Dynamic Nuclear Polarization of 13C in Aqueous Solutions Under Ambient Conditions

The direct enhancement of the 13C NMR signal of small molecules in solution through Overhauser-mediated dynamic nuclear polarization (DNP) has the potential to enable studies of systems where enhanced signal is needed but the current dissolution DNP approach is not suitable, for instance if the sample does not tolerate a freeze-thaw process or if continuous flow or rapid re-polarization of the molecules is desired. We present systematic studies of the 13C DNP enhancement of 13C-labeled small molecules in aqueous solution under ambient conditions, where we observe both dipolar and scalar-mediated enhancement. We show the role of the three-spin effects from enhanced protons on 13C DNP through DNP experiments with and without broadband 1H decoupling and by comparing DNP results with H2O and D2O. We conclude that the efficiency of 13C Overhauser DNP in small molecules strongly depends on the distance of closest approach between the electron and 13C nucleus, the presence of a scalar contribution to the coupling factor, and the magnitude of the three-spin effect due to adjacent polarized protons. The enhancement appears to depend less on the translational dynamics of the 13C-labeled small molecules and radicals.