To optimize the DNP process it is crucial to understand the EPR properties of the polarizing agents. This article demonstrates the need of multi-frequency, high-field EPR spectroscopy to gain a deep understanding of all EPR parameters and how they influence the DNP process.
Soetbeer, Janne, Peter Gast, Joseph J Walish, Yanchuan Zhao, Christy George, Chen Yang, Timothy M Swager, Robert G Griffin, and Guinevere Mathies. “Conformation of Bis-Nitroxide Polarizing Agents by Multi- Frequency EPR Spectroscopy,”
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.