23 Nov 2021

23 Nov 2021

Review status: this preprint is currently under review for the journal MR.

Correction of field instabilities in biomolecular solid-state NMR by simultaneous acquisition of a frequency reference

Václav Římal1, Morgane Callon1, Alexander A. Malär1, Riccardo Cadalbert1, Anahit Torosyan1, Thomas Wiegand1, Matthias Ernst1, Anja Böckmann2, and Beat H. Meier1 Václav Římal et al.
  • 1Physical Chemistry, ETH Zurich, Zurich, CH-8093, Switzerland
  • 2Molecular Microbiology and Structural Biochemistry, UMR 5086 CNRS/Université de Lyon, 69367 Lyon, France

Abstract. With the advent of faster magic-angle spinning (MAS) and higher magnetic fields, the resolution of biomolecular solid-state nuclear magnetic resonance (NMR) spectra has been continuously increasing. As a direct consequence, the always narrower spectral lines, especially in proton-detected spectroscopy, are also becoming more sensitive to temporal instabilities of the magnetic field in the sample volume. Field drifts in the order of tenths of ppm occur after probe insertion or temperature change, during cryogen refill, or are intrinsic to the superconducting high-field magnets, particularly in the months after charging.

As an alternative to a field‒frequency lock based on deuterium solvent resonance rarely available for solid-state NMR, we present a strategy to compensate non-linear field drifts using simultaneous acquisition of a frequency reference (SAFR). It is based on the acquisition of an auxiliary 1D spectrum in each scan of the experiment. Typically, a small-flip-angle pulse is added at the beginning of the pulse sequence. Based on the frequency of the maximum of the solvent signal, the field evolu-tion in time is reconstructed and used to correct the raw data after acquisition, thereby acting in its principle as a digital lock system. The general applicability of our approach is demonstrated on 2D and 3D protein spectra during various situations with a non-linear field drift. SAFR with small-flip-angle pulses causes no significant loss in sensitivity or increase in exper-imental time in protein spectroscopy. The correction leads to the possibility of recording high-quality spectra in a typical biomolecular experiment even during non-linear field changes in the order of 0.1 ppm h−1 without the need for hardware solu-tions, such as stabilizing the temperature of the magnet bore. The improvement of linewidths and peak shapes turns out to be especially important for 1H-detected spectra under fast MAS, but the method is suitable for the detection of carbon or other nuclei as well.

Václav Římal et al.

Status: open (until 21 Dec 2021)

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Václav Římal et al.

Václav Římal et al.


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Short summary
Through the advent of fast magic-angle spinning and high magnetic fields, the spectral resolution of solid-state NMR spectra has recently been greatly improved. To take full advantage of this gain, the magnetic field must be stable over the experiment time of hours or even days. We thus monitor the field by simultaneous acquisition of a frequency reference (SAFR), and use this information to correct multidimensional spectra improving resolution and availability of productive magnet time.