Tailoring of Polarizing Agents in the bTurea Series for Cross-Effect Dynamic Nuclear Polarization in Aqueous Media.
Chemistry
; 22(16): 5598-606, 2016 Apr 11.
Article
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| MEDLINE
| ID: mdl-26992052
ABSTRACT
A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors É ((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 É ((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 É ((1)H). Our study also shows that the molecular structure of AMUPol is not optimal and its É ((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 É ((1)H) than AMUPol itself (by a factor of ca. 1.2).
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