Sample rotor magic-angle spinning

Finally, it is worth keeping in mind that the protein may be perturbed reversibly by the conditions of the NMR experiment. For example, a recent study demonstrated that removal of the bulk buffer that typically separates from the sample during magic angle spinning reversibly altered the conformation of the selectivity filter of the KcsA potassium channel addition of buffer to the rotor restored the conformation.101 Based on this observation of a hydration-induced shift in conformation, it seems prudent to maintain high hydration levels for NMR studies of proteins in general. 

This is the basic concept of Magic Angle Spinning NMR the sample is placed in a rotor, spinning at high speed around an axis at 54,7° to the axis of the applied magnetic field. 

Solid-state experiments use a dry sample that is packed into a rotor and spun at high frequency inside the spectrometer s magnetic field. This approach is termed cross-polarization magic angle spinning (CPMAS), and is the standard protocol for solids. It relies on transfer of magnetization from protons to C (or other nuclei) in order to achieve rapid analyses with reasonably narrow spectral lines. Cody et al. 

Keywords Magic-Angle Spinning NMR Sample rotation Rotor size Selectivity Sensitivity Spectral resolution... 

C-cross polarization magic angle spinning (CP-MAS) spectra were obtained on a Bruker CXP-100 instrument. A rotor consisting of a barrel of boron nitride and a base of Kel-F was used. Rotor speed was 3.8 kHz. Recycle time was varied from 0.3 to 1 s. A variety of contact times from 0.5 to 3ms were employed. The Hartmann-Hahn condition was set using a sample of hexamethylbenzene. Chemical shifts were measured with respect to external hexamethylbenzene (by storing the hexamethylbenzene spectrum in another computer memory block) but are quoted with respect to TMS. It is assumed that the chemical shifts of hexamethylbenzene with respect to TMS are the same in solution as in the solid state. 

Rotation of powdered samples involves another difficulty [27,41,42] magic-angle spinning makes the orientation of each crystallite time-dependent. In the presence of substantial chemical shift anisotropy, this modulation leads, after Tr/2 pulses, to unwanted rotor lines [41], which appear in both dimensions and form cross-peaks of their own. The situation becomes even more complicated when or pulses or a train of different pulses is involved, especially if the magic angle is set incorrectly [42]. In addition, the rotational sidebands can be enhanced and the observed peaks correspondingly broadened for certain MAS frequencies [27]. 

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