Solid-State Dynamic Nuclear Polarization at 263 GHz: Spectrometer Design and Experimental Results

Bruker BioSpin Corporation, 15 Fortune Drive, Billerica, MA 01821, USA.
Physical Chemistry Chemical Physics (Impact Factor: 4.49). 06/2010; 12(22):5850-60. DOI: 10.1039/c003685b
Source: PubMed


Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz (1)H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water-glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.

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Available from: W.E.J.R. Maas, Oct 01, 2015
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    • "Solid-state NMR and DNP experiments ssNMR and DNP experiments were conducted using 3.2 mm triple-resonance ( 1 H, 13 C, 15 N) MAS probe heads at static magnetic fields ranging from 9.4 to 18.8 T corresponding to proton/electron resonance frequencies of 400 MHz/263 GHz (Rosay et al. 2010), 700 or 800 MHz/ 527 GHz (Bruker BioSpin). Data were recorded at 100 K (LT) and at 273 K (referred to as ambient temperature, AT) employing MAS rates between 8 and 15 kHz. "
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    ABSTRACT: Dynamic nuclear polarization (DNP) has become a powerful method to enhance spectroscopic sensitivity in the context of magnetic resonance imaging and nuclear magnetic resonance spectroscopy. We show that, compared to DNP at lower field (400 MHz/263 GHz), high field DNP (800 MHz/527 GHz) can significantly enhance spectral resolution and allows exploitation of the paramagnetic relaxation properties of DNP polarizing agents as direct structural probes under magic angle spinning conditions. Applied to a membrane-embedded K(+) channel, this approach allowed us to refine the membrane-embedded channel structure and revealed conformational substates that are present during two different stages of the channel gating cycle. High-field DNP thus offers atomic insight into the role of molecular plasticity during the course of biomolecular function in a complex cellular environment.
    Full-text · Article · Oct 2014 · Journal of Biomolecular NMR
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    • "The recent availability of commercial DNP spectrometers operating at 9.4 T ( 1 H/e À frequencies of 400 MHz/263 GHz) [17] has greatly expanded access to DNP [18] [19] [20]. However, for many contemporary NMR experiments, 9.4 T is a relatively low field for optimal spectral resolution and it is therefore important to extend DNP to higher fields. "
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    ABSTRACT: We describe the design and implementation of the instrumentation required to perform DNP-NMR at higher field strengths than previously demonstrated, and report the first magic-angle spinning (MAS) DNP-NMR experiments performed at 1H/e− frequencies of 700 MHz/460 GHz. The extension of DNP-NMR to 16.4 T has required the development of probe technology, cryogenics, gyrotrons, and microwave transmission lines. The probe contains a 460 GHz microwave channel, with corrugated waveguide, tapers, and miter-bends that couple microwave power to the sample. Experimental efficiency is increased by a cryogenic exchange system for 3.2 mm rotors within the 89 mm bore. Sample temperatures ⩽85 K, resulting in improved DNP enhancements, are achieved by a novel heat exchanger design, stainless steel and brass vacuum jacketed transfer lines, and a bronze probe dewar. In addition, the heat exchanger is preceded with a nitrogen drying and generation system in series with a pre-cooling refrigerator. This reduces liquid nitrogen usage from >700 l per day to <200 l per day and allows for continuous (>7 days) cryogenic spinning without detrimental frost or ice formation. Initial enhancements, ε = −40, and a strong microwave power dependence suggests the possibility for considerable improvement. Finally, two-dimensional spectra of a model system demonstrate that the higher field provides excellent resolution, even in a glassy, cryoprotecting matrix.
    Full-text · Article · Nov 2012 · Journal of Magnetic Resonance
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    • "The black circle is a fundamental forward wave oscillation, the square is a second harmonic forward wave oscillation and the diamond is a fundamental backward wave (BW) oscillation. Traditionally, gyrotrons interact with the fundamental forward wave [13], [14], [20]; however, the second harmonic forward wave is useful for higher frequency experiments [15], [16], [58], [59] and the BW interaction is used at the first or second harmonic to provide the frequency tunability in gyrotrons for DNP NMR experiments. "
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    ABSTRACT: Dynamic nuclear polarization (DNP) increases the sensitivity of nuclear magnetic resonance (NMR) spectroscopy by using high frequency microwaves to transfer the polarization of the electrons to the nuclear spins. The enhancement in NMR sensitivity can amount to a factor of well above 100, enabling faster data acquisition and greatly improved NMR measurements. With the increasing magnetic fields (up to 23 T) used in NMR research, the required frequency for DNP falls into the THz band (140- 600 GHz ). Gyrotrons have been developed to meet the demanding specifications for DNP NMR, including power levels of tens of watts; frequency stability of a few megahertz; and power stability of 1% over runs that last for several days to weeks. Continuous gyrotron frequency tuning of over 1 GHz has also been demonstrated. The complete DNP NMR system must include a low loss transmission line; an optimized antenna; and a holder for efficient coupling of the THz radiation to the sample. This paper describes the DNP NMR process and illustrates the THz systems needed for this demanding spectroscopic application. THz DNP NMR is a rapidly developing, exciting area of THz science and technology.
    Full-text · Article · Oct 2011 · IEEE Transactions on Terahertz Science and Technology
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