|Author||Considine, Thomas A.|
|Thesis Title||On-Site Profiling and Speciation of Subsurface Pollutants at Hazardous Waste Sites.|
|Advisor||Albert Robbat, Jr|
|Abstract||Summary: This two-part thesis describes the analysis of environmental pollutants at depth without bringing sample to the surface. In the first part a new high temperature transfer line, membrane inlet probe (HTTL-MIP) coupled to a photoionization detector (PID) and gas chromatograph/mass spectrometer (GC/MS) was used to rapidly profile and speciate polycyclic aromatic hydrocarbons (PAH) in the subsurface. PID signals were in agreement with GC/MS results. Correlation coefficients of 0.92 and 0.99 were obtained for discrete and composite samples collected from the same exact location. Continuous probe advancement with PID detection found coal tar, a dense nonaqueous liquid, in soil channels and saturated media. When samples were collected conventionally, split, solvent extracted and analyzed in the field and confirmation lab, GC/MS measurement precision and accuracy were indistinguishable; despite the fact the field lab produced data five times faster than the lab using standard EPA methods. No false positive/negatives were found. Based on these findings, increased confidence in site conceptual models should be obtained, since PID response indicated total PAH presence/absence in "real-time," while GC/MS provided information as to which PAH was present and at what concentration. Incorporation of this tool into a dynamic workplan will provide more data at less cost enabling environmental scientists, engineers, and regulators to better understand coal tar migration and its impact on human health and the environment.|
The second part of this thesis is based on an improved 3-stage Peltier freeze trap, which efficiently pre-concentrates volatile coal tar and petroleum hydrocarbons, and an integrated system for detecting pollutants on-line, in real time by photoionization detection and quantitation by gas chromatography/mass spectrometry (GC/MS) as the probe is advanced into the subsurface. Findings indicate measurement precision and accuracy for volatiles meet EPA criteria for hazardous waste site investigations. When a Teflon membrane inlet is used to detect contaminants in groundwater, its 140
|Author||Baber, Ashleigh E.|
|Thesis Title||Nanoscale Studies of Molecules, Metals and Alloys.|
|Advisor||Charles H. Sykes|
|Abstract||Summary: The advent of scanning tunneling microscopy (STM) changed the way the world perceived surfaces at the nanoscale. STM is a technique capable of imaging surfaces at the atomic scale, allowing researchers to probe surface processes with previously unavailable resolution. While STM is a powerful tool used in many nanoscale applications, this thesis examines three overarching themes: the investigation of adsorbate interactions, the elucidation of surface structures, and the study of catalytically active sites in alloys. STM maps the topographic and electronic structures of surfaces and adsorbates.|
STM can therefore be used to image the atomic structure of a surface as well as the shape and positions of individual molecular adsorbates. We have used this capability to study the effect that adsorbates have on a surface and each other, and to measure the processes that molecules undergo on a surface. In particular, the following chapters outline how STM studies reveal an inert substrate restructuring in the presence of a weakly adsorbed molecule (Chapter 3), dipole-driven ferroelectric ordering on a metal surface (Chapter 4), extended hydrogen-bonded networks and their structural dependence on coverage (Chapter 5), and the rotational dynamics of individual surface-bound molecules (Chapter 6). Capturing interactions and dynamics on a molecular level allows us to reveal the driving forces behind these processes.
The high resolution of STM at low temperatures allows us to image the structure of a static lattice of surface atoms. Using STM, we were able to elucidate the atomic structure of chiral sites at the step edges of an intrinsically chiral surface (Chapter 7), which has previously been shown to enantiospecifically interact with adsorbates. Identifying the surface structure allows us to better predict how adsorbates or reactants will interact with such metals. Additionally, we characterized the temperature dependence of the geometric structure of a catalytically relevant Pd/Au bimetallic alloy (Chapter 8). Because STM measures the geometric and electronic structure of surfaces, we were able to measure the charge transfer of individual Pd atoms when alloyed into an inert Au surface. These results demonstrated the first simultaneous geometric and electronic measurements of individual atoms in a Pd/Au alloy.
Finally, using STM we identified the subtle structural differences between two bimetallic alloys (Pd/Cu and Pd/Au) and studied their interaction with hydrogen (Chapter 9). Hydrogen was used to probe the active Pd sites of the alloys. Although dissociation and spillover of hydrogen occurred over individual Pd atoms in Cu, H uptake was only observed on Au when Pd was present as large particles on the surface. The strong Pd-H interaction on Au resulted in Pd segregation on top of the Au surface. These results show how the surface affects the adsorbates and how adsorbates can influence the surface structure. Each of these examples will show how STM can be used to study systems unlike any other instrument and deepen our understanding of atomic-scale phenomena.
|Thesis Title||Iron(II) Aminopyridine Macrocycles in Catalytic Oxidations: Structures, Reactivity, and Mechanistic Studies of Reactive Intermediates.|
|Advisor||Elena V. Rybak-Akimova|
|Abstract||Summary: This document comprises the results of research in the area of olefin epoxidation with new aminopyridine iron(II) macrocycles: identifying structural features and mechanistic studies of reactive intermediates. Advances in our understanding of the mechanistic role of Fe(IV)-oxo intermediates of non-heme iron enzymes, and the accessibility of similar iron(IV) intermediates in synthetic model complexes have prompted investigators to view high-valent iron(IV)-oxo species as key intermediates responsible for oxygen atom transfer to organic substrates in catalytic oxidations. Our lab is interested in using pyridine-macrocycle ligands (PyMACs) to explore the reactivity of the iron complexes in catalytic oxidations. These ligands combine the advantages of macrocyclic polyamines, which prevent iron loss under catalytic conditions by forming thermodynamically and kinetically stable iron complexes, and those of aminopyridine ligands that are capable of stabilizing a variety of iron-based intermediates. The macrocyclic nature of PyMACs may also stabilize the complexes with respect to irreversible oxidative ligand destruction during catalytic processes.|
In Chapter 2, the study is focused on the role of iron(IV) intermediates in oxidations catalyzed by an iron(II) complex with a pyridine-containing 14-membered macrocyclic ligand L (L = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene). This complex generates an iron(IV)-oxo intermediate using isopropyl 2-iodoxybenzoate as an oxidant. Spectroscopic characterization of iron(IV)-oxo intermediate and its reactivity with substrates (olefins or triarylphosphines) under stoichiometric or catalytic conditions are reported.
In Chapter 3, iron(II) complexes with ligand L (L = 3,7,11-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene) is synthesized and characterized. tert-Butyl hydroperoxide (TBHP) and hydrogen peroxide (H2O2), are used as terminal oxidants in the stoichiometric oxidation of substrates (triaryl phosphines or olefins) and in catalytic epoxidation studies. TBHP and isopropyl 2-iodoxybenzoate were used to generate alkylperoxoiron(III) and oxoiron(IV) species, respectively. The formation and reactivity of these intermediate species was investigated by the stopped-flow methodology. The mechanism of oxygen transfer to organic substrates involving reaction of oxoiron(IV) intermediate was elucidated on the basis of spectroscopic and kinetic data.
In Chapter 4, a comparative study examining iron(II) complexes with a series of pyridine-containing macrocycles is reported, including a newly synthesized mononuclear non-heme iron(II) complex bearing a pyridine-containing 17-membered macrocyclic ligand, (L = 7-(3-propylacetamide)-2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), in order to explore the relevance between the ligand environment and the reactivity of the corresponding oxoiron(IV) species in oxygen atom transfer reactions. The oxidative reactivity studies of oxoiron(IV) intermediate with organic substrates are carried out by monitoring spectral changes of the intermediates under stoichiometric conditions.
|Author||Tierney, Heather L.|
|Thesis Title||Understanding and Controlling Rotation at the Single-Molecule Level.|
|Advisor||E. Charles H. Sykes|
|Abstract||Summary: As technology goes to smaller and smaller dimensions, efficient parts are needed on the same size scale in order to create nano- and micro-machines. Just as motors are needed on the macroscale to run engines, motors are needed in these tiny machines in order to convert a fuel source into productive work. While scientists have had trouble creating synthetic nano- and micro-scale motors, these sorts of "machines" are prevalent throughout biology. This thesis describes studies into the fundamental properties of rotating thioether molecules. Symmetric thioethers have been studied in detail as a function of chain length to clarify the similarities between the rotational barriers for thioethers with two or more carbons in each alkyl tail. Molecules were electrically-driven using pulses from the STM tip, and the mechanism for this rotation was elucidated. Using theoretical methods both the adsorption site and rotational pathway were revealed. Asymmetric thioethers were studied using both experimental and theoretical methods. Adsorption site dependent rotational properties were found, and the barrier to rotation was calculated to have an asymmetric torsional potential. It was discovered that due to the intrinsic chirality of even bare metal STM tips, that directed rotation could be achieved in the electrically-induced motion of surface-bound enantiomers. This directed motion shows for the first time that an electrically-driven rotary motor can be made from a single molecule.|
|Thesis Title||Design and engineering of new glucagon-like-peptide-1 analogues .|
|Abstract||Summary: Diabetes mellitus (DM) is a chronic disease that manifests itself with impaired control of blood glucose levels and affects hundreds of millions of individuals worldwide. Among various types of DM, Type II diabetes is the most prevalent. In addition to conventional therapies for diabetes, increasing knowledge of the pathology of the disease has led to new leads for drug targets. Recent studies have revealed that glucagon like peptide (GLP-1), an incretin hormone, has a significant effect in lowering blood glucose levels without serious side effects. A naturally occurring analog of GLP-1, Exenedin-4 (ByettaRTM), is an approved compound for diabetes. It stimulates insulin secretion in a glucose-dependent manner, and prevents and reverses the destruction of pancreatic beta-cells.|
We have synthesized new analogs of GLP-1 using two different strategies. First, we synthesized a lipidated GLP-1 analog with a flexible linker that can anchor itself in cell membranes. Restriction of GLP-1 to the membrane could increase the effective concentration of the peptide around the GLP-1 receptor (GLP-1R); resulting in a more efficient binding and activation of downstream signals. Our results showed that lipidated construct had the same efficacy as the GLP-1, however it exhibited less potency. Nevertheless this construct includes a number of sites for optimization, which may allow for enhanced activity.
We have also investigated longer lasting analogues of GLP-1 by installing unnatural amino acids at the sites that are susceptible to proteolysis. Using beta-amino acid substitutions at the hydrolytic site, we intended to overcome protease degradation caused by dipeptidyl peptidase-IV (DPP-IV) enzyme. In addition to using well-known side chains of glutamic acid (Glu) and leucine (Leu), we also introduced a fluorinated amino acid with a hexafluoroleucine side chain. We describe a novel and efficient synthesis of beta-hexafluoroleucine along with its use in other systems. Our results indicate that betaGLP-1 analogues had similar efficacies as GLP-1, but showed diminished activities. Protease stability assays for beta-Glu and beta-Leu substituted analogs have revealed that they were more stable than GLP-1, which holds promise for overall insulinotropic effects of these molecules in vivo.
Our strategies for designing longer lasting analogues of GLP-1 have been successful to create ligands with similar efficacy for GLP-1R, although stimulation of signal transduction has not been sufficiently achieved. Due to their efficient binding to the cognate receptor, GLP-1 analogues described in this study may still yet exhibit improved glucose lowering properties in vivo. Moreover, using these constructs as models, further modifications can be made to engineer superior GLP-1 analogues.
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