Browsing by Subject "Structure"
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Item Arc-related Mesozoic basins of northern Mexico : their origin, tectonic inversion and influence on ore localization(2016-05) Lyons, James Irwin, 1948-; Kyle, J. Richard; Lawton, Timothy Frost; Cloos, Mark; Horton, Brian K; Elliott, BrentNew structural mapping and radiometric dating in northern Mexico integrated with previous studies indicate the need for revision of current regional tectonic models. The Mezcalera Marginal Basin, an autochthonous Jurassic-Lower Cretaceous basin exposed from southern Arizona to Guerrero replaces accreted terrane models. The lack of significant documentable offsets of this marginal basin provides evidence that contradict proposed major Mexican transform faults in northern Mexico. A left-lateral Cenomanian transpressional fault along which the Caborca and related terranes and offset Bisbee Group strata were displaced is documented by east-directed thrusting of the translated basement and supracrustal strata over the autochthonous Mezcalera Basin strata. Oxfordian (149 Ma) submarine volcanic domes at Batopilas, Chihuahua indicates the Nazas arc of central Mexico migrated across the Mezcalera Marginal Basin, and 124 to 138 Ma dates on Bisbee Group Morita Formation tuffs indicate Alisitos arc volcanism to the west. The well documented Late Cretaceous through Miocene arc migration can thus be projected to the Early Jurassic. Oceanic plate rollback toward the Pacific from the Jurassic through the Early Cretaceous explains the observed arc migration as well as the resulting extension of the Mexican continent. A previously unrecognized intracratonic basin, the Carrizal Basin, a probable northern extension of the Mexican Basin, is documented west of the Chihuahua Basin. The older usage Aldama Platform is divided into the Casas Grandes Platform to the west and the Florida-Aldama Ridge to the east of the Carrizal Basin. Basin inversion as defined by mapping of bivergent out-of-the-basin thrusting along both sides of both the Carrizal and Mexican Intracratonic Basins suggests inversion as the principal tectonic process that produced the Sierra Madre Oriental fold belts. Stratigraphic relationships document the inception of tectonic shortening as Late Cenomanian and a folded 43.7 Ma rhyolite flow at Division de Norte, Chihuahua documents continuing basin inversion well into the Eocene. Previous observations of spatial correlations between structurally complex basin margins and numerous major Cretaceous through Miocene mineral deposits are enhanced by the discovery of the large Cinco de Mayo polymetallic carbonate deposit hosted in stacked west-directed out-of-the-basin thrusting on the west margin of the Carrizal Basin.Item Assessment training for faculty: effects on k-12 teacher preparedness(Texas Tech University, 2009-05) Elliott, Robert Wayne; Paton, Valerie O.; Wang, Eugene W.; Murray, John P.; Fox, KellyAs the condition of education remains a critical issue among policymakers and researchers, both nationally and internationally, assessment and accreditation of teacher education programs throughout the United States is a growing concern. This experimental design study uses structural equation modeling (SEM) to confirm the effects that assessment training for teacher education faculty have on preservice teacher candidate assessment scores as indicators of teacher preparedness. Data were collected from teacher education faculty and preservice teacher candidates using surveys to examine the associations between candidate characteristics and perceptions of the assessment learning environment, and measures of candidates’ assessment ability. Results of this study are intended to inform administrators and policymakers for making decisions about the accountability, assessment, and accreditation of teacher education programs at postsecondary institutions.Item A comprehensive numerical model of Io's chemically-reacting sublimation-driven atmosphere and its interaction with the Jovian plasma torus(2012-05) Walker, Andrew Charles; Varghese, Philip L.; Goldstein, David Benjamin, doctor of aeronautics; Trafton, Laurence M.; Raja, Laxminarayan; Johnson, Robert; Spencer, JohnIo has one of the most dynamic atmospheres in the solar system due in part to an orbital resonance with Europa and Ganymede that causes intense tidal heating and volcanism. The volcanism serves to create a myriad of volcanic plumes across Io's surface that sustain temporally varying local atmospheres. The plumes primarily eject sulfur dioxide (SO₂) that condenses on Io's surface during the relatively cold night. During the day, insolation warms the surface to temperatures where a global partially collisional atmosphere can be sustained by sublimation from SO₂ surface frosts. Both the volcanic and sublimation atmospheres serve as the source for the Jovian plasma torus which flows past Io at ~57 km/s. The high energy ions and electrons in the Jovian plasma torus interact with Io's atmosphere causing atmospheric heating, chemical reactions, as well as altering the circumplanetary winds. Energetic ions which impact the surface can sputter material and create a partially collisional atmosphere. Simulations suggest that energetic ions from the Jovian plasma cannot penetrate to the surface when the atmospheric column density is greater than 10¹⁵ cm⁻². These three mechanisms for atmospheric support (volcanic, sublimation, and sputtering) all play a role in supporting Io's atmosphere but their relative contributions remain unclear. In the present work, the Direct Simulation Monte Carlo (DSMC) method is used to simulate the interaction of Io's atmosphere with the Jovian plasma torus and the results are compared to observations. These comparisons help constrain the relative contributions of atmospheric support as well as highlight the most important physics in Io's atmosphere. These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO₂ on the exposed non-frost surface, and surface temperature distribution are investigated. Circumplanetary flow is predicted to develop from the warm dayside toward the cooler nightside. Io's rotation leads to a highly asymmetric frost surface temperature distribution (due to the frost's high thermal inertia) which results in circumplanetary flow that is not axi-symmetric about the subsolar point. The non-equilibrium thermal structure of the atmosphere, specifically vibrational and rotational temperatures, is also examined. Plasma heating is found to significantly inflate the atmosphere on both the dayside and nightside. The plasma energy flux causes high temperatures at high altitudes, but plasma energy depletion through the dense gas column above the warmest frost permits gas temperatures cooler than the surface at low altitudes. A frost map (Douté et al., 2001) is used to control the sublimated flux of SO₂ which can result in inhomogeneous column densities that vary by nearly a factor of four for the same surface temperature. A short residence time for SO₂ molecules on the non-frost component is found to smooth lateral atmospheric inhomogeneities caused by variations in the surface frost distribution, creating an atmosphere that looks nearly identical to one with uniform frost coverage. A longer residence time is found to agree better with mid-infrared observations (Spencer et al., 2005) and reproduce the observed anti-Jovian/sub-Jovian column density asymmetry. The computed peak dayside column density for Io agrees with those suggested by Lyman-[alpha] observations (Feaga et al., 2009) assuming a surface frost temperature of 115 K. On the other hand, the peak dayside column density at 120 K is a factor of five larger and is higher than the upper range of observations (Jessup et al., 2004; Spencer et al., 2005). The results of the original DSMC simulations of Io's atmosphere show that the most important and sensitive parameter is the SO₂ surface frost temperature. To improve upon the original surface temperature model, we constrain Io's surface thermal distribution by a parametric study of its thermophysical properties. Io's surface thermal distribution is represented by three thermal units: sulfur dioxide (SO₂) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in the thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, the thermal model solves the one-dimensional heat conduction equation in depth into Io's surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO₂ frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo photo-polarimeter (PPR) brightness temperature. The thermophysical parameters are the frost Bond albedo, and thermal inertia, as well as the non-frost surface Bond albedo, and thermal inertia. The best fit parameters are found to be [equations] for the SO2 frost surface and [equations] for the non-frost surface. These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io's sublimation-driven atmosphere using DSMC. The DSMC simulations show that the sub-Jovian hemisphere is significantly affected by the daily solar eclipse. The SO₂ surface frost temperature is found to drop ~5 K during eclipse but the column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO₂ vapor pressure on the SO₂ surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO₂ frost surface. These opposing winds meet and are collisional enough to form stagnation point flow. The simulations are compared to Lyman-[alpha] observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-Jovian and sub-Jovian hemispheres. A composite "average dayside atmosphere" is formed from a collisionless simulation of Io's atmosphere throughout an entire orbit. The composite "average dayside" atmosphere without the effect of global winds indicates that the sub-Jovian hemisphere should have lower average column densities than the anti-Jovian hemisphere (with the strongest effect at the sub-Jovian point) due entirely to the diurnally averaged effect of eclipse. Lastly, a particle description of the plasma is coupled with the sophisticated surface thermal model and a final set of global DSMC atmospheric simulations are performed. The particle description of energetic ions from the Jovian plasma torus allows for momentum transfer from the ions to the neutral atmosphere. Also, the energetic ions (or solar photons) can dissociate the neutral atmosphere and cause sputtering of SO₂ on the surface. SO₂ remains the dominant dayside species (>90%) despite being dissociated by ions and photons to form O, O₂, S, and SO. SO₂ remains the dominant atmospheric species on the nightside between dusk and midnight due to sputtering of SO₂ surface frosts by energetic ions as well as the high thermal inertia of SO₂ frosts that cause the surface temperature to cool slowly and thus sublime a thicker SO₂ atmosphere. O₂ becomes the dominant atmospheric species above coldest areas of the surface because it is non-condensable at Io's surface temperatures and other species are sticking to the surface. SO and O are present in similar gas fractions because they are created together via the same ion and photo-dissociation reactions. Sulfur column densities are the lowest throughout the atmosphere because S is created slowly via direct dissociation of SO₂; it is instead created primarily through dissociation of SO. The momentum transfer from the plasma is found to have substantial effect on the global wind patterns. The interaction between the plasma pressure and day-to-night pressure gradient is highly dependent on Io's subsolar longitude. Similar to previous simulations, the westward winds reach higher Mach numbers and wind speeds than the eastward winds. This is because the westward winds are accelerated by a larger day-to-night pressure gradient due to the very cold surface temperatures that exist prior to dawn. Eastward equatorial winds on the nightside are accelerated by the plasma pressure and condense out near the dawn terminator after traveling ~3/4 of the circumference of Io. O₂ is pushed to the nightside by the circumplanetary winds where it builds-up until it reaches an equilibrium column density. On the nightside, O₂ is destroyed by ion dissociation. On the nightside, a shear layer develops between the equatorial eastward winds and stagnant non-condensable species at mid-latitudes. This shear layer generates lateral vorticity which is especially visible in O₂ streamlines. Large cyclones develop in the northern and southern hemispheres and are most apparent in the O₂ wind patterns because other species condense out on the nightside.Item Landuse and hydroperiod influences on amphibian community structure and the role of larval amphibians in the playa food web(Texas Tech University, 2007-08) Torrence, Shannon M.; Smith, Loren M.; Wilde, Gene R.; Allen, Linda J. S.; Densmore, Llewellyn D.; McMurry, Scott T.Playa wetlands are the primary breeding sites for amphibians in the Southern High Plains (SHP). Most playas have cultivated watersheds, which causes sediments to accumulate in playa basins, reducing their hydroperiods. Little is known about the influences of landuse practices and hydroperiod on amphibian community structure. I investigated potential influences on amphibian populations for cumulative (entire sampling season) and short term sampling period 2-day values. For cumulative analyses, I examined the influence of landuse (cropland versus native grassland), hydroperiod (how long a playa contains water), vegetation cover, and playas size on cumulative amphibian metamorph diversity (H’), amphibian adult diversity, Spea spp. metamorph abundance, and Spea spp. adult abundance. To examine short-term influences on amphibian communities, I tested for the influence of landuse, time (week within a season), weekly water loss (ranging from positive (i.e., water gains) to negative (water loss) values), vegetation cover, and playa size on sampling period amphibian diversity, sex ratios (proportion of males), metamorph Spea spp. abundance, and adult Spea spp. abundance by developing predictive models. I also examined how landuse influenced metamorph survival, capture and recapture rates, emigration probability, and population size. Because aquatic food web studies can provide useful information regarding relationships among trophic levels and wetland conservation efforts, I tested hypotheses that concerning how hydroperiod and landuse influenced playa trophic structure, and whether this structure is best modeled as a web or a chain. I sampled 12 playas each year in 2003, 2004, and 2005 in the SHP. I did not observe a landuse effect on cumulative amphibian metamorph diversity, amphibian adult diversity, Spea spp. metamorph abundance, or Spea spp. adult abundance. However, H’ for metamorphs was greater in 2004 than 2003. Spea spp. metamorph and adult abundance was greater in 2003 than 2004. I also did not observe a hydroperiod, playa size, or vegetation cover effect on cumulative amphibian metamorph diversity, Spea spp. metamorph abundance, and Spea spp. adult abundance. Hydroperiod and amphibian adult H’ were positively associated in 2003 or for both years combined. Results are difficult to interpret because power is low in these tests due to high variation and low sample size. Contrary to other studies, I did not observe a landuse effect on sampling period Spea spp. abundance. I also observed no evidence of landuse effects on sampling period amphibian diversity or sex ratios. Sampling period metamorph diversity increased with weekly water loss, whereas adult diversity decreased with weekly water loss. Sampling period proportion of males decreased with weekly water loss. Sampling period Spea spp. metamorph abundance increased with weekly water loss, whereas sampling period Spea spp. adult abundance decreased with weekly water loss. There were weekly differences in Spea spp. abundance, but no differences in weekly diversity or sex ratios. Vegetation cover was positively related to metamorph diversity and had no influence on adult diversity. Vegetation cover had an overall positive influence on sampling period proportion of males. Spea spp. metamorph abundance was positively associated with playa vegetation cover in 2003 and negatively associated with vegetation cover in 2004. Spea spp. metamorph abundance was positively associated with vegetation cover for both years combined. Vegetation cover had no influence on adult abundance. Playa size had no influence on Spea spp. abundance and proportion of males, but had a positive influence on diversity of adults in 2004. However, the relevance of results related to adult proportion of males, diversity, and Spea spp. abundance are unclear because I was unable to sample playas immediately following rain events, when adults were most abundant. Spea spp. metamorph survival could not be estimated effectively using current model estimation techniques, as data were overdispersed. Whereas thousands of metamorphs were initially captured, few metamorphs (mean among playas = 1.86%) were recaptured among weeks. When emigration could be estimated, it was usually high (> 60%). Population size also varied considerably among playas. Because of poor model fit, testing for differences in population parameters between cropland and grassland landuses could not be performed. The hypothesis that playa trophic structure was best modeled as a food web over a food chain was supported. However, models that included environmental variables such as hydroperiod and vegetation cover had higher Akaike weight than food web or chain models alone, meaning food web models should include foraging functional group and environmental variables. Hydroperiod was an important variable in structuring playa communities, wheras landuse was not. Long hydroperiod playas had high Ambystoma tigrinum mavortium and low tadpole densities, compared to short hydroperiod playas. Short hydroperiod playas had higher proximate invertebrate and amphibian richness. Because all playas dry completely and refill at some point, this food web study is applicable to data collected over a short time period. Long-term sampling of playas would reveal that long hydroperiod playas initially have communities similar to short hydroperiod playas and then shift to communities of long hydroperiods. Therefore, long hydroperiod playas should have a cumulative richness greater than short hydroperiod playas. My results suggest the proximate mechanism influencing amphibian communities in playas is hydroperiod. Anything that alters hydroperiod, such as playa sedimentation or excavation, could negatively influence amphibian communities, and those activities should be avoided. Given their high densities, larval amphibians are important members of the playa community. Therefore, conservation efforts for playas should include amphibians. Because amphibians use terrestrial and aquatic habitats, I suggest that the best strategy requires conservation of playas and adjacent upland areas. Conservation of uplands will reduce playa sedimentation and ensure amphibians have upland habitat to forage, emigrate, and hibernate.Item On the crushing of honeycomb under axial compression(2010-12) Wilbert, Adrien; Kyriakides, S.; Ravi-Chandar, KrishnaswamyThis thesis presents a comprehensive study of the compressive response of hexagonal honeycomb panels from the initial elastic regime to a fully crushed state. Expanded aluminum alloy honeycomb panels with a cell size of 0.375 in (9.53 mm), a relative density of 0.026, and a height of 0.625 in (15.9 mm) are laterally compressed quasi statically between rigid platens under displacement control. The cells buckle elastically and collapse at a higher stress due to inelastic action. Deformation then first localizes at mid-height and the cells crush by progressive formation of folds; associated with each fold family is a stress undulation. The response densifies when the whole panel height is consumed by folds. The buckling, collapse, and crushing events are simulated numerically using finite element models involving periodic domains of a single or several characteristic cells. The models idealize the microstructure as hexagonal, with double walls in one direction. The nonlinear behavior is initiated by elastic buckling while inelastic collapse that leads to the localization observed in the experiments occurs at a significantly higher load. The collapse stress is found to be mildly sensitive to various problem imperfections. For the particular honeycomb studied, the collapse stress is 67% higher than the buckling stress. It was also shown that all aspects of the compressive behavior can be reproduced numerically using periodic domains with a fine mesh capable of capturing the complexity of the folds. The calculated buckling stress is reduced when considering periodic square domains as the compatibility of the buckles between neighboring cells tends to make the structure more compliant. The mode consisting of three half waves is observed in every simulation but its amplitude is seen to be accented at the center of the domains. The calculated crushing response is shown to better resemble measured ones when a 4x4 cell domain is used, which is smoother and reproduces decays in the amplitude of load peaks. However, the average crushing stress can be captured with engineering accuracy even from a single cell domain.Item Regional structure, stratigraphy, and hydrocarbon potential of the Mexican sector of the Gulf of Mexico(2011-08) Rodriguez, Anthony Byron; Mann, Paul, 1956-; Galloway, William E.I have compiled digital seismic and well data over a region of approximately 700,000 km² to better improve the correlation of the Mexican sector of the Gulf of Mexico (MGOM) with the better studied and more explored U.S. sector. I have ~25,000 km of regional 2D lines that were collected by the University of Texas in the 1970's. I have digitized data from published PEMEX data from the MGOM using SEG-Y converter software and incorporated these data into my seismic grid. Using these data, I interpreted and correlated 20 surfaces that range in age from Late Jurassic to Recent. The combined shelf-slope-basin dataset from the MGOM allows for correlation of units from the deepwater MGOM, across into the Mexican Ridges passive margin foldbelt, and onto the Mexican shelf. I have also incorporated seismic data from the offshore Chicxulub crater and correlated units in the Yucatan platform area with the deepwater MGOM. This regional data set indicates that normal, growth faulting linked with downdip toe thrusts and folds of the Mexican Ridges initiated in post-Middle Miocene time and are therefore unrelated to the earlier Paleogene Laramide uplift deformation phase. Shelf-slope-deep basin seismic facies of Eocene and Oligocene units show an influx of clastic materials linked with regional uplift and volcanic events affecting central Mexico during this period. I propose that the deepwater folds of the Mexican Ridges accompanied shelf-edge gravity sliding and normal faulting activated during accelerated Oligo-Miocene uplift, regional volcanic activity, and erosion of the Mexican landmass. Downdip sliding occurred on the seaward-dipping top Cretaceous carbonate unit (7° to 13°) along with overlying horizons that range in dip from 1° to 2°. Shelf-slope-deep basin seismic facies of the Paleocene units around the Yucatan peninsula suggest a sediment-starved and slide-free carbonate margin with a current basinward dip of approximately 12° and significantly greater than those dips observed along the present-day eastern Gulf of Mexico. Based on the seismic interpretations and plate reconstructions, I propose four major tectonosequences fill the Gulf of Mexico basin: 1) A Late Jurassic to Late Cretaceous passive margin phase; 2) a Late Cretaceous to Late Eocene Laramide deformational phase; 3) a Late Eocene to Middle Miocene passive margin phase; and 4) a Late Miocene to Recent Neogene deformational phase.Item Reliability-based Design of Offshore Mooring Systems(2013-02-05) Mousavi, Mir EmadThis study proposes new methods for the reliability-based design of structural systems, with emphasis on offshore mooring systems. After a brief introduction to the mooring systems, two main objectives are discussed in this dissertation. The first objective is the calculation of the probability of failure of a structural system, which is an important input for a reliability-based design or any quantitative risk assessment. Two different methods are proposed for calculation of the probability of failure: a method based on the Monte Carlo simulations and a method based on the basic rules of probability, which is called the Progressive Reliability Method (PRM). Both methods are flexible to the definition of system failure. For example, the probability of a serviceability or ultimate-strength failure can be assessed using any of the two methods. It is shown that the two methods produce similar results, but PRM is preferred because it is exact and usually faster to implement. The second main objective in this dissertation is to develop a method for the optimization of the design of a structural system, given a target probability of failure. In this method, using the structural analysis of a preliminary design, the ratio of the optimal to the preliminary mean capacity of each component, which is called the Optimality Factor, is determined. Two design strategies are considered. First, an optimal design is intended to achieve the maximum system integrity. System integrity is defined as the balanced contribution of system components to its reliability. To quantify the system integrity, the Integrity Index is defined, and its calculation for various systems is discussed. Second, a designated failure scenario is considered, where some components serve as a fuse to protect some other components. This design strategy is especially applicable to mooring systems with drag anchor foundations because normally, if a drag anchor is pulled out from the seabed, it can cause significant damages to nearby subsea facilities. Using the rules of probability, a method is then developed to calculate the optimality factor of each component.Item Structural studies of Mycobacterium tuberculosis KatG, an INH drug activator, and Brucella abortus VirB11, an ATPase of type IV translocation system(2009-05-15) Yu, HongCatalase-peroxidase (KatG) of Mycobacterium tuberculosis is a bifunctional heme enzyme that has been shown to play an important role in the activation of a first line drug, isoniazid (INH), used in the treatment of tuberculosis infection. Mutations in the katG gene have been found to be associated with INH resistance. The most commonly encountered mutation is the Ser315Thr point mutation. In this dissertation, the x-ray crystallographic structures of MtbKatG and the mutant enzyme KatG[S315T] are presented to explore the molecular basis of the INH activation and resistance. The structure is dimeric and contains a heme cofactor in each subunit of the dimer. The most important change in KatG[S315T] is due to the presence of the methyl group of the threonine 315 side chain, which is located at the narrowest part of the substrate channel. The protruding methyl group effectively constricts the accessibility to the heme by closing down the dimensions of the channel, constraining the substrate entrance. VirB11 of Brucella abortus is a hexameric ATPase that belongs to the type IV secretion system. The crystal structure of BaVirB11 was found to contain six molecules per asymmetric unit. The Walker A (P loop), His box, and Glu box from the C-terminal domain are located at the interface of the N- and C-terminal domain. A large conformational change was found in the linker region when compared with that of HP0525 structure, the VirB11 analogous from H. pylori. To elucidate the functional role of each domain, seven functional mutations were generated and used for biochemical studies. The GER motif and the linker region were found to be crucial for ATP hydrolysis activity of BaVirB11. Mutations in the GER motif (R101Q) and the linker region (R120E) of BaVirB11 completely abolish the ATP hydrolysis activity of the enzyme. The binding affinities of the two mutants to the ATP; however, are similar to that of the wild-type enzyme, indicating that mutation in the GER motif or the linker region has no effect on ATP binding.Item Structure-based drug mechanism study and inhibitor design targeting tuberculosis(2009-05-15) Wang, FengThe increase of multi-drug resistant and extensively drug resistant tuberculosis (TB) cases makes it urgent to develop a new generation of TB drugs to counter resistance and shorten treatment. Structural biology, which allows us to ?visualize? macromolecules, is now playing a key role in drug discovery. In this work, a structure-based approach was applied to the study of the mode of action of current TB chemotherapies, the identification of potential therapeutic targets, and the design of new inhibitors against TB. Knowledge of the precise mechanisms of action of current TB chemotherapies will provide insights into designing new drugs that can overcome drug-resistant TB cases. Structural biology combined with biochemical and genetic approaches was used to elucidate the mechanisms of actions of isoniazid, ethionamide and prothionamide. The active forms of these anti-TB prodrugs were identified by protein crystallography and the target-inhibitor interactions were revealed by the complex structures. Although these drugs are activated through two completely different routes, they all inhibit InhA, an essential enzyme in mycolic acid biosynthesis, by modification of the enzyme cofactor NAD, which unveils a novel paradigm of drug action. Isoniazid, ethionamide and prothionamide all target InhA, which validates the enzyme as a superb drug target. A structure-based approach was adopted to design new inhibitors targeting InhA, using triclosan as the scaffold. Guided by the InhA-inhibitor complex structures, two groups of triclosan analogs were identified with dramatically increased inhibitory activity against InhA. Structural biology has also provided fundamental knowledge of two potential therapeutic targets, Mtb ?-lactamase (BlaC) and fatty-acyl-CoA thioesterase (FcoT). Mtb ?-lactamase has been proposed to be the most significant reason for mycobacterial resistance to ?-lactam antibiotics. The determination of Mtb BlaC structure not only demonstrates the mechanism of drug resistance but also provides a solid base for the design of new ?-lactamase inhibitors that could be used with ?-lactam antibiotics as a new regimen to treat tuberculosis. The crystal structure of FcoT provided crucial information in identification of the function of this previously hypothetical protein. The characterization of FcoT revealed an important pathway that is critical for Mtb?s survival in host macrophages.Item Structures and Reactivity of Transition-Metal Compounds Featuring Metal-Ligand Multiple Bonds(2014-07-25) Xu, ZhenggangThis dissertation presents the results from density functional theory (DFT) calculations on three major projects I have been working on over the past several years. The first system is focused on the structure and reactivity of a novel osmium silylyne compounds featuring an Os?Si triple bond. NMR simulation confirmed the existence of this compounds and bonding analysis like NBO and ETS-NOCV proved its triple bond character. The structures of the [2+2] cyclo-addition product from silylyne and other small molecules (PhC?CPh and P?C^(t)Bu) were also determined from possible isomers by energetic results and NMR simulations. The cycloaddition reactions were found to be under kinetic control and steric effects should be a major reason for that. Furthermore, the geometric and electronic structures of the osmium silylyne analogues (M?E, M = Ru and Os; E = Si, Ge and Sn) are studied computationally and their similarities and distinctions are discussed. Both the second and third systems are related to the formation of transition metal imido compound (M=NR). In the second system a cationic oxorhenium(V) complex reacts with a series of arylazides (N_(3)Ar) to give cationic cis-rhenium(VII) oxo imido complexes. Inductive effect is found in the reaction rates but it leads to different trends for electron-donating and electron-withdrawing substituted phenyl azides. Computations found all the substituted phenyl imido products are formed through the same pathway. The reason for the different trend is due to the competitions between two major energy barriers along the reaction pathway. For the electron-withdrawing substituents, N_(2) extrusion is rate determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett ? values; thus, the Hammett plot appears with a break in slope. In the third system a series of vanadium (III) terminal organoazides were converted to vanadium (V) imido compounds and small quantities of diazenes were found formed in the reactions. Computational studies were conducted to examine serveral possible pathways and a mechanism in which nitrene radicals released from metal-azide decompositions is supported based on two reasons: 1) The two-electron reduction on vanadium center during decomposition of azide generates two fragments both in their ground states (triplet states), which might drag the activation barrier down; 2) This conversion is spin-allowed since vanadium center and nitrene nitrogen are antiferromagnetically coupled in transition state. The ligand simplification study indicates that the bulky mesityl groups on the supporting ligand prevents the reactions from going through the direct N_(2) extrusion pathway.Item Synthesized polyimide membranes for pervaporation separations of toluene/iso-octane mixtures(2005-12) Xu, Wen Yuan; Paul, Donald R.; Koros, William J., 1947-Separation of aromatic/aliphatic hydrocarbon mixtures by pervaporation has been of increasing interest in recent decades. Dozens of polymer materials have been reported for separations of benzene/cyclohexane and toluene/iso-ocatne mixtures. However, fundamental understanding of material structure and transport relations is not adequate to generalize guidelines for materials screening. The goals of this study are to tailor the structure of the polyimide materials, correlate the structure and transport relations, and establish guidelines for future materials. The 3, 5-Diaminobenzoic acid (DABA) containing polyimides were synthesized by both chemical and thermal solution imidization. The synthesized polyimides were formed into dense films by solution casting. The physical properties of the polyimides synthesized with monomers: 2, 2-bis (3, 4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), 4, 6-trimethyl-1, 3-phenylendiamine (DAM) and DABA, were characterized by DSC, WAXD, GPC and density. The chemical structures were assessed by FTIR and NMR. The pervaporation and sorption of the synthesized polyimide membranes were tested in toluene/iso-octane mixtures at 100°C. The structure- transport property relations were established for the 6FDA-DAM/DABA membranes. The 6FDA-DAM/DABA polyimides were crosslinked by ethylene glycol. The pervaporation and sorption of the crosslinked membranes were tested in toluene/iso-octane mixtures at 100°C. Thermal imidization was found to give a higher imidization degree than chemical imidization. As a result, the polyimides made by chemical imidization contain a higher percentage of carboxylic acid groups than those made by thermal imidization. Chemical imidization gives higher film density, glass transition temperature and lower flux and higher selectivity for the toluene/iso-octane pervaporation than the thermally imidized membranes because of the higher carboxylic acid concentration. The chemically imidized membranes are slightly brittle after the crosslinking. Only the thermal imidization membranes have good flexibility and its pervaporation selectivity improves significantly after the crosslinking. Solubility selectivity and diffusivity selectivity of the 6FDA-DAM/DABA membranes were correlated with solubility parameters and fractional free volume, respectively. The structure-mass transport relations show that for the 6FDA-DAM/DABA membranes, both solubility selectivity and diffusivity selectivity contribute to the pervaporation selectivity. For the chemically imidized membranes, increased DABA concentration has a positive effect on solubility selectivity and diffusivity selectivity. For the thermally imidized membranes, increased DABA concentration has a significant effect on diffusivity selectivity only.Item The structure of global economic integration and conflict(2009-08) Patterson, Erik James; McDonald, Patrick J., 1973-; Moser, RobertEmpirical examinations have reached a number of conclusions regarding the relationship between global economic integration and interstate conflict. Most studies find that increased economic relations dampen the likelihood of interstate violence. Yet these studies have focused largely on bilateral trade measures and domestic-level causal mechanisms. In this paper I take the discussion to the structural level and construct systemic variables for global economic openness and concentration. I then test the relationships among these variables and international conflict using an autoregressive model and event count regression methods. The outcomes suggest that high levels of hegemonic protection and global economic concentration are associated with low levels of systemic violence, a finding that runs counter to much of the existing empirical and theoretical literature surrounding the international market and conflict.Item The Walk : holding space in the face of crisis, failure, and fear(2015-05) Hutchinson, Jessica Rae; Rasmussen, Sarah; Carlson, Andrew; Dietz, Steven; Engelman, ElizabethThis thesis is about fear, failure, and faith. The tools I have developed during my graduate study have enabled me first to tolerate and then to welcome the energy of these forces into my artistic process. By examining and establishing Structure, creating and empowering Ensembles, and insisting upon an Inquiry-driven process, I can encounter fear and failure as I seek out the unpredictable, unrepeatable transcendence of the living play. By cultivating and inviting uncertainty, I hold space for the emergence of grace.