Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results.
ABSTRACT 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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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.Journal of Biomolecular NMR 10/2014; 60(2-3):157-68. · 3.31 Impact Factor
- ACS Macro Letters 08/2014; 3:922-925. · 5.24 Impact Factor
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ABSTRACT: Aluminas (Al 2 O 3) are ubiquitous functional materials. In particular, the γ-alumina form is extensively used in research and industry as a catalyst and catalyst support. Nevertheless, a full structural description, which would aid in comprehension of its properties, is lacking and under large debate. Solid-state NMR has been used previously to study γ-alumina but is limited for certain applications, such as surface studies, due to intrinsic low sensitivity. Here, we detail the implementation of low temperature (∼100 K) magic angle spinning combined with dynamic nuclear polarization (MAS-DNP) to significantly enhance the sensitivity of solid-state NMR experiments and gain structural insights into this important material. Notably, we analyze hydrophilic and hydrophobic sample preparation protocols and their implications on the sample and resulting NMR parameters. We show that the choice of preparation does not perturb the spectrum, but it does have a large effect on NMR coherence lifetimes, as does the corresponding required (hyper)polarizing agent. We use this preliminary study to optimize the absolute sensitivity of the following experiments. We then show that there are no detectable hydroxyl groups in the bulk of the material and that DNP-enhanced 1 H → 27 Al cross-polarization experiments are selective to only the first surface layer, enabling a very specific study. This primostrato NMR is integrated with multiple-quantum magic angle spinning (MQMAS) and it is demonstrated, interestingly, that pentacoordinated Al 3+ ions are only observed in this first surface layer. To highlight that there is no evidence of subsurface pentacoordinated Al 3+ , a new bulk-filtered experiment is described that can eliminate surface signals.The Journal of Physical Chemistry C 10/2014; 118(43):25065–25076. · 4.84 Impact Factor