Browsing by Subject "Nuclear Magnetic Resonance"
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Item Mixing laws and fluid substitution for interpretation of magnetic resonance measurements(2016-12) Ravi, Vivek R; Torres-Verdin, CarlosNuclear magnetic resonance (NMR) relaxation time measurements are affected by pore structure and saturating fluids. Interpretation of NMR distributions as pore-size distributions and estimation of permeability from NMR logs using methods such as the Schlumberger Doll Research (SDR) model assume a homogeneous surface relaxivity and remain reliable only for measurements obtained from homogeneous single-fluid saturated rocks. However, heterogeneous rock formations commonly consist of laminations, vugs, and a mixed solid composition, which result in non-uniform values of surface relaxivity. Furthermore, most rock formations penetrated by wells contain multiple fluids and are commonly affected by mud-filtrate invasion. Therefore, presence of spatial heterogeneity and multiple fluids in rock formations render the petrophysical interpretation and analysis of longitudinal relaxation time T1 and transverse relaxation time T2 measurements challenging. Thus, it is necessary to correct for spatial heterogeneity by decomposing the NMR response of the heterogeneous formation into that of its homogeneous components. Presence of multiple fluids is corrected by replacing the hydrocarbon NMR response in the original logs with the corresponding water response in order to obtain NMR distributions of the 100% water saturated formation. Subsequently, the petrophysical quantities of interest such as permeability and pore-size distribution are determined. NMR mixing laws define the physics of how NMR data from different homogeneous components combine. It was observed that a linear mixing law best describes a laminated formation and a non-linear mixing law best describes a dispersed formation. In this work, NMR mixing laws are derived and applied to give a better interpretation of NMR logs obtained from highly heterogeneous formations by extracting the NMR distributions of each homogeneous component. A physics-based NMR fluid substitution method is also developed, which takes into account capillary-pressure effects and pore-size distributions and does not require knowledge of permeability and surface relaxivity. The method consists of two steps. First, the hydrocarbon NMR response is removed from the initially water-hydrocarbon saturated NMR data. Next, the NMR distribution of the resulting hydrocarbon-depleted system is transformed to that of a completely water-saturated system. Several laboratory measurements and field cases are used to successfully verify the mixing laws and the fluid substitution method.Item Nuclear magnetic resonance imaging and analysis for determination of porous media properties(Texas A&M University, 2007-04-25) Uh, JinsooAdvanced nuclear magnetic resonance (NMR) imaging methodologies have been developed to determine porous media properties associated with fluid flow processes. This dissertation presents the development of NMR experimental and analysis methodologies, called NMR probes, particularly for determination of porosity, permeability, and pore-size distributions of porous media while the developed methodologies can be used for other properties. The NMR relaxation distribution can provide various information about porous systems having NMR active nuclei. The determination of the distribution from NMR relaxation data is an ill-posed inverse problem that requires special care, but conventionally the problem has been solved by ad-hoc methods. We have developed a new method based on sound statistical theory that suitably implements smoothness and equality/inequality constraints. This method is used for determination of porosity distributions. A Carr-Purcell-Meiboom-Gill (CPMG) NMR experiment is designed to measure spatially resolved NMR relaxation data. The determined relaxation distribution provides the estimate of intrinsic magnetization which, in turn, is scaled to porosity. A pulsed-field-gradient stimulated-echo (PFGSTE) NMR velocity imaging experiment is designed to measure the superficial average velocity at each volume element. This experiment measures velocity number distributions as opposed to the average phase shift, which is conventionally measured, to suitably quantify the velocities within heterogeneous porous media. The permeability distributions are determined by solving the inverse problem formulated in terms of flow models and the velocity data. We present new experimental designs associated with flow conditions to enhance the accuracy of the estimates. Efforts have been put forth to further improve the accuracy by introducing and evaluating global optimization methods. The NMR relaxation distribution can be scaled to a pore-size distribution once the surface relaxivity is known. We have developed a new method, which avoids limitations on the range of time for which data may be used, to determine surface relaxivity by the PFGSTE NMR diffusion experiment.Item The Application of Dynamic Nuclear Polarization Enhanced NMR to Non-Equilibrium Systems(2012-02-14) Bowen, Sean MichaelNuclear magnetic resonance (NMR) yields remarkably detailed structural information about virtually any molecule. However, its application to non-equilibrium systems is hampered by a lack of sensitivity. To increase the amount of signal that can be obtained from a NMR experiment, various hyperpolarization schemes have been previously introduced. One such technique is dynamic nuclear polarization (DNP), which can enhance NMR sensitivity by several orders of magnitude. The work detailed here focuses on the development of methods utilizing DNP to study non-equilibrium systems such as chemical and biochemical reactions in real-time. To work with hyperpolarized samples, we have designed and constructed a rapid injection and mixing system. This system allows samples to be transported between superconducting magnets used for polarization and for NMR spectroscopy in less than two seconds. Rapid transport is essential for successful use of samples with short spin-lattice relaxation times. For the study of reactions under non-equilibrium conditions, the system provides the additional capability for samples to be mixed with a second, unpolarized reagent. A chromogenic trypsin catalyzed ester hydrolysis reaction was used to validate the DNP-NMR technique as a tool for kinetic analysis. It is shown that the DNP-NMR method agrees with the conventional UV method within the uncertainty of the measurement. Hyperpolarization in this modality presents both challenges and opportunities, each of which motivate the development of new NMR techniques. In addition to the determination of kinetics, DNP-NMR is amenable to mechanistic analysis of a reaction. We have developed a technique based on selective inversion of spin-polarization, which allows for mapping of atoms between reactant and product of a reaction. This scheme was applied to a Grignard reaction, demonstrating applicability to organic reactions. Signal averaging, as it is applied for conventional multi-dimensional correlation spectroscopy cannot always be applied easily when using hyperpolarized sample. For the rapid measurement of heteronuclear correlation spectra, we have developed a technique utilizing the differential scaling of scalar coupling under off-resonance irradiation. Although DNP-NMR yields spectra of outstanding quality even with small quantities of sample, peak intensities are not quantitative. It is nevertheless possible to compare peak multiplets obtained from fractionally isotope labeled samples. Using biosynthetically labeled lipids from E. Coli cells, we showed that the resulting labeling patterns reflect their biosynthetic pathways. As a final case-study employing several of these newly developed methods, the uronate isomerase catalyzed isomerization of glucuronate into fructuronate was studied. The ability to follow the reaction in real-time while directly observing all anomeric forms of the reactant and product permits the independent determination of kinetics for each anomeric form of substrate and product. This study revealed the anomeric specificity of the enzyme.Item The Metal-dependent Function of C2?: A Conditional Membrane Domain from Protein Kinase C?(2013-11-26) Morales-Rivera, Krystal A.Protein Kinase C (PKC) isoforms function in signaling pathways responsible for controlling cell proliferation, survival and apoptosis. Up or down-regulation of PKCs has been implicated in cancer progression, cardiovascular dysfunction, and neurological disorders. Moreover, the conventional Protein Kinase Calpha (PKCalpha) has also been identified as an important molecular target in Pb2+ poisoning. Two out of three Pb^(2+) sites in full-length PKCalpha were mapped onto its C2 domain (C2alpha), which associates with anionic membranes in response to binding Ca^(2+) ions in the first step of activation. The objective of this work was to determine the specific role of divalent metal ions on the modulation of C2alpha structure, function, and interactions with other PKCalpha domains. Nuclear magnetic resonance (NMR) and F?rster resonance energy transfer were used to characterize the specific role of divalent metal ions in C2alpha membrane-interactions. Pb^(2+) and Cd^(2+) ions bind C2alpha with high affinity. Pb^(2+) drives C2alpha association with lipid-membranes, whereas Cd^(2+) does not support membrane-binding. This work provides direct evidence for the specific role of divalent metal ions in mediating protein-membrane interactions and illustrates the opposite responses produced by toxic metal ions in a single molecular target. The structures of metal-free (1.9 ?) and Pb2+-bound (1.5 ?) C2alpha were determined using X-ray crystallography. These revealed a remarkable coexistence of hemi- and holo-directed coordinated geometries for the two Pb^(2+) ions that are bound to the Ca^(2+) binding loops (CBLs). The overall backbone conformation does not change upon metal-binding. However, elevated B-factors were observed in the CBLs of metal-free C2alpha, suggesting that this region is dynamic. NMR techniques were used to identify the dynamic regions and to quantify the timescales of C2alpha motions in different states of metal ligation. Metal-binding quenches the microsecond-timescale motions of the CBLs but results in elevated millisecond-timescale dynamics of the N- and C-terminal regions. These regions are implicated in the interactions of C2 with other regulatory domains of PKC. Our data suggest that the changes in protein dynamics is a mechanism by which the information about the metal-binding event propagates to other regions of the protein. We then extended our structural and dynamical studies to the two-domain construct of PKCalpha comprising two regulatory domains, C1B-C2. NMR chemical shift perturbation and dynamic studies showed that these domains do not behave as independent modules in solution. The linker region connecting the functional modules shows significant chemical shifts and conformational dynamic changes induced by Ca^(2+)-binding. These results support our hypothesis that Ca^(2+)-binding triggers the rearrangement of the C1B and C2 domains. The strategy of using paramagnetic relaxation enhancement experiments to refine the C1B-C2 structure is presented. A prevailing view in the PKC field is that the function of metal ion is to alter the electrostatic potential of the C2 domain and thereby facilitate the protein insertion into the negatively charged membranes. The results of our work support the multi-faceted role of metal ions, which includes the formation of specific coordination bonds with lipid head groups, as well as the modulation of protein dynamics and inter-domain orientation.