Pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) has become a method of choice for the determination of random motion (i.e., translational diffusion or self-diffusion) of molecules and small particles. This thesis focuses on the development of advanced PGSE NMR methods for conducting self-diffusion measurements on magnetically inhomogeneous (i.e., containing materials with different magnetic susceptibilities), aqueous or slowly diffusing samples. PGSE NMR diffusion measurements rely on the accurate encoding of the position of each diffusing particle by the use of pulses of spatially well-defined magnetic gradients. However, for a magnetically inhomogeneous sample, local magnetic field gradients (i.e., background gradients) with unknown intensities and directions, which are normally generated by the differences in magnetic susceptibilities inside and/or around the sample, can also lead to the position encoding of the diffusing particles. This position encoding may couple with the position encoding caused by the applied pulsed gradients and thus cause errors in PGSE NMR diffusion measurements. Although in numerous cases background gradients are assumed to be spatially and temporally constant in order to simply the analysis of the background gradient based position encoding, in general the background gradient experienced by each diffusing particle is non-constant both spatially and temporally because of the complex sample structure and particles diffusing through different magnetic environments. To suppress the deleterious effects of the (non-constant) background gradients, a new stimulated-echo (STE)-based PGSE method with the intensities of pulsed gradients at a magic ratio (MAG), MAG-PGSTE, was developed for the determination of self-diffusion in magnetically inhomogeneous samples. The method was tested on two water saturated glass bead packs (i.e., 212-300 μm and less than 106 μm glass beads). The MAG-PGSTE method was compared to the magic asymmetric gradient STE-based PGSE (MAGSTE or MPFG) method, which is newly developed for the suppression of the deleterious effects of the (non-constant) background gradients in only one transient, and Cotts 13-interval method, one of the most successful suppression methods based on the assumption of constant background gradients, using both glass bead samples. The MAG-PGSTE and MAGSTE method outperformed the Cotts 13-interval method in the measurement of diffusion coefficients; more interestingly, for the less than 106 μm glass bead sample with smaller bead sizes and thus higher background gradients, the MAG-PGSTE method provided higher signal-to-noise ratios and thus better diffusion measurements than the MAGSTE and Cotts 13-interval methods. In addition, when using relatively long pulsed gradients (e.g., 3 ms), the MAG-PGSTE method provided good bead size characterizations. Due to solubility problems, limited sample availability and/or aggregation, solvent (e.g., water) signals in NMR are typically 4-5 orders of magnitude higher than the solute signals. The huge solvent signal not only overlaps with the signals of interest but also saturates the NMR receiver and thus prevents the detection of the weak signals from the molecules of interest. To achieve a high degree of water suppression in PGSE NMR diffusion measurements, a new STE-based PGSE method incorporating the WATERGATE method, which is based on the selective manipulation of the water signal by the use of a binomial-like selective inversion sequence (i.e., a group of radio frequency (RF) pulses with symmetric pulse duration arrangement separated by equal delays), PGSTE-WATERGATE, was developed. The method is simple to set up and particularly suited to measuring diffusion coefficients in aqueous solution, which is commonly required in pharmaceutical and combinatorial applications. The method was tested on a sample containing lysozyme in 10% 2H2O and 90% 1H2O and a sample containing sucrose in 10% 2H2O and 90% 1H2O and provided superb water suppression and accurate diffusion measurements. Importantly, the new method provided the high degree suppression of the phase distortions in NMR spectra caused by the use of selective inversion pulses or sequences. In the NMR spectrum of an aqueous (bio-molecular) sample, especially a protein sample, the water signal almost always overlaps with the signals of interest. Therefore, a good water suppression technique should target only the water signal. The selectivity of the PGSTE-WATERGATE method depends on the selectivity of the binomial-like selective inversion sequence. The traditional way to enhance the selectivity of a binomial-like sequence is to increase the number of RF pulses contained in the binomial-like sequence. This type of selectivity enhancement is very effective but leads to longer sequence durations and thus the signal loss due to NMR relaxation. In this research, two 6-pulse phase-modulated binomial-like inversion sequences were developed by simultaneously optimizing the RF pulse durations and phases instead of increasing the number of the RF pulses. In combination with the excitation sculpting method, which contains two WATERGATE unit and affords the suppression of phase distortions caused by the phase modulations on the binomial-like sequences, both of the new binomial-like sequences outperformed the well-known 3-9- 19 sequence, a 6-pulse binomial-like sequence with a pulse duration ratio of 3:9:19:19:9:3, in selectivity and inversion width. The new sequences provided the similar selectivity and inversion width to the W5 sequence, a 10-pulse binomial-like sequence, but with significantly shorter sequence durations. When used in PGSTEWATERGATE, they afforded highly selective water suppression in diffusion experiments. The hydrodynamics of slowly diffusing molecules (e.g., protein) and small particles (e.g., protein aggregates) has long been of scientific interest due to its importance in the understanding of many biological and biophysical processes such as protein crystallization. However, the diffusion measurements on these molecules and small particles confront the PGSE NMR methods with huge challenges because the diffusion coefficients of these molecules and small particles are normally at or under the lower diffusion determination limit of the PGSE NMR methods, which is mainly controlled by the maximum pulsed gradient strength of the NMR probe (i.e., high pulsed gradient strengths are preferred for measuring slow diffusion), the pulsed gradient durations (i.e., long gradient durations are preferred for measuring slow diffusion), and the observation time (i.e., long observation times are preferred for measuring slow diffusion) which is limited by the NMR relaxation times of the molecules and small particles. To overcome this problem, multi-quantum NMR coherences may be utilized in PGSE NMR experiments as these coherences are more sensitive to the encoding by the pulsed gradients than normal single quantum coherences (e.g., the encoding effect of a pulsed gradient on a single quantum coherence may be doubled by using a double-quantum coherence). In this research, five new multi-quantum coherence encoding STE-based PGSE methods were successfully developed. The new multi-quantum PGSE methods were tested on a sample containing L-[3-13C]-alanine in 2H2O. The quadruple quantum and triple quantum coherences from 13CH3, which can greatly enhance the encoding by pulsed gradients, were successfully selected using these new sequences. This thesis contains eight chapters. In Chapter 1, the focus of this thesis, translational diffusion, is introduced and the difficulties in the PGSE NMR diffusion measurements on magnetically inhomogeneous, aqueous, and slowly diffusing samples are also introduced. In Chapter 2, to facilitate the understanding of how PGSE NMR diffusion measurements work, both the basic principles and simulation methods of NMR and the fundamentals of PGSE NMR diffusion measurements are given. In Chapter 3, a brief review on the current advanced PGSE NMR methods for measuring diffusion in magnetically inhomogeneous, aqueous, and slowly diffusing samples is given. In Chapter 4, the experimental details of the research work in this thesis are given. From Chapter 5 to Chapter 7, newly developed PGSE NMR methods and their applications are discussed in detail. In Chapter 8, the general conclusions are given. / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:ADTP/233140 |
| Date | January 2008 |
| Creators | Zheng, Gang, University of Western Sydney, College of Health and Science, School of Biomedical and Health Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
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