Closed-Loop MAS-DNP-NMR at 30 K for Affordable Dramatic Gains in S/N

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 DESCRIPTION (provided by applicant): NMR is arguably the most powerful analytical technique for structure determination and function elucidation in molecules of all types, but there are complex challenges for biological macromolecules. Dynamic Nuclear Polarization (DNP) with Magic Angle Spinning (MAS) has recently demonstrated S/N gains of up to two orders of magnitude at ~100 K (the lower-temperature bound using N2-MAS technology) compared to conventional NMR-MAS in many solids. Despite this enormous benefit, the adaptation rate of DNP will be severely limited by its very high price tag ($2-4M), mostly because DNP experiments at field strengths greater 3.5 T and operating in the 90-110 K range require an expensive gyrotron for the high-power millimeter-wave (mmw) irradiation, and that in turn requires a special magnet with superconducting sweep coils. Preliminary experiments at ~30 K (that consumed liquid helium at the rate of up to 250 L/day) have shown the potential for the use of low-power solid-state mmw sources and order-of-magnitude higher S/N for DNP - if suitable MAS-DNP probes could be developed. However, 30K-MAS-DNP will necessitate helium recycling, improved sample spinner designs, and a number of additional technical advances. This SBIR project will enable 30K-MAS-DNP by developing: (A) the needed high-stability 30-K helium-spinner technology, (B) the needed high-efficiency closed-loop cryogenic helium recycling system, (C) complete quad-resonance and quint-resonance multi-nuclear (1H-19F/X/Y/e- and 1H-19F /X/Y/Z/e-) DNP probes with auto sample exchange and variable temperature (VT) operation from 30 K to 400 K. The probes will be demonstrated at 7 T, 11.7 T, 14 T, and 16.4 T using a combination of facilities at DSI, the National High Magnetic Field Laboratory, and customers. Compared to current state-of-the-art (100 K) MAS-DNP instrumentation, the proposed fully optimized MAS-DNP probes at 30 K will often improve sample throughput by one to two orders of magnitude. The information needed for determining detailed molecular structures could then be obtained on many complex biomolecules, catalysts, and other solids in a few hours rather than days or weeks. As demonstrated during the Phase I: the combination of increased Boltzmann factor, dramatically reduced rf circuit noise temperature, and substantially increased circuit Q, combine to yield most of these throughput gains. Additional significant gains will come from incorporation of a gradient coil for coherence selection. Reduction in system cost will stem from the dramatic increase in T1e below 50 K, enabling the use of low- cost solid-state millimeter-wave sources and standard wide-bore magnets. The helium purification and recycling system this project will develop to enable 30K-DNP will also make it straightforward and cost-effective for the facility to implement recycling fo all the helium it requires for its cryo-magnets. The additional hardware required for helium liquefaction will be relatively minor. Hence, not only will this project advance throughput of solis NMR by more than an order of magnitude, it will also enable cost-effective recycling of helium at the larger NMR facilities.