Browsing by Subject "Small angle neutron scattering"

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    Block copolymer self-assembly in solution: structure and dynamics.
    (2010-08) Choi, Soo-Hyung
    Block copolymers can self-assemble into micelles or vesicles when dispersed in a selective solvent. In this study, spherical micelles were formed by poly(styrene-bethylene- alt-propylene) (PS-PEP) in squalane, highly selective to PEP blocks, leading to PS cores and swollen PEP coronas. The micelle structure was characterized by dynamic light scattering (DLS) and small-angle x-ray scattering (SAXS). The experimental results provide a detailed picture of micelle structure and intermicelle interaction as a function of block copolymer molecular weight and composition, concentration, and temperature. Based on this structural information, the single molecular exchange kinetics between the spherical micelles in dilute solution was examined by time-resolved small-angle neutron scattering (TR-SANS). Two pairs of structurally matched partially protonated and deuterated micelles were prepared and each pair was blended to provide an initially isotopically segregated state in solution. The SANS intensity is directly related to the concentration of protonated chains in the micelle cores. Therefore, a reduction in the measured scattering intensity can be quantitatively correlated with the exchange of chains. This measurement was aimed at probing the dependence of molecular exchange kinetics on temperature, molecular weight, and concentration. The temperature dependence of the chain exchange rate R(t) can be explained based on the core block dynamics, while the documented quasi-logarithmic decay of R(t) is shown to be consistent with single chain exchange that is hypersensitive to the core degree of polymerization and therefore polydispersity. Complementary measurements were also conducted with concentrated solutions where the micelles pack onto a body-centered cubic lattice. Based on a first-principles model, the exchange kinetics are expected to be independent of micelle concentration. However, slower dynamics in ordered micelles were observed. These results suggest that contributions from factors other than core block dynamics can come into play in the exchange kinetics for ordered micelles.
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    Excepient Phase Transformation In Frozen And Freeze-Dried Formulations And Their Impact On Protein Stability
    (2022-03) Sonje, Jayesh
    Freeze-thaw and freeze-drying are common unit operations in manufacturing of biotherapeutics. Excipients such as sugars, buffers, and surfactants, each with an intended functionality, aid in preventing protein destabilization against stresses encountered during processing or storage. Sugars, specifically added to stabilize proteins, can only be effective if retained in the amorphous state. Selective crystallization of a buffer component during freezing, drying or storage can result in in a pH shift. The first objective of this thesis was to investigate the impact of stress associated with pH shift on protein (lactate dehydrogenase, LDH) conformation in presence of a crystallizing (sodium phosphate) and non-crystallizing (histidine) buffer, during the freezing and thawing stages. This was accomplished using small angle neutron and dynamic light scattering. The significant findings were: (i) LDH, at high concentration, had self-stabilizing effect and exhibited reversible aggregation after 5 freeze-thaw cycles, irrespective of buffer used. (ii) At low LDH concentrations, only with the selection of an appropriate buffer, irreversible aggregation could be avoided. Crystallization of certain excipients such as mannitol, is desirable, from a processing as well as product quality perspective. Mannitol is widely used in freeze-dried formulations and is known to crystallize as an unstable mannitol hemihydrate (MHH). Although, the conditions of formation of MHH are well established, its dehydration kinetics and the impact of the release of lattice water on the formulation stability is not known. The next set of objectives were (i) to investigate MHH dehydration kinetics at different conditions (relative humidity and temperature) in lyophilized mannitol formulations (with and without sucrose), and (ii) to determine the impact of MHH dehydration followed by sucrose crystallization on protein stability. Finally, in protein drug substances which are stored frozen, the use of mannitol as a crystallizing excipient and its role in generating a homogeneous freeze-concentrate was explored. Overall, the thesis highlights the importance of a multidisciplinary approach, using a variety of complementary characterization tools, to gain insights into protein stability.