Magic angle spinning NMR spectroscopy: a versatile technique for structural and dynamic analysis of solid-phase systems

Abstract

Magic Angle Spinning (MAS) NMR spectroscopy is a powerful method for analysis of a broad range of systems, including inorganic materials, pharmaceuticals, and biomacromolecules. The recent developments in MAS NMR instrumentation and methodologies opened new vistas to atomic-level characterization of a plethora of chemical environments previously inaccessible to analysis, with unprecedented sensitivity and resolution.

Figure 1

Left: Schematic representation of MAS NMR setup. The sample is placed into an NMR probe at the “magic” angle of 54.74° with respect to the static magnetic field, and spun rapidly. In practice, MAS frequencies of 10 – 62 kHz are used, and the highest currently attainable MAS frequency in 2014 is 110 kHz.^, The bottom photograph illustrates MAS NMR rotors of various diameters ranging from 1.3 to 4 mm. MAS NMR rotors of diameters of 5 and 7 mm are also available from NMR manufacturers, Bruker, Doty Scientific, and others. Middle: Illustration of the effect of MAS on NMR lineshapes in a spin-1/2 nucleus. The broad ¹³C powder pattern resulting from chemical shift interaction in a static sample (a) is broken up into a series of spinning sidebands (b), which represent the Fourier components of the MAS frequency; and averaged out into an isotropic peak (c) when the MAS frequency exceeds the magnitude of the anisotropic interaction. Right: Illustration of the effect of MAS on NMR lineshapes in a spin – 7/2 nucleus. The broad ⁵¹V powder pattern resulting from a combined effect of quadrupolar and chemical shift interactions and spanning the central and satellite transitions in a static sample (d) is broken up in to a series of spinning sidebands (e and f), which represent the Fourier components of the MAS frequency. Note that at 60 kHz, neither the CSA nor the quadrupolar interaction are averaged out, which is a common case in half-integer quadrupolar nuclei in low-symmetry environments.

Figure 2

Rotational Echo Double Resonance – REDOR. (a) The pulse scheme on the top signifies the basic repeat unit of the reference experiment. The local field experience by spin S is shown at the bottom. (b) The pulse scheme on the top signifies the basic repeat unit of the dipolar-dephased experiment. The local field experience by spin S is not averaged to zero and results in a net loss of magnetization due to the I-S dipolar interaction. A recoupling curve is obtained by repeating the experiment for multiples of the basic two-rotor-period units shown above. Reprinted in part from Figure 2 of. Copyright © 1998 John Wiley & Sons, Inc.

Figure 3

(a) Physical dimensions of the rf coils (in millimeters) and their integration into a MAS stator of a low-E probe; (b) photograph of the probe head. The Teflon coil platform and two pairs of leads can be seen at the bottom of the stator. (c) and (d) A comparison of sample heating in single-solenoid and low-E probes at 600 MHz. (c) RF loss in saline sample per B₁² (mW/kHz²) at 600 MHz as function of salt concentration in a 5 mm tube. Remaining losses in the low-E probe are mostly of inductive nature. Note that dielectric loss in the solenoid has nonlinear dependence on sample conductivity. Reduction of conservative electric fields results in 10-fold decrease in sample heating for samples with < 200 mM NaCl, a value that is higher than the buffer strength in the majority of protein sample preparations. (d) Temperature rise in a saline sample (100 mM NaCl buffer doped with 20 mM TmDOTP⁵⁻) as a function of time-averaged RF power that reaches the coil under actual experimental conditions during the ¹H decoupling pulse. The figure and the caption are reproduced with permission from ref and.

Figure 4

Proton-detected HN correlation spectra of E. coli [¹³C,¹⁵N]-single-strand DNA binding protein (SSB, 4 × 18 kDa), at a ¹H NMR frequency of 800 MHz. MAS rates were respectively 22.5 (a), 40 (b), 50 (c) and 60 kHz (d). Note the dramatic increase in resolution in the indirect ¹H dimension with the increase in spinning frequency. (e) Representative 2D H-C planes of the ¹³C-detected 3D HNCOCX (purple) and HNCA (green) spectrum of U-¹³C, ¹⁵N-LC8 at δ_15N = 128.3 ppm acquired at the magnetic field of 19.9 T (850 MHz ¹H frequency) and a MAS frequency of 62 kHz. The spectra were recorded with 3.1 mg of sample. The experimental times are 3.5 days for HNCA and 7 days for HNCOCX. The corresponding NCA and NCOCX experiments recorded with conventional 3.2 mm probes at MAS frequencies of 24 kHz and below require 15–30 mg of sample. Panels a–d and e of the figure and the caption are reproduced with permission from refs^,,,, respectively.

Figure 5

(a) HRTEM images and (b) ⁷Li 50 kHz MAS NMR spectra of pristine and chemically delithiated Li_1−xMnBO₃. Monoclinic lithium manganese borate is of interest for its usage as cathode material. The figure and the caption are reproduced with permission from.

Figure 6

(a) Pulse sequence used for 1D CP MAS NMR spectroscopy; (b) different covalently incorporated aromatic substrates on the silica material; ¹³C CP MAS spectra with (top and/or middle) and without (bottom) microwave (MW) irradiation at 263 GHz to induce DNP of I (c) and II (d); (e) contour plots of a DNP-enhanced 2D ¹H-¹³C spectrum of II recorded using MW irradiation of 263 GHz. ε_H and ε_C denote the experimental enhancements gained by DNP for ¹H and ¹³C nuclei. The figure and the caption are reproduced with permission from.