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Dynamic Nuclear Polarization Methods Development for Achieving High Nuclear Magnetic Resonance Signal Sensitivity


Solid-state nuclear magnetic resonance (NMR) is an essential tool for the study of biological solids, catalysts, and other functional materials. However, NMR has intrinsically low signal sensitivity and dynamic nuclear polarization (DNP) is one of the most effective approaches to enhance NMR sensitivity. DNP enhances NMR signal sensitivity through transferring the high polarization of electron spins to nuclear spins using microwave (mw) irradiation as a perturbation via different DNP mechanisms. As current DNP efficiency is still far from the theoretical limit (660 for 1H NMR), a major focus in DNP research is to develop methods that can maximize DNP enhancements at conditions germane to solid-state NMR, at high magnetic fields, with fast magic angle spinning (MAS), and under variable temperatures. There are many aspects involved in DNP methods development, including the DNP mechanisms clarification and improvement, instrumentation development, better samples preparation methods, and new data processing techniques.

DNP mechanisms development is one of the most important aspects in the DNP methods development area. Current diagnostics of DNP mostly rely on the analysis of the nuclear spin dynamics as a function of mw irradiation parameters which provides incomplete (sometimes even misleading) insights into the mechanism diagnosis process. With the help of continuous wave and pulsed electron paramagnetic resonance (EPR) spectrometers at various fields (from 0.35 to 7 Tesla) as well as quantum mechanical simulations, I have developed and standardized a workflow to diagnose DNP mechanisms and improve their efficiencies. Using the developed workflow, I have improved the existing Cross Effect (CE) DNP efficiency significantly by rationally tuning the the EPR spectral density of mixed broad (TEMPO) and narrow (Trityl)-line radicals, suggesting a novel polarizing agent design of one Trityl tethered to at least two TEMPO moieties. With the help of this novel workflow, a new truncated CE DNP mechanisms was discovered that has the apparent features of an Overhauser Effect. This discovery not only expanded the scope of the theoretical understanding of DNP mechanisms but also provided a technique to probe paramagnetic materials with very fast electron spin lattice relaxation rates. Furthermore, I have discovered an unexpected 1H Thermal Mixing (TM) DNP mechanism with narrow line radicals Trityl and BDPA, providing a new direction of future DNP radical design utilizing the TM DNP mechanism.

Instrumentation development is another key aspect in the DNP methods development. In this aspect, being able to operate at ultra low temperature (ULT, 100 K) is one of the major challenges. In collaboration with JEOL RESONANCE Inc., Japan and JEOL USA Inc, we have successfully installed the first commercial 14.1 Tesla NMR spectrometer equipped with a closed-cycle helium ULT-MAS system. To demonstrate the feasibility of doing DNP/NMR at ULT using the newly installed system, I conducted a comprehensive NMR characterization of a hydrated [U-13C]alanine standard sample at variable temperatures (25 – 100 K) and different spinning speeds (1.5 – 100 kHz). I confirmed that the 13C CP-MAS NMR of [U-13C]alanine obtained a large sensitivity gain at ULT resulting from the Boltzmann factor, radio frequency circuitry quality factor improvement and the suppression of its methyl group rotation. I further observed that the addition of organic biradicals widely used for CE DNP significantly shortened the 1H T1 spin lattice relaxation time at ULT via the two-electron-one-nucleus triple flip transition, without further broadening the 13C spectral linewidth. My experimental observations suggest that the prospects of DNP/NMR under ULT conditions established with a closed-cycle helium MAS system are bright.

With the DNP mechanisms improvements and instrumentation advancements, I have further applied DNP enhanced NMR to study biological samples as well as inorganic silica nano-particles, where tens to hundreds of DNP enhancements have been achieved in all the tested samples. Preliminary DNP study has already provided unprecedented information of the presence of a minor –Si(OH)3 species in the hydrated natural abundant silica nanoparticle materials. However, the applications of DNP in bio-solid materials need more than sensitivity enhancement, where NMR spectral resolution is another critical factor. To eventually apply DNP in bio-solids, better sample preparation methods to obtain homogeneous Tau amyloid protein fibrils and better data processing using wavelet denoising techniques are underdevelopment.

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