Abstract
The chemical structure of polarizing agents critically determines the efficiency of dynamic nuclear polarization (DNP). For cross-effect DNP, biradicals are the polarizing agents of choice and the interaction and relative orientation of the two unpaired electrons should be optimal. Both parameters are affected by the molecular structure of the biradical in the frozen glassy matrix that is typically used for DNP/MAS NMR and likely differs from the structure observed with X-ray crystallography. We have determined the conformations of six bis-nitroxide polarizing agents, including the highly efficient AMUPol, in their DNP matrix with EPR spectroscopy at 9.7 GHz, 140 GHz, and 275 GHz. The multi-frequency approach in combination with an advanced fitting routine allows us to reliably extract the interaction and relative orientation of the nitroxide moieties. We compare the structures of six bis-nitroxides to their DNP performance at 500 MHz/330 GHz.
Highlights
Dynamic Nuclear Polarization (DNP) is a powerful method to enhance the sensitivity of NMR spectroscopy
In magic-angle spinning (MAS) NMR/DNP, the NMR sample is doped with a paramagnetic species, which is often a stable organic radical and called a polarizing agent, and polarization is transferred in situ by high-power, high-frequency microwaves
A blind test of the fitting routine between JS and GM resulted in simulations of high quality and reproduced the structural parameters of a hypothetical bis-nitroxide well within the uncertainty range estimated from the w2 curves, see Fig. SI-4 (ESI†)
Summary
Dynamic Nuclear Polarization (DNP) is a powerful method to enhance the sensitivity of NMR spectroscopy. In DNP, microwave irradiation induces a polarization transfer from unpaired electrons to nuclei, thereby enhancing the NMR signal. In combination with magic-angle spinning (MAS) NMR, DNP enables structural studies in materials science and biology that would otherwise be out of reach due to insufficient sensitivity. In MAS NMR/DNP, the NMR sample is doped with a paramagnetic species, which is often a stable organic radical and called a polarizing agent, and polarization is transferred in situ by high-power, high-frequency microwaves. Polarizing agents are dissolved in a glass-forming solvent that is frozen after addition to the molecular system of interest. The frozen glassy matrix assures that dipolar couplings are available for electron–1H a Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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