Effects of glassing matrix deuteration on the relaxation properties of hyperpolarized C spins and free radical electrons at cryogenic temperatures.

Glassing matrix deuteration could be a beneficial sample preparation method for C dynamic nuclear polarization (DNP) when large electron paramagnetic resonance (EPR) width free radicals are used. However, it could yield the opposite DNP effect when samples are doped with small EPR width free radicals. Herein, we have investigated the influence of solvent deuteration on the C nuclear and electron relaxation that go along with the effects on C DNP intensities at 3.35 T and 1.2 K. For C DNP samples doped with trityl OX063, the C DNP signals decreased significantly when the protons are replaced by deuterons in glycerol:water or DMSO:water solvents. Meanwhile, the corresponding solid-state C T relaxation times of trityl OX063-doped samples generally increased upon solvent deuteration. On the other hand, C DNP signals improved by a factor of ∼1.5 to 2 upon solvent deuteration of samples doped with 4-oxo-TEMPO. Despite this C DNP increase, there were no significant differences recorded in C T values of TEMPO-doped samples with nondeuterated or fully deuterated glassing matrices. While solvent deuteration appears to have a negligible effect on the electron T relaxation of both free radicals, the electron T relaxation times of these two free radicals generally increased upon solvent deuteration. These overall results suggest that while the solid-phase C DNP signals are dependent upon the changes in total nuclear Zeeman heat capacity, the C relaxation effects are related to H/H nuclear spin diffusion-assisted C polarization leakage in addition to the dominant paramagnetic relaxation contribution of free radical centers.