Development of Solid State NMR Methods and Technology

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Progress in FY2013 was made in the following areas: (1) LOW-TEMPERATURE DYNAMIC NUCLEAR POLARIZATION (DNP). DNP is a phenomenon in which irradiation of electron spin transitions with microwaves leads to enhancements of nuclear spin polarizations, and hence enhancements of NMR signals. We have constructed a new magic-angle spinning probe with DNP capabilities that operates down to 20 K. DNP enhancements greater than 20X have been achieved, with microwave powers around 30 mW, at 20-25 K and with MAS at 7 kHz. With a new "extended interaction oscillator" (EIO) source that produces 700 mW of microwave power, DNP signal enhancements up to 200X have been achieved. This technology now permits solid state NMR studies of a variety of systems that were previously inaccessible due to low signal-to-noise. Demonstrations and tests on transient intermediate species in amyloid fibril formation pathways and peptide/protein complexes (M13/calmodulin) have been performed and will be pursued further in FY14. (2) THEORY OF DNP. We have developed the first comprehensive theoretical understanding of dynamic nuclear polarization under magic-angle spinning (MAS). The theory takes into account the periodic time dependence of nuclear and electron spin energy levels due to sample rotation and shows that DNP (i.e., transfer of electron spin polarizations to nuclear spins) occurs through a series of population transfers at time-dependent level crossings. The theory also predicts that MAS alone can perturb nuclear spin polarizations in nitroxide-radical-doped samples, with application of microwaves. This effect has been verified by experiments. (3) DEVELOPMENT OF TRIRADICAL POLARIZING AGENTS FOR DNP. We have synthesized and tested a series of tri-nitroxide compounds with related chemical structures and varying solubilities, as well as bi-nitroxide and tetra-nitroxide compounds. We find that tri-nitroxide compounds produce the largest DNP enhancements at temperatures around 25 K under MAS, and thus are the preferred compounds for our DNP experiments. (4) DNP-ENHANCED MRI MICROSCOPY. We have initiated a project to design and construct a magnetic resonance imaging system capable of sub-micron resolution. This MRI system will operate at low temperatures (<10 K) and use dynamic nuclear polarization to enhance proton NMR signals, ultimately enabling signals from voxels with < 1 micron dimensions to be detected. A prototype has been constructed and tested at room temperature, based on a surprisingly simple design in which magnetic field gradient coils and RF coils are mounted on a series of stacked sapphire plates. 3D images of "phantom" samples have been obtained successfully, so far with spatial resolution of approximately 5 microns in each direction. Attempts to reach 2 micron resolution at room temperature are in progress, which would represent a world record for MRI microscopy. Low temperature experiments will be performed in FY14.