Development of Solid State NMR Methods and Technology

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Progress in FY2014 was made in the following areas: (1) MECHANISMS OF DYNAMIC NUCLEAR POLARIZATION IN LOW-TEMPERATURE SOLID STATE NMR. Dynamic nuclear polarization (DNP) is a physical phenomenon in which couplings between electron spins and nuclear spins allows the large spin polarization of electrons to be transferred to nuclei when the electron spins are irradiated by resonant microwaves. The large spin polarization greatly enhances the strength of nuclear magnetic resonance signals, allowing NMR measurements to be carried out on much smaller or more dilute samples. In recent work, we developed a comprehensive theory that explains the principal DNP mechanism in low-temperature solid state NMR experiments. In FY2014, we demonstrated that rapid sample rotation alone, even in the absence of microwave irradiation, leads to a strong perturbation of nuclear spin polarizations. Rapid sample rotation is almost always employed in biomolecular solid state NMR measurements, so these results have important practical consequences. (2) DEVELOPMENT OF TRIRADICAL POLARIZING AGENTS FOR DNP. We have synthesized a family of chemical compounds, each containing three nitroxide free-radical groups, that can be used in DNP measurements. The new compounds have improved solubilities in aqueous solutions near neutral pH, making them compatible with most biomolecular solid state NMR measurements. We find that these triradical compounds produce larger net NMR signal enhancements and more rapid build-up of signals than related compounds that are used by other groups. (3) MRI MICROSCOPY. We have designed and construct a magnetic resonance imaging (MRI) system for studies of single cells and cell clusters. The MRI system contains a radio-frequency microcoil (150 micron diameter) and three-axis field gradient coils, all mounted within a set of stacked sapphire plates within a volume of approximately 1 cubic centimeter. Field gradients on the order of 1 Gauss per micron can be produced with 100 amp current pulses, implying that 3D images with isotropic spatial resolution below 1 micron are feasible, in principle. Test images of "phantom" samples (20-micron polystyrene beads in water) indicate that 3D isotropic resolution of 5 microns can be obtained at room temperature, limited by signal-to-noise considerations. Experiments at low temperatures are planned, where signal-to-noise will be inherently higher and image resolution can consequently also be higher, especially with DNP enhancements of signal strengths.