Field Dependent Dynamic Nuclear Polarization with Radicals in Aqueous Solution

The dynamic nuclear polarization (DNP) effect between the polarizers TEMPOL and trityl and the 1H protons of water solution was investigated at two different magnetic fields corresponding to electron pumping frequencies of 9 and 94 GHz. For TEMPOL, large DNP enhancements up to about −100 (9 GHz) and −20 (94 GHz) were observed by continuous microwave irradiation on one of the three nitroxide hyperfine lines. These enhancements are considerably larger than those ever reported for this system and also larger than the ones we obtained with the second proposed polarizer agent, the trityl radical, under the same conditions. 1H Overhauser coupling factors were extracted from NMRD relaxation dispersion experiments and used to estimate the degree of effective saturation of the TEMPOL EPR line. Our results show that nitroxide radicals are well-suited polarizers for DNP experiments in aqueous solutions at variable fields. The large enhancements open up attractive perspectives for the application potential of DNP in solution NMR with macromolecular complexes.