Effects of glassing matrix deuteration on the relaxation properties of hyperpolarized 13C spins and free radical electrons at cryogenic temperatures

Abstract

Glassing matrix deuteration could be a beneficial sample preparation method for ¹³C 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 ¹³C nuclear and electron relaxation that go along with the effects on ¹³C DNP intensities at 3.35 T and 1.2 K. For ¹³C DNP samples doped with trityl OX063, the ¹³C DNP signals decreased significantly when the protons are replaced by deuterons in glycerol:water or DMSO:water solvents. Meanwhile, the corresponding solid-state ¹³C T₁ relaxation times of trityl OX063-doped samples generally increased upon solvent deuteration. On the other hand, ¹³C DNP signals improved by a factor of ∼1.5 to 2 upon solvent deuteration of samples doped with 4-oxo-TEMPO. Despite this ¹³C DNP increase, there were no significant differences recorded in ¹³C T₁ values of TEMPO-doped samples with nondeuterated or fully deuterated glassing matrices. While solvent deuteration appears to have a negligible effect on the electron T₁ relaxation of both free radicals, the electron T₂ relaxation times of these two free radicals generally increased upon solvent deuteration. These overall results suggest that while the solid-phase ¹³C DNP signals are dependent upon the changes in total nuclear Zeeman heat capacity, the ¹³C relaxation effects are related to ²H/¹H nuclear spin diffusion-assisted ¹³C polarization leakage in addition to the dominant paramagnetic relaxation contribution of free radical centers.

FIG. 1.

Structures of the free radicals and ¹³C-labeled compound used in this study.

FIG. 2.

Effect of glassing solvent deuteration on trityl OX063-doped ¹³C DNP samples at 3.35 T and 1.2 K: (a) Top: Representative ¹³C DNP buildup curves for ¹³C samples with nondeuterated (ND) and fully deuterated (FD) glassing solvents. Bottom: Comparative bar graphs of extrapolated ¹³C DNP intensities for ND and FD glycerol:H₂O or DMSO:H₂O glassing solvents. (b) Top: Representative hyperpolarized ¹³C T₁ decay curves for ¹³C samples with ND and FD solvents. Bottom: Relative bar graphs of ¹³C T₁ values of frozen ¹³C DNP samples with ND and FD glassing solvents.

FIG. 3.

Influence of glassing solvent deuteration on ¹³C DNP samples doped with 4-oxo-TEMPO free radical: (a) Top: Representative ¹³C DNP buildup curves for ¹³C samples with nondeuterated (ND) and fully deuterated (FD) glassing solvents. Bottom: Comparative bar graphs of ¹³C DNP intensities for ND and FD glycerol:H₂O or DMSO:H₂O glassing solvents. (b) Top: Representative hyperpolarized ¹³C T₁ decay curves for ¹³C samples with ND and FD solvents. Bottom: Relative bar graphs of ¹³C T₁ values of frozen ¹³C DNP samples with ND and FD solvents. Mean values (N = 3) and standard deviations of ¹³C DNP signals and ¹³C T₁ values are displayed. All these DNP measurements were taken at 3.35 T and 1.2 K.

FIG. 4.

W-band EPR measurements of ¹³C DNP samples: (a) Top: Representative EPR spectra of trityl OX063 with ND and FD glassing solvents. Bottom: Temperature dependence of trityl OX063 electron spin-lattice relaxation rate (1/T₁) for ¹³C DNP samples with ND and FD glycerol:H₂O or DMSO:H₂O glassing matrices. (b) Top: Representative EPR spectra of 4-oxo-TEMPO in ND and FD solvents. Bottom: 4-Oxo-TEMPO electron T₁ relaxation rates vs temperature in ND and FD glassing solvents. The dashed lines denote the apparent temperature power-law dependence of electron 1/T₁.

FIG. 5.

(a) Representative electron T₂ transverse magnetization dephasing curves for trityl OX063 in ND and FD glycerol:water solvents at W-band and 5 K. The solid curves are fits to an exponential decay equation. (b) Bar graphs showing the increase in electron T₂ values of trityl OX063 and 4-oxo-TEMPO at W-band and 5 K upon deuteration of the glassing solvents.