Browsing by Subject "Crystallization"
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Item Basaltic volcanism : deep mantle recycling, Plinian eruptions, and cooling-induced crystallization(2010-08) Szramek, Lindsay Ann; Lassiter, John C.; Gardner, James Edward, 1963-; Carlson, William; Houghton, Bruce; Rowe, Michael C.Mafic magma is the most common magma erupted at the surface of the earth. It is generated from partial melting of the mantle, which has been subdivided into end-members based on unique geochemical signatures. One reason these end members, or heterogeneities, exist is subduction of lithospheric plates back into the mantle. The amount of elements, such as Cl and K, removed during subduction and recycled into the deep mantle, is poorly constrained. Additionally, the amount of volatiles, such as Cl, that are recycled into the deep mantle will strongly affect the behavior of the system. I have looked at Cl and K in HIMU source melts to see how it varies. Cl/Nb and K/Nb suggest that elevated Cl/K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle. After the mantle has partially melted and mafic melt has migrated to the surface, it usually erupts effusively or with low explosivity because of its low viscosity, but it is possible for larger eruptions to occur. These larger, Plinian eruptions, are not well understood in mafic systems. It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur. Plinian eruptions require fragmentation to occur, which means the melt must undergo brittle failure. This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding. To test this, I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions. One eruption has a fast ascent, faster than those seen in more silicic melts, whereas the other eruption is unable to be reproduced in the lab, however it began with a increased viscosity in the partly crystallized magma. After fragmentation and eruption, it is generally thought that tephra do not continue to crystallize. We have found that crystallinity increases from rim to core in two basaltic pumice. Textural data along with a cooling model has allowed us to estimate growth rates in a natural system, which are similar to experimental data.Item Crystallization and mutational studies of carbon monoxide dehydrogenase from moorella thermoacetica(Texas A&M University, 2004-09-30) Kim, Eun JinCarbon Monoxide Dehydrogenase (CODH), also known as Acetyl-CoA synthase (ACS), is one of seven known Ni containing enzymes. CODH/ACS is a bifunctional enzyme which oxidizes CO to CO2 reversibly and synthesizes acetyl-CoA. Recently, X-ray crystal structures of homodimeric CODH from Rhodospirillum rubrum (CODHRr) and CODH from Carboxydothermus hydrogenoformans (CODHCh) have been published. These two enzymes catalyze only the reversible oxidation of CO to CO2 and have a protein sequence homologous to that of the ? subunit of heterotetrameric ?2?2 enzyme from Moorella thermoacetica (CODHMt), formerly Clostridium thermoaceticum. Neither CODHRr nor CODHCh contain an ?-subunit as is found in CODHMt. The precise structure of the active site for acetyl-CoA synthase, called the A-cluster, is not known. Therefore, crystallization of the ? subunit is required to solve the remaining structural features of CODH/ACS. Obtaining crystals and determining the X-ray crystal structure is a high-risk endeavor, and a second project was pursued involving the preparation, expression and analysis of various site-directed mutants of CODHMt. Mutational analysis of particular histidine residues and various other conserved residues of CODH from Moorella thermoacetica is discussed. Visual inspection of the crystal structure of CODHRr and CODHCh, along with sequence alignments, indicates that there may be separate pathways for proton and electron transfer during catalysis. Mutants of a proposed proton transfer pathway were characterized. Four semi-conserved histidine residues were individually mutated to alanine. Two (His116Mt and His122Mt) were essential to catalysis, while the other two (His113Mt and His119Mt) attenuated catalysis but were not essential. Significant activity was "rescued" by a double mutant where His116 was replaced by Ala and His was also introduced at position 115. Activity was also rescued in double mutants where His122 was replaced by Ala and His was simultaneously introduced at either position 121 or 123. Activity was also "rescued" by replacing His with Cys at position 116. Mutation of conserved Lys587 near the C-cluster attenuated activity but did not eliminate it. Activity was virtually abolished in a double mutant where Lys587 and His113 were both changed to Ala. Mutations of conserved Asn284 also attenuated activity. These effects suggest the presence of a network of amino acid residues responsible for proton transfer rather than a single linear pathway.Item Crystallization of amorphous solid films(2003-05) Safarik, Douglas Joseph; Mullins, C. B.Below ~130 K, H2O can exist for prolonged periods in a thermodynamically unstable, non-crystalline solid form known as amorphous solid water (ASW). When warmed to above 135 K, ASW crystallizes to the thermodynamically favored state, cubic ice I, on a laboratory time scale. Despite the relevance of ASW crystallization to a variety of scientific problems ranging from astrophysical phenomena to cryopreservation, the kinetics of this transformation are largely uncharacterized, and its mechanism is not fully understood. In the present work, the crystallization kinetics of vapor-deposited, nonporous ASW films less than one micron thick are investigated experimentally near 140 K. The amorphous to crystalline transition is characterized using a probe molecule, chlorodifluoromethane (CHF2Cl), whose adsorbed states and hence desorption kinetics are sensitive to the crystallinity of solid water surfaces. The transformation kinetics of very thick ASW films are found to be both independent of specimen size and consistent with simultaneous homogeneous nucleation and isotropic growth of crystalline ice grains. As the ASW film thickness is reduced from 385 nm to 55 nm, however, the rate of surface crystallization decelerates, in apparent conflict with a homogeneous nucleation and growth mechanism. In an attempt to explain this behavior, a geometrical model of phase transition kinetics at the surface of solids, with special consideration of finite specimen size in one dimension, is constructed. For materials in which nucleation occurs spatially randomly, phase change is predicted to decelerate when film thickness is reduced below the mean crystal grain size. This phenomenon originates from a reduction in the number of crystallites available to transform the surface as the sample becomes thinner. Good quantitative agreement between this simple model and the experimental data is attained using a minimum of kinetic parameters, suggesting it captures the essential physics of ASW crystallization. These model fits also yield preliminary estimates of crystalline ice growth and nucleation rates. Finally, an experimental protocol and corresponding model of phase change that together permit accurate quantification of nucleation and growth kinetics (when both processes occur simultaneously) is developed. Using the outlined methodology, crystalline ice growth and nucleation rates near 140 K are found to be ~1 Å/s and ~1010 cm-3s -1, respectively, and to exhibit Arrhenius temperature dependencies with activation energies of ~50 and ~170 kJ/mol.Item Crystallization of the Bacillus cereus 5/B/6 Metallo-β-lactamase(2012-08) Williams, Elizabeth; Shaw, Robert W.; Nes, William Davidβ-lactamases pose a great threat in clinical settings due to their resistance to classical antibiotics. The worst offenders of this group include class B β-lactamases, which are metal dependent enzymes. They require one or two zinc ions for optimal activity. Bacillus cereus 5/B/6 produces a metallo-β-lactamase that has not previously had its structure reported. Structural information is crucial for determining the chemical mechanism of action of the enzyme inhibitors. The research conducted involved finding proper conditions in which 5/B/6 metallo-β-lactamase would produce crystals suitable buffer for further x-ray diffraction studies The ideal conditions were in bis-tris propane with a pH range from 7.5 to 8.5, and a range of 5% to 10% polyethylene glycol (PEG) 5000 monomethyl ether (MME). The initial x-ray diffracted crystals were grown in 0.1 M bis-tris propane pH 8.5 with 6% PEG 5000 MME. The current diffraction led to a structure of 2.7 Å resolution structure under these conditions.Item Gas transport properties of poly(n-alkyl acrylate) blends and modeling of modified atmosphere storage using selective and non-selective membranes(2007-12) Kirkland, Bertha Shontae, 1976-; Paul, Donald R.The gas transport properties of side-chain crystalline poly(n-alkyl acrylate) and poly(m-alkyl acrylate) blends are determined as a function of temperature for varying side-chain lengths, n and m, and blend compositions. The side chains of poly(n-alkyl acrylate)s crystallize independently of the main chain for n [is greater than or equal to] 10 which leads to an extraordinary increase in the permeability at the melting temperature of the crystallites. The compatibility of these polymers are examined and macroscopic homogeneity is observed for a small range of n and m when the difference /n - m/ is between 2 - 4 methylene units. Thermal analysis shows that the blend components crystallize independently of one another; at the same time, the crystallization of each component is hindered by the presence the other component. The permeation responses of these blends show two distinct permeation jumps as the crystallites from each component melt at their respective melting temperatures. Blends with continuous permeation responses are found to have higher effective activation energies than observed for common polymers. Thermal analysis proved to be a useful tool to help predict the permeation response for poly(alkyl acrylates); thus the thermal behavior of poly(n-alkyl acrylate) blended with n-aliphatic materials and random copolymers of poly(n-alkyl acrylates) are briefly examined. A bulk modified atmospheric storage design is proposed where produce is stored in a rigid chamber that is equipped with both selective and non-selective membrane modules that help regulate the oxygen entering and the carbon dioxide leaving the produce compartment. The design enables control of the atmosphere inside the chamber by modulating gas flow, i.e. the gas flow rate and composition, through the non-selective membrane by delivering fresh air upstream of the non-selective membrane. The model shows that the choice of materials for the selective and non-selective membranes dictate the range of concentrations achievable; however, the air flow rate allows the control between these ranges. The method to design a practical chamber from this model is also described.Item Reversible Attraction-Mediated Colloidal Crystallization on Patterned Substrates(2009-05-15) Fernandes, GregoryIn this dissertation we used tunable particle-particle and particle-substrate attraction to achieve reversible two-dimensional crystallization of colloids on homogeneous and patterned substrates. Total internal reflection and video microscopy techniques were used to quantify the interparticle and particle-substrate interactions in these colloidal systems. Equilibrium and dynamic simulations were then utilized to link these colloidal interactions to the experimental colloidal phase behaviour. The importance of the nature of the attractive interaction in successfully crystallizing colloids has also been documented. The first set of experiments demonstrates the use of temperature and specific ion effects to reversibly control the net particle-substrate van der Waals (vdW) attraction. Colloidal stabilization was achieved via the use of adsorbed polymer brush layers. By using evanescent wave microscopy, we directly and precisely measured how temperature and specific ion effects control the dimensions of adsorbed polymer layers and hence the net van der Waals attraction in between the colloids and the substrate. However, the magnitude of the van der Waals attraction decays very rapidly with increasing surface separation and is therefore not conducive to the self assembly of colloidal crystals. We successfully used thermoresponsive polymer nanoparticles to control the depletion attraction between micron sized silica particles and thereby induced reversible crystallization of the micron sized silica colloids on homogeneous substrates. Video and evanescent wave microscopy techniques were used to measure the nanoparticle-induced attractive interaction as a function of temperature. The experimentally observed phase behaviour was verified via simulations that utilized knowledge of the measured colloidal depletion interactions. Finally, patterned surface topologies were used to position attractive colloidal crystals. Simulations were used to link the measured colloidal interactions to experimental phase behaviour as well as substrate topology. An extension of the concepts developed in this dissertation might suggest a general strategy to assemble colloidal particles into robust and annealable crystals contributing to the fabrication of photonic bandgap materials.Item X-Ray Study of The Transition of Amorphous Nib63s-Pdb17s-Pdb20s Alloy to a Crystalline State(Texas Tech University, 1972-08) Chen, Hua May LinNot Available.