Dynamic nuclear polarization at 9T using a novel 250GHz gyrotron microwave source

Abstract

In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9T (250 GHz for g=2 electrons, 380 MHz for 1H). In these experiments, 1H enhancements of up to 170+/-50 have been observed in 1-13C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20K; in addition, we have observed significant enhancements in 15N spectra of unoriented pf1-bacteriophage. Finally, enhancements of approximately 17 have been obtained in two-dimensional 13C-13C chemical shift correlation spectra of the amino acid U-13C, 15N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.

Fig. 1

Cross-polarization with continuous microwave irradiation. In all cases, the electron resonance offset was set to maximize the ¹H enhancement.

Fig. 2

DNP CP spectra of static 1-¹³C-glycine (0.39 M) dispersed in a 60:40 water/glycerol solution containing 80mM 4-amino TEMPO. The spectra were recorded at 20K with (upper trace) and without (lower trace) microwave irradiation. A ¹H DNP enhancement of 170 ± 50 was observed with ~1.0 W of microwave power incident on the sample. Eight transients were recorded. The peak at ~75 ppm arises from glycerol at natural abundance.

Fig. 3

Dependence of the DNP enhancement (solid circles) on electron resonance offset, superimposed on the simulated 4-amino TEMPO EPR spectrum (solid trace). Because the gyrotron oscillator frequency is fixed, the static magnetic field was swept in this experiment. For the simulation of the TEMPO spectrum we employed g_ii and A_ii values from Budil et al. [30]; specifically, g_xx = 2.00860, g_yy = 2.00622, and g_zz = 2.00233 and A_xx = 0.66 mT, _Ayy = 0.52 mT and A_zz = 3.54 mT.

Fig. 4

Sequences for two-dimensional homonuclear chemical shift correlation spectroscopy. Following cross polarization from ¹H–¹³C, the system evolves under the ¹³C chemical shift for a period t₁. Correlations are established using mixing via: (a) proton-driven spin diffusion or (b) SPC5 homonuclear double-quantum mixing as described elsewhere [28,31], and detected during t₂.

Fig. 5

Two-dimensional ¹³C–¹³C correlation spectra of U-¹³C, [¹⁵N]proline, In (a), the SPC5 dipolar recoupling sequence was applied during a double quantum mixing period. The MAS frequency was 6 kHz, the mixing time was 1.33 ms, and the temperature was regulated at 97 ± 0.8 K. In (b), correlations were established by proton-driven spin diffusion for a mixing time of 10 ms. The MAS frequency was 7 kHz, and the temperature was unregulated but remained within the range 98–101 K. In both cases, 16 transients were acquired for each of 128 increments in the t₁ dimension, and the DNP enhancement was approximately 17. In (c) and (d), we show one dimensional ¹³C MAS spectra obtained with and without DNP, respectively, using SPC5 recoupling with a double quantum phase cycle; (e) and (f) are CP spectra with and without DNP. The apparent intensity differences between the spectra in (c) and (e) are due to recoupling dynamics at short excitation times [31].