Ultra-low-temperature (6 K) static NMR-DNP for metalloproteins, proteins in cells, and materials

Ultra-low-temperature (6 K) static NMR-DNP for metalloproteins, proteins in cells, and materials Abstract The critical importance of solid-state NMR (ssNMR) was recently demonstrated by, after nearly two decades of intense efforts, yielding the first atomic-resolution structures of the A40 and A42 amyloid fibrils that play a crucial role in Alzheimer’s Disease (AD). Challenges posed by the inherently low sensitivity of NMR can be improved by reducing the sample and circuit temperature to below 100 K, or preferably below 35 K – also known as Ultra Low Temperature (ULT). Combining ULT NMR with another emerging technique, denoted as dynamic nuclear polarization (DNP), shows enormous promise for expanding the role of ssNMR by providing significant S/N enhancements in many cases. However, despite the huge gain in S/N that is possible from DNP, generally performed at ~100 K, there are still only a handful of high-field MAS-DNP systems in the U.S. – primarily because they are so expensive ($3-20M). Further, no commercially available instrumentation appears to be available for versatile NMR experiments below 90 K, let alone ULT experiments combined with DNP. A huge step forward would include the development of a high-mode THz cavity for more efficient microwave delivery to the sample, thus enabling the use of a lower power low-cost solid-state microwave source (ss-source). Any area of ssNMR that employs the use of non-spinning (static) samples would benefit immensely from the availability of such a commercially built ULT probe, with an option for inclusion of DNP. Further, the static probe can serve as a prototype for a MAS version of such a probe. The Phase-I effort showed feasibility of a robust, highly effective, tunable, THz cavity compatible with ULT WL and MAS NMR wide-bore probes, but it also revealed that such required the ability to tune and match their THz impedance, at operating conditions, to prevent excessive microwave reflection, which otherwise could destroy the solid-state source. This requires a low-loss high-directivity HE11 bidirectional coupler, but such are not com- mercially available and have not previously been publicly reported. The Phase-I effort showed feasibility of the needed THz directional coupler, and it also showed feasibility of a novel approach to ULT WL and MAS probe design with exceptional cryogenic, microwave, and multi-nuclear RF performances. The developments pro- posed under this Phase-II project aim to reduce entry level cost into both ULT and DNP by more than an order of magnitude while reducing certain operational challenges.