly with substrates, and in others becomes further activated via the cleavage of the O–O bond, leading to the generation of a potent high-valent oxidant that is the active oxidant for the cycle. Peroxo-diferric intermediates are of high interest because they are crossroads between the use of peroxo-diferric or high-valent oxo intermediate as the active oxidant in diiron-cluster-mediated oxidase and oxygenase chemistry. Understanding this O2 activation process requires structural characterization of enzymatic peroxo-diferric species. Spectroscopic methods, like electronic absorbance, X-ray absorption (XAS), and resonance Raman (rR) spectroscopies are used to probe a rich landscape of oxygen-activated intermediates and obtain detailed structures of these species. Through systematic study, insight can be gained into the mechanisms of these biological systems and ultimately this insight can be used to understand how Nature has chosen to use peroxo-diferric intermediates for a variety of different functions. In Chapter 2, X-ray diffraction and XAS were used to characterize various form of the enzyme CmlA to understand how O2 is regulated in the presence and in the absence of its non-ribosomal peptide synthetase (NRPS) bound substrate. In Chapter 3, the intermediate species on the O2 activation pathway of the human enzyme deoxyhypusine hydroxylase (hDOHH), including the µ-1,2-peroxo species, were studied using XAS. The structural analysis of the active sites of the various hDOHH species provided insight into the reaction mechanism for the system. In Chapter 4, XAS and rR studies on the unusual peroxo-diferric species of the N-oxygenase CmlI were carried out. The spectroscopic analysis of the peroxo intermediate describes a new peroxo binding geometry for diiron enzymes, a µ-1,1-peroxo species. In Chapter 5, detailed XAS characterization of various synthetic peroxo-diferric and oxoiron(IV) model complexes is described. Overall, this thesis demonstrates the power of structural characterization by complementary spectroscopic methods to support and generate enzymatic mechanistic hypotheses.
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    Spectroscopic and Structural Characterization of Synthetic Models of Dioxygen-Activating Nonheme Diiron and Monoiron Systems
    (2022-11) Abelson, Chase
    Using monoiron and diiron active sites, Nature has found a way to activate O2 toperform powerful oxidations. Upon the binding of O2 into the active site, the iron centers are oxidized to high-valent intermediates, and these highly oxidized species are able to break strong C-H bonds, such as those found in methane (104.5 kcal/mol). There has been a great interest in understanding the mechanistic cycle of these reactive oxidants as well as how nature can craft active sites that can perform difficult transformations. Understanding how enzymes activate O2 requires the employment of a variety of techniques, including structural characterization through X-ray absorption spectroscopy (XAS), single crystal X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). Other spectroscopic techniques, like electronic absorbance, resonance Raman, and electron paramagnetic resonance (EPR) help further our understanding of the wide variety of these oxygen-activating enzyme active sites. Due to the complexity of handling these enzymes, biomimetic synthetic complexes have been synthesized and investigated, with well over 100 characterized high-valent iron complexes. These small molecules allow for a greater understanding of why Nature employs iron centers to perform biologically vital transformations. In Chapter 2, ultraviolet-visible spectrophotometry (UV-Vis) and resonance Raman spectroscopy have been employed to better understand the role of a proton in helping to regulate the O—O bond cleavage step to unleash a powerful high-valent oxoiron oxidant(V) in a synthetic complex. Chapter 3 is an investigation of synthetic diiron systems whereby the structures of complexes have been structurally characterized using XAS and other techniques. This work is an effort in helping to better understand the mechanism by which diiron enzymes can form high-valent iron centers through the activation of O2. In Chapter 4, a combination of reactivity and spectroscopy has been employed to better understand how electronic parameters and the steric environments can perturb the oxidizing potential of FeIV(O) species. Overall, this thesis demonstrates the power of combining a variety of spectroscopic techniques to help generate and support hypotheses for enzyme mechanisms.
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    Structural- and Spectroscopic-Reactivity Relationships of Nonheme Oxoiron(IV) Complexes
    (2019-05) Rasheed, Waqas
    Non-heme oxoiron(IV) motifs have been identified as key intermediates that activate strong C—H bonds. Unlike the enzymatic intermediates however, most oxoiron(IV) complexes in synthetic chemistry have a triplet ground spin state and thus differ in their functional and electronic properties from the S = 2 units characterized in the enzymes. One striking exception is the complex [FeIV(O)(TQA)(L)]2+, where TQA = tris(2-quinolylmethyl)amine, which has Mössbauer parameters that closely resemble those of TauD-J, an enzymatic intermediate that has been relatively well-characterized. This oxoiron(IV) complex contains quinoline donors, and its thermal instability precludes its structural characterization (half-life = 15 minutes at 233 K). In this dissertation, several oxoiron(IV) complexes supported by pentadentate and tetradentate ligands are characterized, and examined for their reactivity and spectroscopic features. Crystallographic characterization of a few of these molecules is also reported. The structurally characterized oxoiron(IV) complexes along with some previously reported oxoiron(IV) complexes are used to set up structure-reactivity and spectroscopic-reactivity relationships, and show linear correlations with increasing isomer shifts, λmax values as well as metal-ligand distances. In addition, this thesis also uses 1H-NMR spectroscopy as an effective tool to identify solution-state structure as well the spin state of oxoiron(IV) complexes. We also characterize the first example of a spin crossover oxoiron(IV) complex, examples of which are only seen in iron(II) and iron(III) complexes.