Thurber, K.R., et al., Solid state nuclear magnetic resonance with magic-angle spinning and dynamic nuclear polarization below 25K. J. Magn. Reson., 2013. 226(0): p. 100-106.
We describe an apparatus for solid state nuclear magnetic resonance (NMR) with dynamic nuclear polarization (DNP) and magic-angle spinning (MAS) at 20-25K and 9.4Tesla. 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 30mW circularly polarized microwave source at 264GHz, MAS at 6.8kHz, and 21K 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.