Frequency-swept dynamic nuclear polarization

• Published in: Journal of magnetic resonance (1997) Vol. 353; p. 107511
• Main Authors: Mardini, Michael, Palani, Ravi Shankar, Ahmad, Iram M., Mandal, Sucharita, Jawla, Sudheer K., Bryerton, Eric, Temkin, Richard J., Sigurdsson, Snorri Th, Griffin, Robert G.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.08.2023
• ISSN: 1090-7807; 1096-0856; 1096-0856
• DOI: 10.1016/j.jmr.2023.107511

[Display omitted] •Considerations for the construction of solid-state microwave sources for pulsed DNP.•Advantages of arbitrary-waveform driven microwave sources.•Results from a 250 GHz MAS-DNP spectrometer in several contexts.•Outlook and future directions for high-field pulsed DNP. Dynamic nuclear polarization (DNP) improves the sensitivity of NMR spectroscopy by the transfer of electron polarization to nuclei via irradiation of electron-nuclear transitions with microwaves at the appropriate frequency. For fields > 5 T and using g ∼ 2 electrons as polarizing agents, this requires the availability of microwave sources operating at >140 GHz. Therefore, microwave sources for DNP have generally been continuous-wave (CW) gyrotrons, and more recently solid state, oscillators operating at a fixed frequency and power. This constraint has limited the DNP mechanisms which can be exploited, and stymied the development of new time domain mechanisms. We report here the incorporation of a microwave source enabling facile modulation of frequency, amplitude, and phase at 9 T (250 GHz microwave frequency), and we have used the source for magic-angle spinning (MAS) NMR experiments. The experiments include investigations of CW DNP mechanisms, the advantage of frequency-chirped irradiation, and a demonstration of an Overhauser enhancement of ∼25 with a recently reported water-soluble BDPA radical, highlighting the potential for affordable and compact microwave sources to achieve significant enhancement in aqueous samples, including biological macromolecules. With the development of suitable microwave amplifiers, it should permit exploration of multiple new avenues involving time domain experiments.