Browsing by Subject "crystallization"

Now showing 1 - 5 of 5
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    Crystallization study of the resistance to cobalt and nickel repressor (RcnR) protein in complex with double-strand DNA
    (2019-12) Li, Chao
    This thesis included my work on the crystallization, data analysis and phasing attempts regarding RcnR in complex with a ds-DNA molecule .RcnR is a transcription factor that regulates the homeostasis of cobalt and nickel in bacterial cells. We crystallized Escherichia coli RcnR with DNA that encompasses the DNA binding site. X-ray diffraction data were collected to 2.9 Å. The crystal belongs to space group P61/522, with unit cell parameters a = b = 73.65 Å, c= 153.77Å, α=β=90°, γ = 120°. The second and third part included my work on MauG and PqqB projects.
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    Data for Crystallinity-independent toughness in renewable poly(L-lactide) triblock plastics
    (2024-03-18) Krajovic, Daniel M; Haugstad, Greg; Hillmyer, Marc A; hillmyer@umn.edu; Hillmyer, Marc A; Hillmyer Research Group
    Poly(L-lactide) (PLLA)’s broad applicability is hindered by its brittleness and slow crystallization kinetics. Among the strategies for developing tough, thermally resilient PLLA-based materials, the utilization of neat PLLA block polymers has received comparatively little attention despite its attractive technological merits. In this work, we comprehensively describe the microstructural, thermal, and mechanical properties of two compositional libraries of PLLA-rich PLLA-b-poly(γ-methyl-ε-caprolactone) (PγMCL)-b-PLLA (“LML”) triblock copolymers. The rubbery PγMCL domains microphase separate from the matrix in the melt and intercalate between PLLA crystal lamellae on cooling. Despite the mobility constraints associated with mid-block tethering, the PLLA end-blocks crystallize as rapidly as a PLLA homopolymer control of similar molar mass. Independent of their degree of crystallinity, LML triblocks exhibit vastly improved tensile toughnesses (63-113 MJ m-3) over that of PLLA homopolymer (1.3-2 MJ m-3), with crystallinities of up to 55% and heat distortion temperatures (HDTs) as high as 148 °C. We investigated the microstructural origins of this appealing performance using X-ray scattering and microscopy. In the case of a largely amorphous PLLA matrix, the PγMCL domains cavitate to enable concurrent PLLA shear yielding and strain-induced crystallization. In highly crystalline PLLA matrices, PγMCL facilitates a lamellar-to-fibrillar transition during tensile deformation, the first such transition reported for PLLA drawn at room temperature. These results highlight the unique attributes of PLLA block polymers and prompt future architectural and processing optimizations to achieve ultratough, high-HDT PLLA block polymer plastics after a simple thermal history on economical timescales.
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    The effect of additives on the molecular mobility, physical stability and dissolution of amorphous solid dispersions
    (2018-01) Fung, Michelle
    Amorphization provides an avenue for improving the oral bioavailability of poorly water-soluble compounds. However, crystallization of the amorphous phase during storage or dissolution could negate the solubility advantage. The objectives were to (i) develop an accelerated testing method for predicting the solid-state physical stability of drugs in amorphous solid dispersions (ASD), (ii) gain a mechanistic understanding of the stabilization in the amorphous state brought about by small molecule excipients, and (iii) investigate the relationship between solid-state properties and dissolution performance of ASDs. Utilizing glycerol as a plasticizer, an accelerated physical stability testing method of ASD was developed. The acceleration in crystallization brought about by glycerol expedited the determination of the coupling between molecular mobility and crystallization. This approach is especially useful for ASDs with high polymer content where drug crystallization is extremely slow at relevant storage temperature. The ability of several organic acids to stabilize an amorphous API, ketoconazole (KTZ), was next investigated. Oxalic (OXA), citric (CIT), tartaric (TAR) and succinic (SUC) acids were chosen based on their relative strengths (pKa values). Coamorphous systems of KTZ with each acid exhibited ionic and/or hydrogen bonding interactions. An increase in the strength of KTZ-acid interactions translated to a reduction in molecular mobility. However, molecular mobility could not completely explain the crystallization propensity of the systems. When in contact with water, coamorphous KTZ-citric and KTZ-tartaric were exceptionally stable while KTZ-succinic and KTZ-oxalic systems crystallized more readily than KTZ. The dissolution performance of the coamorphous systems were compared using the areas under the curve (AUC) obtained from the concentration-time profiles. KTZ-OXA exhibited the highest AUC, while it was about the same for KTZ-TAR and KTZ-CIT and the lowest for KTZ-SUC. Coamorphization with acid caused at least a 2-fold increase in AUC when compared with amorphous KTZ. In ternary KTZ-acid-polyvinylpyrrolidone (PVP) ASDs, the interactions between drug and acid each acid influenced the solid-state stability as well as dissolution performance.
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    Effect of the Thermodynamic and Physical State of the Freeze-Concentrate on Protein Stability
    (2017-12) Jena, Sampreeti
    In this dissertation research, specific interactions (excipient-excipient, excipient/protein-ice, protein-excipient) governing protein conformational stability and crystallization behavior of excipients in the freeze concentrate, were explored. Furthermore, the effects of formulation composition (type and mole fractions of excipients in the formulation) on afore-mentioned interactions, during freeze-thaw and freeze-drying of protein formulations, was investigated. Concentration dependent effects of excipients including the bulking agent, lyo/cryo-protectant and surfactant on the nucleation and growth of crystalline phases in the freeze concentrate were characterized and quantified. Changes in the secondary and tertiary conformations of model proteins (such as Bovine Serum Albumin and Immunoglobulin) due to crystallization of excipients, were determined as a function of formulation composition during freeze-thaw and freeze-drying. Infrared (IR) Spectroscopy was used to detect onset of crystallization the bulking agent and lyo/cryo-protectant. X-Ray Diffractometry (XRD) was used to characterize the polymorphic form of crystalline phases. Far UV circular Dichroism (CD) was used to characterize secondary conformation of protein in thawed and reconstituted (freeze-dried) formulations. IR Spectroscopy was used to characterize secondary conformation of protein in frozen and freeze-dried formulations. A bulking agent – lyo/cryo-protectant – protein system, a typical freeze-drying formulation, was chosen for characterization of frozen and freeze-dried formulations. It was observed that high concentrations of non-crystallizing components such as the protein and lyo/cryo-protectant (usually a disaccharide such as trehalose) inhibited crystallization of the (otherwise readily crystallizing) bulking agent (such as mannitol) and vice versa. At low concentrations, surfactants such as Polysorbate 20, prevented growth of crystalline phases due to amphiphilic interface coverage, but when their concentrations exceeded the critical micelle concentration (CMC), they enhanced degree of crystallinity in the formulation. Structural unfolding of the protein was detected upon crystallization of the lyo/cryo-protectant and micelle formation (when surfactant concentration exceeded CMC). Detection of protein aggregates in reconstituted solutions, confirmed that unfolding induced during freezing, thawing and drying processes, did not reverse upon reconstitution. Presence of ice surfaces and other crystalline interfaces (such as those introduced by the bulking agent) significantly contributed to protein degradation. In our model system, thawing induced stresses such as recrystallization were found to be more detrimental than the stresses induced by freezing and desiccation and hence, freeze-drying yielded better structural recovery of the protein than freeze-thaw in our model system. Secondary relaxations arising from the flexible polar groups on the protein surface (millisecond time scales) and dynamic ring flips of the monosaccharide units about the glycosidic linkage (microsecond time scales) of disaccharides (indicating flexibility of glycosidic linkage) were detected in our model freeze-dried system using Frequency Domain Dielectric Spectroscopy. In the presence of protein, flexibility of the glycosidic linkage was decreased and likewise, presence of disaccharides slowed down the dynamics of flexible protein groups, up to a critical protein to disaccharide mass ratio (= 0.5). Surfactant and higher protein to disaccharide mass ratios (≥ 0.5) produced the opposite effect. These secondary relaxations govern conformational stability of the protein and propensity of the disaccharide to crystallize during storage below the Tg. In the final part of the thesis, effects of slow freezing on lyo/cryo-protectant-protein formulations during cryo-vitrification was investigated. Chemical toxicity of cell penetrating lyo/cryo -protectants such as Dimethyl Sulfoxide (DMSO), frequently used for cryo-vitrification of organs and tissues, was shown to be dictated by their hydrogen bonding behavior (characterized by IR Spectroscopy). At temperatures where hydrogen bonding interactions between lyo/cryo-protectant and water were unfavorable, the lyo/cryo-protectant directly partitioned in the hydration shell of the protein and caused unfolding of the protein, potentially due to hydrophobic interactions. It was also ascertained that when the freeze concentrate is vitrified during freezing, rapid thawing is a necessity to minimize ice recrystallization during devitrification to minimize the damage to the proteins. This dissertation research has enhanced an overall understanding of interactions between the excipients, protein and crystalline interfaces (of ice and crystalline excipients such as bulking agent) as well as protein dynamics in the freeze-concentrate. This information is needed to identify stresses arising in the protein micro-environment that lead to conformational destabilization (and loss of activity) during preservation of protein formulations and is currently absent in literature.
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    An in-situ analytical scanning and transmission electron microscopy investigation of structure-property relationships in electronic materials
    (2014-08) Wagner, Andrew
    As electronic and mechanical devices are scaled downward in size and upward in complexity, macroscopic principles no longer apply. Synthesis of three-dimensionally confined structures exhibit quantum confinement effects allowing, for example, silicon nanoparticles to luminesce. The reduction in size of classically brittle materials reveals a ductile-to-brittle transition. Such a transition, attributed to a reduction in defects, increases elasticity. In the case of silicon, elastic deformation can improve electronic carrier mobility by over 50%, a vital attribute of modern integrated circuits. The scalability of such principles and the changing atomistic processes which contribute to them presents a vitally important field of research. Beginning with the direct observation of dislocations and lattice planes in the 1950s, the transmission electron microscope has been a powerful tool in materials science. More recently, as nanoscale technologies have proliferated modern life, their unique ability to spatially resolve nano- and atomic-scale structures has become a critical component of materials research and characterization. Signals produced by an incident beam of high-energy electrons enables researchers to both image and chemically analyze materials at the atomic scale. Coherently and elastically-scattered electrons can be collected to produce atomic-scale images of a crystalline sample. New specimen stages have enabled routine investigation of samples heated up to 1000 °C and cooled to liquid nitrogen temperatures. MEMS-based transducers allow for sub-nm scale mechanical testing and ultrathin membranes allow study of liquids and gases. Investigation of a myriad of previously "unseeable" processes can now be observed within the TEM, and sometimes something new is found within the old. High-temperature annealing of pure a Si:H films leads to crystallization of the film. Such films provide higher carrier mobility compared to amorphous films, offering improved photovoltaic performance. The annealing process, however, requires exceptionally high temperature (> 600 °C) and time (tens of hours), limiting throughput and costing energy. In an effort to fabricate polycrystalline solar cells at lower cost, large (~30 nm) silicon nanocrystals were incorporated into hydrogenated amorphous silicon (a Si:H) thin films. When annealed, the embedded nanocrystals were expected to act as heterogeneous nucleation sites and crystallize the surrounding amorphous matrix. When observed in the TEM, an additional and unexpected event was observed. At the boundary between the nanocrystal and amorphous matrix, nanocavities were observed to form. Continued annealing resulted in movement of the cavities away from the nanocrystal while leaving behind a crystalline tail. The origins and fundamental mechanisms of this phenomenon were examined by in-situ heating TEM and ex-situ crystallographic TEM techniques. We demonstrate a mechanism of solid-phase crystallization (SPC) enabled by nanoscale cavities formed at the interface between an hydrogenated amorphous silicon film and embedded 30 nm to 40 nm Si nanocrystals. The nanocavities, 10 nm to 25 nm across, have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while "pulling behind" a crystalline tail. It is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence the direction of nanocavity propagation. HASH(0x7febe3ca40b8) The mechanical properties and mechanisms of plasticity in these same silicon nanocubes have also been investigated. The strain-dependent mechanical properties and the underlying mechanisms governing the elastic-plastic response are explored in detail. Elastic strains approaching 7% and flow stresses of 11 GPa were observed, significantly higher than that observed in other nanoscale volumes of Si. In-situ imaging revealed the formation of 5 nm dislocation embryos at 7% strain, giving way at 20% strain to continuous nucleation of leading partial dislocations with {111}-habit at the embryo surface.