Because of the inherently low sensitivity of solid-state NMR experiments, measurements on biological solids are often restricted to relatively small model compounds. Recent developments in Dynamic Nuclear Polarization (DNP) offer the potential of substantially larger signal/noise ratios in biological systems. DNP couples the high spin polarization of unpaired electrons to nuclear spins, resulting in a potential NMR signal enhancement of up to three orders of magnitude. Recent advances in high field (140 GHz EPR / 211 MHz NMR) DNP have been made in the application of DNP under high-resolution, MAS conditions for large biological. We have developed an aqueous solvent system in which signal enhancements of up to a factor of 185 can be achieved for biological solutes in frozen solution. The system consists of the nitroxide spin label 4-amino TEMPO in a water/glycerol solution. Irradiation off the center of the nitroxide EPR line polarizes the proton spins coupled to the radical. Spin diffusion among proton spins transfers the high polarization throughout the solvent, followed by cross-polarization to low-gamma nuclei. An enhancement of approximately 185 was obtained in the static CP/MAS of '3C-carbonyl glycine in 60:40 glycerol water at 14 K. In order to achieve high-resolution spectra, we have extended this experiment to incorporate magic-angle spinning (MAS). Because the polarization transfer is most efficient at temperatures well below 100 K (due to the increased electron and nuclear relaxation times), a DNP/MAS probe for low temperatures was constructed. Pressurized helium gas, cooled through a heat exchanger in liquid helium, was used to drive a standard Chemagnetics rotor. Transmission line tuning was incorporated to avoid exposure of internal capacitors to helium, which has a low breakdown voltage. Temperatures as low as 20 K at speeds up to 5 KHz were achieved with the low temperature DNP setup. We have obtained an enhancement of 20 in the ~ CPMAS spectrum of uniformly labeled L-arginine in TEMPO/water/glycerol at 50 K. Enhancements up to a factor of 100 have been obtained at 25 K. This experiment can be easily extended to larger biological solutes. Because the electron-proton polarization transfer step occurs primarily to solvent protons, this step is minimally affected by the size of the solute. The only inherent limit on the size of the solute arises from the efficiency of proton spin diffusion, which delivers the enhanced polarization throughout the protons of the solute. Estimates of these rates, compared with typical proton T1 relaxation rates, suggest that proteins up to several hundred kDa should be amenable to this technique. We have obtained an enhancement of approximately 50 in the '5N CPMAS spectrum of '5N-Ala T4-lysozyme, an 18.7 kD lytic protein in a TEMPO/water/glycerol solution.
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