Tailoring of Polarizing Agents in the bTurea Series for Cross-Effect Dynamic Nuclear Polarization in Aqueous Media.

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ɛ (1H) 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 ɛ (1H) 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 ɛ (1H). Our study also shows that the molecular structure of AMUPol is not optimal and its ɛ (1H) 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 ɛ (1H) than AMUPol itself (by a factor of ca. 1.2). [ABSTRACT FROM AUTHOR]