UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item THE ASSEMBLY STATE OF A BACTERIAL MICROCOMPARTMENT AND ITS RELATED STRUCTURES(2022) Trettel, Daniel; Winkler, Wade; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bacterial microcompartments (BMCs) are polyhedral, protein-based organelles present in a wide range of bacteria. This mode of compartmentalization is highly modular and can accommodate a wide range of chemistries within them, including carbon fixation. These aspects make them a promising target to serve as bioplatforms for commodity chemical synthesis and enhanced carbon fixation. However, it is challenging to investigate the structure and function of BMCs using classical methods. As such, the native structure of BMCs remains largely enigmatic, hampering their synthetic adaption. This dissertation addresses these concerns by describing the assembly state of the model 1,2-propanediol (Pdu) BMC using a variety of approaches. Chemical probing reveals the Pdu BMC is surprisingly permeable to and permissive of derivatization. This insight enabled application of crosslinking mass spectrometry to describe its protein interactome. The interactome map reveals that small domains called encapsulation peptides dominate interior interactions while reporting on the organization of the outer protein shell. Laser scanning confocal approaches were developed to study the solution behavior of BMCs. These experiments heavily suggest that the Pdu BMC is a dynamic entity that exchanges protein elements; a result we primarily attribute to the protein shell. These confocal microscopy approaches were further used to study the super-structures formed by individual shell proteins and to describe their interactions with one another. Together, the results from this project give important insight on the assembly state of the model Pdu BMC including its biogenesis, organization, and behavior. These data answer some of the open questions concerning the assembly of BMC structures, which will help innovate the next generation of BMC-based biotechnology tools.Item The Novel Use of Nitroxide Antioxidants as Free Radical Scavengers in Ultra-High Molecular Weight Polyethylene (UHMWPE) for Total Joint Replacements(2010) Chumakov, Marina Konstantinovna; Al-Sheikhly, Mohamad; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-high molecular weight polyethylene (UHMWPE) has been the standard load-bearing material used in total joint replacements since the 1960s. However, oxidative degradation can lead to premature aging and wear of UHMWPE, requiring implant revision. The novel use of nitroxide antioxidants to prevent oxidation in UHMWPE was proposed and the resulting structure and property changes were evaluated in this work. Standard sterilization and crosslinking methods of Co-60 gamma or high energy electron beam radiation produce alkyl free radicals on the polymer chain. Alkyl radicals react to form bimolecular crosslinks and long-lived allyl radicals at high dose rates; at low dose rates they tend to react with oxygen to form peroxyl radicals. The peroxyl radicals further interact with the polymer chain producing hydroperoxides and more free radicals, leading to oxidative degradation. As an alternative to post-irradiation remelting, which allows radical recombination but reduces fatigue strength, antioxidants can be introduced into UHMWPE to scavenge residual radicals. Nitroxides are stable organic compounds that have a strong paramagnetic signal and are very efficient in preventing lipid peroxidation and in providing radioprotection in biological tissues. The nitroxides used are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (Tempol). Through radical-radical interactions, radiolytically-produced alkyl radicals in UHMWPE are scavenged by the nitroxide radical. This is demonstrated through Electron Paramagnetic Resonance (EPR) spectroscopy where the paramagnetic nitroxide signal decays as it interacts with carbon-centered radicals in UHMWPE. Pulse radiolysis kinetics studies also show that alkyl radicals in UHMWPE preferentially react with nitroxides in the absence of oxygen. Controlled infiltration of UHMWPE with nitroxides is also observed using EPR. The resulting crosslink densities were investigated using Thermomechanical Analysis. It was observed that the addition of nitroxides after irradiation does not alter the crosslink density. Tensile testing of crosslinked and nitroxide-doped UHMWPE demonstrates increased ultimate tensile strength and toughness and the material exhibits an increase in crystallinity. Additionally, accelerated aging of specimens containing trace levels of nitroxide show very low oxidation levels when placed in an aggressive oxygen environment. Consequently, low concentrations of nitroxides diffused into UHMWPE after crosslinking produce an oxidation-resistant and highly crosslinked material for improved implant performance.