Sci. Adv. 2021
Experimental setup of the MAS solid-state NMR experiment of insoluble proteins.
(A) The sample is placed inside a rotor that is oriented at 54.7° with respect to the static magnetic field and rotated within a solenoid coil, which allows the application of rf pulses to manipulate nuclear magnetization. Upon sample rotation, molecules experience periodical modulations of the rf field due to spatial inhomogeneity. Magnetic field lines are drawn schematically. (B) Protein molecules are contained in microcrystals that are randomly oriented in a powder. (C) Atomic level protein structure with arrows illustrating NCA and NCO magnetization transfer pathways between the amide nitrogen and Cα/C′ carbons (NCA/NCO transfers) of the protein backbone. The relative orientation of a bond vector with respect to the external static magnetic field determines the size of the dipolar interaction between the two atoms. The scale on the right indicates typical order of magnitude for object dimensions.
Zdeněk Tošner and Bernd Reif Research Groups
Significance
Dipolar recoupling is a central concept in the nuclear magnetic resonance spectroscopy of powdered solids and is used to establish correlations between different nuclei by magnetization transfer. The efficiency of conventional cross-polarization methods is low because of the inherent radio frequency (rf) field inhomogeneity present in the magic angle spinning (MAS) experiments and the large chemical shift anisotropies at high magnetic fields. Very high transfer efficiencies can be obtained using optimal control–derived experiments. These sequences had to be optimized individually for a particular MAS frequency. We show that by adjusting the length and the rf field amplitude of the shaped pulse synchronously with sample rotation, optimal control sequences can be successfully applied over a range of MAS frequencies without the need of reoptimization. This feature greatly enhances their applicability on spectrometers operating at differing external fields where the MAS frequency needs to be adjusted to avoid detrimental resonance effects.
Tosner, Z.; Brandl, M. J.; Blahut, J.; Glaser, S. .J. & Reif, B.: Maximizing efficiency of dipolar recoupling in solid-state NMR using optimal control sequences, Sci. Adv. 2021, 7(42), abj5913, https://doi.org/10.1126/sciadv.abj5913