Abstract
Magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments at ultra low temperature (ULT) (≪ 100 K) have demonstrated clear benefits for obtaining large signal sensitivity gain and probing spin dynamics phenomena at ULT. ULT NMR is furthermore a highly promising platform for solid-state dynamic nuclear polarization (DNP). However, ULT NMR is not widely used, given limited availability of such instrumentation from commercial sources. In this paper, we present a comprehensive study of hydrated [U-13C]alanine, a standard bio-solid sample, from the first commercial 14.1 Tesla NMR spectrometer equipped with a closed-cycle helium ULT-MAS system. The closed-cycle helium MAS system provides precise temperature control from 25 K to 100 K and stable MAS from 1.5 kHz to 12 kHz. The 13C CP-MAS NMR of [U-13C]alanine showed 400% signal gain at 28 K compared with at 100 K. The large sensitivity gain results from the Boltzmann factor, radio frequency circuitry quality factor improvement, and the suppression of its methyl group rotation at ULT. We further observed that the addition of organic biradicals widely used for solid-state DNP significantly shortens the 1H T1 spin lattice relaxation time at ULT, without further broadening the 13C spectral linewidth compared to at 90 K. The mechanism of 1H T1 shortening is dominated by the two-electron-one-nucleus triple flip transition underlying the Cross Effect mechanism, widely relied upon to drive solid-state DNP. Our experimental observations suggest that the prospects of MAS NMR and DNP under ULT conditions established with a closed-cycle helium MAS system are bright.
| Original language | English (US) |
|---|---|
| Article number | 107090 |
| Journal | Journal of Magnetic Resonance |
| Volume | 333 |
| DOIs | |
| State | Published - Dec 2021 |
Funding
We thank JEOL RESONANCE Inc. Japan; JEOL USA Inc.; and Cryovac Corp. Japan, for closely collaborating with us on the installation of the 14.1 T JEOL NMR instrument with a closed-cycle helium MAS system. We would especially like to thank Mr. Hiroto Suematsu from JEOL RESONANCE Inc. Japan for the generous customer support and technology development. We would like to thank the Department of Chemistry and Biochemistry at the University of California Santa Barbara for providing the lab space and policies that allowed for this successful installation during the COVID-19 pandemic. We would like to thank Prof. Yangping Liu at the School of Pharmacy, Tianjin Medical University, Tianjin, China, for synthesizing and providing the TEMTriPol-1 radical. This work was supported by the National Science Foundation Grant CHE CMI #2004217. We thank JEOL RESONANCE Inc., Japan; JEOL USA Inc.; and Cryovac Corp., Japan, for closely collaborating with us on the installation of the 14.1 T JEOL NMR instrument with a closed-cycle helium MAS system. We would especially like to thank Mr. Hiroto Suematsu from JEOL RESONANCE Inc., Japan for the generous customer support and technology development. We would like to thank the Department of Chemistry and Biochemistry at the University of California Santa Barbara for providing the lab space and policies that allowed for this successful installation during the COVID-19 pandemic. We would like to thank Prof. Yangping Liu at the School of Pharmacy, Tianjin Medical University, Tianjin, China, for synthesizing and providing the TEMTriPol-1 radical. This work was supported by the National Science Foundation Grant CHE CMI #2004217.
Keywords
- Closed-cycle helium circulation
- Dynamic Nuclear Polarization
- ULT DNP
- ULT NMR
ASJC Scopus subject areas
- Condensed Matter Physics
- Nuclear and High Energy Physics
- Biophysics
- Biochemistry