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Enhancing the sensitivity of biomolecular MAS NMR by new methods of Dynamic Nuclear Polarization

Subject Area Analytical Chemistry
Solid State and Surface Chemistry, Material Synthesis
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 321027114
 
Final Report Year 2024

Final Report Abstract

Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) is a tremendously informative spectroscopic technique for anyone with a question about a chemical system in the solid state. MAS NMR explores structure and dynamics at the atomic level, often in situations where other techniques fail. MAS NMR is, however, not very sensitive and this restricts practical applications. Dynamic nuclear polarization (DNP) is an effective means to enhance the sensitivity of MAS NMR. In DNP, the relatively plentiful magnetization of unpaired electron spins is transferred to nuclear spins by means of microwave irradiation. Currently this microwave irradiation is always applied continuously and with the highest possible power. While this strategy has, without question, increased the versatility and impact of MAS NMR, it also has clear limitations. These are most dearly felt in the study of complex biomolecules, which require, besides sensitivity, a very high resolution and consequently MAS NMR at a very high magnetic field. In this project, we aimed to address these limitations through the development of new methods of DNP. Specifically, we proposed to apply microwave irradiation in the form of a pulse sequence, instead of continuously. Microwave sources for generating these pulse sequences did not (and do not) exist at frequencies compatible with high resolution MAS NMR, with the exception of a few proto-types. We felt that it would nevertheless be important to start to explore pulsed DNP, for the time being at low magnetic field. Over the course of the project, we assembled instrumentation for pulsed DNP at 0.34 T and at 1.2 T. We successfully developed pulse sequences, analytical theory, and numerical simulations. We even got microwave engineers engaged and the first test experiments have been done at 3.4 T. Moreover, the design and construction of a source at 9.4 T is in progress. Magnetization transfer from electron spins to nuclear spins can be considered an optimal control problem, which provides a mathematical argument why pulsed DNP has to be more efficient than continuous-wave DNP. The optimal DNP pulse sequence we have not yet found, but we have laid a foundation for further investigations. The added flexibility of pulsed DNP should make it possible to tailor microwave excitation to a problem at hand. Thus, once pulsed MAS DNP comes of age, it is likely to provide new insight into hitherto unexplored heterogeneous materials and complex biomolecules. To illustrate what is possible today with standard MAS NMR spectroscopy, i.e. without DNP, we present two case studies.

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