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J Magn Reson. 2013 Jan;226:100-6. doi: 10.1016/j.jmr.2012.11.009. Epub 2012 Nov 21.

Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25 K.

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Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520, United States.


We describe an apparatus for solid state nuclear magnetic resonance (NMR) with dynamic nuclear polarization (DNP) and magic-angle spinning (MAS) at 20-25 K and 9.4 Tesla. The MAS NMR probe uses helium to cool the sample space and nitrogen gas for MAS drive and bearings, as described earlier, but also includes a corrugated waveguide for transmission of microwaves from below the probe to the sample. With a 30 mW circularly polarized microwave source at 264 GHz, MAS at 6.8 kHz, and 21 K sample temperature, greater than 25-fold enhancements of cross-polarized (13)C NMR signals are observed in spectra of frozen glycerol/water solutions containing the triradical dopant DOTOPA-TEMPO when microwaves are applied. As demonstrations, we present DNP-enhanced one-dimensional and two-dimensional (13)C MAS NMR spectra of frozen solutions of uniformly (13)C-labeled l-alanine and melittin, a 26-residue helical peptide that we have synthesized with four uniformly (13)C-labeled amino acids.

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