Erforschung und Entwicklung von gepulsten Methoden der dynamischen Kernpolarisation (DNP) in der magnetischen Kernresonanz (NMR)
Final Report Abstract
The research project was focused on the development and investigation of pulsed methods of dynamic nuclear polarization (DNP) in the solid state. DNP is a technique to increase the sensitivity of nuclear magnetic resonance (NMR) by several orders of magnitude by transferring the high electron spin polarization to nuclear spins. Although theoretically the maximum signal enhancement of proton spins is ~660, enhancements of ~240 and ~110 can be routinely obtained at magnetic fields of 5 T and 9 T, respectively, and temperatures around 90 K. This lower than theoretically expected value is mostly limited by a disadvantageous field dependence of the DNP efficiency, scaling with B0-1 or B0-2, depending on the DNP mechanism evoked. By the utilization of coherent transfer pathways between the electron spins and the nuclear spins, this field dependence would be overcome, resulting in efficient DNP even at high NMR fields. For example, in the very promising NOVEL experiment the electron spins are spin-locked in a microwave field matching the nuclear Larmor frequency, resulting in a Hartmann-Hahn-like condition which allows very efficient cross polarization. However, a rather high microwave field strength equivalent to >212 MHz has to be available to fulfill that condition at high magnetic field (>5 T). A 140 GHz gyroamplifier has been developed earlier in the laboratory of Richard E. Temkin (Plasma Science and Fusion Center, MIT). Driven with a solid state pulsed microwave source this amplifier is potentially capable of outputting ~1 kW of microwave power. In order to utilize this amplifier for DNP/EPR experiments, we redesigned an already existing pulsed EPR/DNP spectrometer. We successfully constructed a pulsed microwave bridge delivering 120 mW of microwave power at 140 GHz with fast gating (<1 ns) and full phase control in four channels plus an additional variable frequency channel for double resonance experiments. Since the existing DNP probe suffered from arcing between the microwave waveguide and the rf coil, a doubly-balanced rf circuit for 1H and 13C has been designed and several modifications have been applied to the probe. These improvements resulted in very good rf performance and long-term stability, allowing us to perform detailed investigations of mechanistic aspects of DNP. Using this improved spectrometer which provides complete cross-triggering capabilities between EPR and NMR excitation and detection, we have been able to develop a model describing the polarization transfer and spin-diffusion in Solid Effect DNP based on rate equations in combination with experimental findings. This model might help to explain the complex processes involved in the hyperpolarization of bulk nuclei which are not completely understood yet, and to improve the efficiency of DNP. Although we have fulfilled most of the requirements for implementation of the gyroamplifier into the EPR/DNP spectrometer, several modifications of the amplifier itself are still required to be carried out in the laboratory of R. E. Temkin. Besides this, we have investigated the possibility to utilize paramagnetic metal ions as polarizing agents for DNP, which might allow for site-specific hyperpolarization of nuclei in the vicinity of endogenous or substituted paramagnetic metal sites in biomolecules (e.g. in metalloproteins) or for utilizing paramagnetic ions as polarization sources for DNP in materials science. We have successfully transferred electron spin polarization from high-spin complexes containing Gd3+ and Mn2+ to protons. Furthermore, we investigated the effects of zero-field splitting and hyperfine coupling to the metal nucleus. This first successful hyperpolarization of nuclear spins at high magnetic field by paramagnetic metal ions might potentially open up a new field of DNP in biochemistry and/or materials science using paramagnetic metal sites as endogenous polarizing agents. We have constructed a magic angle spinning (MAS) DNP probe operating at 5 T (140 GHz e–, 212 MHz 1H frequency). It utilizes a custom sample eject system, capable of easily and reliably changing sample rotors under cryogenic (<90 K) conditions without need to warm up the probe even after more than 20 sample ejections during two weeks of continuous cryogenic operation. This probe further allowed us to carry out extensive and detailed studies about the influence and effects of proton concentration as well as radical concentration on achievable enhancements, sensitivity, and nuclear spin relaxation times under DNP for different polarizing agents. The experiments yield great insight into the optimal choice of polarizing agent and the respective concentration as well as the protonation level of the cryoprotecting solvent used for specific experimental requirements. In collaboration with the groups of Tim Swager (MIT) and Paul Tordo (University of Marseille) we also developed a novel rigid biradical polarizing agent soluble in water which outperforms the commonly used TOTAPOL and might potentially become a standard polarizing agent for Cross Effect DNP at high field. Last, we investigated the heterogeneous line broadening generally observed when freezing biomolecules in a cryoprotecting matrix. We observed and characterized several broadening mechanisms in the hydrophilic tripeptide APG which served as a model system. The information obtained in this study might help to understand the broadening observed in larger biomolecules and potentially to retain narrow line width and high resolution even at temperatures as low as 90 K which are generally required for efficient DNP.
Publications
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Resolution and polarization distribution in cryogenic DNP/MAS experiments. Phys. Chem. Chem. Phys. 12, 5861-5867 (2010)
A. B. Barnes, B. Corzilius, M. L. Mak-Jurkauskas, L. B. Andreas, V. S. Bajaj, Y. Matsuki, M. L. Belenky, J. Lugtenburg, J. R. Sirigiri, R. J. Temkin, J. Herzfeld, and R. G. Griffin
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High-Field Dynamic Nuclear Polarization with High-Spin Transition Metal Ions. J. Am. Chem. Soc. (2011)
B. Corzilius, A. A. Smith, A. B. Barnes, C. Luchinat, I. Bertini, and R. G. Griffin
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Microwave Field Distribution in a Magic Angle Spinning Dynamic Nuclear Polarization NMR Probe. J. Magn. Reson., (2011)
E. Nanni, A. B. Barnes, Y. Matsuki, P. P. Woskov, B. Corzilius, R. G. Griffin, and R. J. Temkin
