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 1 H). In these experiments, 1 H enhancements of up to 170 ± 50 have been observed in 1-13 C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in 15 N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ~17 have been obtained in two-dimensional 13 C-13 C chemical shift correlation spectra of the amino acid U-13 C, 15 N-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.

On the role of electron–nucleus contact and microwave saturation in thermal mixing DNP

“…4) by repeating the computation for our reference sample at higher field (parameters were set as follow, according to reference [22]: B 0 = 4.64 T, T = 1.2 K, T 1e = 1 s, ∆ω e = 63 MHz). The extension of our calculations to a model representing different radicals and eventually higher temperatures, for comparison with the experimental observations achieved under such conditions [23][24][25][26], will be faced in a next dedicated study.…”

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

“…A computer-controlled stable CW source, the 250 GHz gyrotron was the first gyro-device specifically designed with the purpose of seamless integration into an NMR spectrometer [44,50]. During the course of this work, the 250 GHz gyrotron has been operated continuously for a period of 21 days yielding a power stability of <1% and frequency stability of better than 400 kHz (1.6 ppm).…”

“…The sample is then cooled further to ~90 K where the attenuated electronic and nuclear spin-lattice relaxation renders the polarization transfer more efficient. Information about the sample preparation and conditions used to generate and trap specific photocycle intermediates is provided elsewhere [62,76,[78][79][80][81]. Briefly, we employ sapphire rotors since they attenuate microwaves less than ZrO 2 .…”

“…Typical pulse sequences for DNP enhanced MAS experiments are described in several papers [29,43,49,50] and the sequence used to acquire the multidimensional spectra presented here is illustrated in Figure 3.…”

“…For the latter we present in this paper DNP enhanced MAS spectra of bacteriorhodopsin (bR), a 26.6 kD membrane protein embedded in a lipid bilayer. This is a more challenging test case for the DNP method than small model compounds such as urea or proline that we have used extensively in other papers demonstrating DNP [36,49,50]. It is also a biologically important system that poses outstanding scientific questions and therefore addresses the applicability of DNP experiments to interesting systems.…”