Sauvee, C., et al., Tailoring of Polarizing Agents in the bTurea Series for Cross-Effect Dynamic Nuclear Polarization in Aqueous Media. Chemistry, 2016. 22(16): p. 5598-606.
A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors varepsilon ((1) H) in cross-effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP-optimized glycerol/water matrix ("DNP juice") have been studied. We observe that varepsilon ((1) H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e-e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e-e distance or too long a T1e can dramatically limit varepsilon ((1) H). Our study also shows that the molecular structure of AMUPol is not optimal and its varepsilon ((1) H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better varepsilon ((1) H) than AMUPol itself (by a factor of ca. 1.2).