Solid-state nitrogen-14 nuclear magnetic resonance enhanced by dynamic nuclear polarization using a gyrotron

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Biophysical Chemistry
• Chemical Physics

Background:

• Solid-state NMR spectroscopy is crucial for determining the structure and dynamics of biomolecules.
• Acquiring high-quality 14N NMR spectra often requires long acquisition times due to low signal-to-noise ratios and long relaxation delays.
• Proton relaxation in certain samples, like proline at room temperature, is inefficient, necessitating extended experimental durations.

Purpose of the Study:

• To investigate the application of dynamic nuclear polarization (DNP) combined with indirect 14N detection to enhance solid-state NMR experiments.
• To assess the improvement in signal-to-noise ratio and reduction in experimental time using DNP.
• To evaluate the efficiency of DNP in overcoming long proton relaxation times in solid samples.

Main Methods:

• Utilized a 400 MHz solid-state NMR spectrometer equipped with a 263 GHz gyrotron for microwave irradiation.
• Employed dynamic nuclear polarization (DNP) with the stable bi-radical TOTAPOL.
• Acquired 14N solid-state NMR spectra of glassy proline samples doped with TOTAPOL, rotating at 15.625 kHz at 110 K.

Main Results:

• Achieved DNP enhancement factors of approximately 40.
• Reduced recovery delays from 60 seconds (without radicals at 300 K) to 6 seconds (with radicals at 110 K).
• Decreased acquisition times for 13C-detected 14N spectra from several days to a few hours.

Conclusions:

• DNP significantly improves signal-to-noise ratios and shortens experimental recovery delays in 14N solid-state NMR.
• This DNP-enhanced approach drastically reduces overall acquisition times, making complex solid-state NMR analyses more feasible.
• The method is particularly effective for samples with inefficient proton relaxation, such as proline.