Nuclear Magnetic Relaxation Dispersion and Pulsed-Gradient Spin Echo Studies of Biomolecular Solutions and Amphiphilic Liquid Crystals

Abstract: Biomolecular solutions and amphiphilic liquid crystals are studied by Nuclear Magnetic Resonance Dispersion (NMRD) and Pulsed-Gradient Spin Echo (PGSE) respectively. A random-flight simulation is performed to obtain the obstructing factors for self-diffusion of small molecules in macrofluids. The accuracy of existing mean-field approximations for the obstruction factors is analyzed. The orientational order and the size of the discoidal micelles in the nematic phase of the binary system CsPFO/water is studied by water self-diffusion measurements using the 2H PGSE NMR technique. The results challenge the theoretical predictions based on hard-particle models, and indicate that soft potentials, such as the anisotropic double-layer repulsion, are important. The 2H NMRD technique is used to characterize interactions of DMSO with globular proteins. The importance of specific binding sites for a stable DMSO-protein complex is demonstrated. The observed NMRD for DMSO/lysozyme solution is found to be due to a single DMSO molecule in the active site of the protein. A quantitative analysis yields information about orientational order and residence time of this DMSO molecule. The hydration of six B-DNA dodecamers with A-tracts of variable length and sequence is investigated via the NMRD of the water 2H and 17O resonances. Highly ordered long-lived water molecules are found in the minor groove of the duplex, with an occupancy which is correlated with the width of the groove. An essentially invariant residence time is found for all long-lived water molecules, suggesting that water exchange occurs from an open state with a uniformly wide minor groove. A model-free approach for analyzing stretched dispersion profiles is proposed. The method is validated with the aid of synthetic relaxation data, and is then applied to water 1H NMRD data from solutions of BPTI. It is demonstrated that a widely used empirical dispersion function for stretched NMRDs, known as ”the Cole-Cole expression”, is unphysical and gives qualitatively different results, as compared to the model-free approach, when used in the analysis of the BPTI data.