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

Abstract

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 parts per million 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 evolution 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 experimental 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 solutions, 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.

High and fast: NMR protein–proton side-chain assignments at 160 kHz and 1.2 GHz

“…S2 †) and was recorded using the simultaneous acquisition of a frequency reference (SAFR). 39,44 As well, additional 3D-hCCH and 3D-HcCH TOBSY experiments were recorded using a WALTZ-16 bloc 40 for the TOBSY transfer with 30 kHz rf amplitude and with a length of 11 ms. The inter-scan delay for the experiments has been set to the value of 1.27T 1 ( 1 H ali ) based on the measured bulk protein T 1 ( 1 H ali ) relaxation time.…”