High field DNP and cryogenic MAS NMR : novel instrumentation and applications

Abstract

Solid State Nuclear Magnetic Resonance (ssNMR) spectroscopy has blossomed over the last two decades. As ssNMR is progressively applied to more challenging systems, the sensitivity remains one of its major limiting factors. Gyrotron based high-field dynamic nuclear polarization (DNP) permits increasing the sensitivity of ssNMR by 1-2 orders magnitude, significantly extending the reach of ssNMR. Successful application of ssNMR/DNP at 5T and 9.4T stimulated interest to extending this technique to higher fields and new applications. Here, the progress toward this goal is presented. It has involved completion of the world highest field magic angle spinning (MAS) DNP spectrometer and a probe for 16.4T, initial DNP experiments on ¹⁷ O nuclei, variable temperature studies of a model tripeptide, and a systematic analysis of a novel approach to high efficiency RF circuit design. The extension of DNP-NMR to 16.4T has required the development of probe technology, cryogenics, gyrotrons, and microwave transmission lines. A novel DNP probe and cryogenic instrumentation permit extended operation at 85-90K and 10kHz MAS. Initial enhancements [epsilon]=-40 and further optimization of experimental conditions is underway. ¹⁷ O detected DNP-NMR of a water/glycerol glass at 5T enabled an 80-fold enhancement of signal intensities at 82K permitting ¹⁷ O- ¹H distance measurements and heteronuclear correlation experiments. Variable temperature MAS NMR studies of a model tripeptide APG in combination with cryogenic calorimetry and XRD revealed a first-order phase transition and severe attenuation of the cross polarization MAS signal in a wide temperature range due to interference between decoupling and 3-fold hopping of the Ala-CH₃ and Ala-NH₃+ groups. A new, efficient strategy for designing balanced transmission line RF circuits for MAS NMR probes based on back propagation of a common impedance node (BPCIN) is presented. In this approach, the impedance node is the sole means of achieving mutual RF isolation and balance in all channels. BPCIN is illustrated using a custom double resonance MAS probe operating at 11.7T.