Theses and Dissertations from UMD
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Item INVESTIGATION OF THE PRODUCTION AND DECAY PATHWAYS OF SUPEROXIDE BY CHROMOPHORIC DISSOLVED ORGANIC MATTER(2022) Le Roux, Danielle Marie; Blough, Neil; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chromophoric dissolved organic matter (CDOM) in natural waters absorbs sunlight which leads to the production of a suite of reactive intermediates and reactive oxygen species (ROS) such as superoxide (O2⦁-) and hydrogen peroxide (H2O2). A significant amount of research over the years has investigated the sources and sinks of these two ROS. The currently accepted sequence of reactions for their production involves photochemically produced one-electron reductants (OER) within CDOM reacting with dissolved oxygen to form O2⦁-, which undergoes self-dismutation to produce H2O2. A previously used method to detect radical species with CDOM has been modified herein to be conducted simply using a fluorometer. Production rates of OER and H2O2 were measured for a variety of samples and correlations between the rates and optical/structural properties of the samples indicate that lower molecular weight species produce more OER and H2O2. Based on the stoichiometry of the mechanism above, the ratio of the production rate of OER to that of H2O2 should be two. However, ratios from five to sixteen were obtained, which suggests that O2⦁- undergoes oxidative reactions that compete with dismutation. The possibility of a light-dependent pathway for O2⦁- decay has been proposed but had yet to be explicitly demonstrated. Herein this sink is directly shown through O2⦁- spiking experiments. Rapid consumption of the O2⦁- spike occurs if injected into a sample during irradiation, as compared to a spike introduced into the sample in the dark, suggesting the presence of a light-dependent sink. Extensive data analysis and kinetic modeling of the O2⦁- decay data has allowed for approximations as to the extent of the sink and its decay rate constant. O2⦁- and H2O2 are environmentally important species, and a significant amount of work has been done on modeling their concentrations in natural waters. Based on the work here, O2⦁- is produced at higher concentrations than previously believed, which has implications on the modeling of O2⦁- and H2O2 in natural waters. Additionally, the light-dependent oxidative sink of O2⦁- could be with moieties within CDOM, providing further insight to the photochemical transformation of DOM during transit from terrestrial sources to marine waters.Item TOWARDS A GENETICALLY-ENGINEERED BACTERIUM FOR GASTROINTESTINAL WOUND HEALING(2017) Virgile, Chelsea; Bentley, William E; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Society and physicians frequently associate the increase of antibiotic-resistant bacteria with the overuse of antibiotics. This proposes a question, “Why use antibiotics to fight bacteria and risk resistance, when one could engineer bacteria to target and kill infectious bacteria?” Bacteria are often thought of as ‘good’ bacteria (e.g., commensals, probiotics) or ‘bad’ bacteria (e.g., pathogens). Synthetic biology enables the augmentation of biosynthetic capabilities and retooling of regulatory structures in the creation of cells with unprecedented ability to make products. One can also, however, think of the cell as the product – a cell that operates in a noisy environment to execute non-native tasks. There have been several recent reports of the rewiring of bacterial cells to function as conveyors of therapeutics. The engineering and rewiring of the bacteria such as E. coli into ‘smart' bacteria potentially allows for a broad range of applications, from the treatment of wounds, the elimination of pathogenic strains, to the delivery of vaccines, particularly in the gastrointestinal (GI) tract. I have engineered smart bacteria as a therapeutic delivery vehicle for wound healing in the GI tract. The approach comprises synthetic biology and microfluidics for the creation of a biological ‘test track' for ensuring the appropriate design and testing of engineered bacteria. Bacterial motility was engineered for response to wound-generating signals such as hydrogen peroxide. Specifically, we have placed a motility enzyme CheZ under the control of the hydrogen-peroxide-responsive oxyR/S gene-promoter pair so that the ‘run’ in the tumble and run scheme of bacterial movement is externally regulated. These engineered cells exploit pseudotaxis for directional swimming towards hydrogen peroxide, a non-native signal. Additionally, the therapeutic enzyme transglutaminase plays an important role in the tissue clotting cascade. Microbial transglutaminase can crosslink fibrinogen, similar in function to human transglutaminases during the clotting cascade, but independently of calcium ions. This allows for a potentially faster, increased wound-healing response. By combining microbial transglutaminase expression with controlling motility and lysis expression using the OxyR/S system, the ‘smart’ bacteria can potentially swim towards and treat at the wound site with subsequent cell lysis. Ultimately, this strategy can lead to new bacterial therapies.Item Synthesis and Reactivity of Monohydrocarbyl Palladium(IV) Complexes Using Hydrogen Peroxide as Oxidant in Protic Solvents(2011) Oloo, Williamson Njoroge; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mild, and selective transition metal catalyzed processes for the functionalization of C-H bonds utilizing environmentally benign and inexpensive dioxygen and/ or HOOH oxidants are extremely attractive, as they render these transformations more atom economical and practical for large-scale syntheses. Our approach towards this end involves optimizing the oxidation and C-X reductive elimination steps of the proposed catalytic cycle using tridentate facially chelating ligands, which include 1-hydroxy-1,1-di(2-pyridyl)methoxide, a derivative of di(2-pyridyl)ketone (dpk) and 6-(2-pyridinoyl)pyridine-2-carboxylic acid (ppc). Oxidation of the dpk- and the ppc-ligated palladacycles with HOOH in water and acetic acid solvents produces the corresponding monohydrocarbyl Pd(IV) complexes quantitatively. The mechanism of oxidation of these complexes was investigated, and was proposed to involve addition of HOOH across the C=O bond of the ligand, followed by heterolytic cleavage of the O-O bond via nucleophilic attack of Pd(II) onto the hydroperoxo adduct. The dpk- and ppc-ligated monohydrocarbyl Pd(IV) complexes undergo C-O reductive elimination at room temperature in acetic acid and/ or water to produce the corresponding phenols and/ or aryl acetates quantitatively. Mechanistic studies led us to propose a C-O reductive elimination reaction that proceeds either from a 5-coordinate intermediate, produced upon dissociation of the pyridine group of the dpk chelate or from a 6-coordinate Pd(IV) species. Addition of HX (X=Cl, Br, and I) to aqueous solutions of the dpk-supported hydroxo-ligated monohydrocarbyl Pd(IV) complexes leads to quantitative formation of C-X bond-coupling products. Some of the corresponding X-ligated monohydrocarbyl Pd(IV) complexes were isolated from these solutions (X=Cl and Br), and could be independently prepared by oxidation of the hydrocarbyl Pd(II) precursors with the corresponding N-halogenosuccinimides (NXS). Palladium catalyzed C-H functionalization reactions were performed in the presence of tridentate, facially chelating bis(6-methyl-2-pyridyl)methanesulfonate ligand. Substituted 2-phenylpyridine substrates underwent predominantly C-C coupling reactions with minor C-O coupling products produced, while 2-benzyl- and 2-phenoxypyridine substrates that form 6-membered palladacycles produced the corresponding C-O coupling products selectively in high yields. These reactions were significantly slower in the absence of the ligand, and no reactions took place in the absence of palladium acetate.