Dynamic nuclear polarization of carbonyl and methyl C-13 spins of acetate using 4-oxo-TEMPO free radical

摘要

Hyperpolarization of C-13-enriched biomolecules via dissolution dynamic nuclear polarization (DNP) has enabled real-time metabolic imaging of a variety of diseases with superb specificity and sensitivity. The source of the unprecedented liquid-state nuclear magnetic resonance spectroscopic or imaging signal enhancements of 10 000-fold is the microwave-driven DNP process that occurs at a relatively high magnetic field and cryogenic temperature. Herein, we have methodically investigated the relative efficiencies of C-13 DNP of single or double C-13-labeled sodium acetate with or without 2H-enrichment of the methyl group and using a 4-oxo- l'EMPO free radical as the polarizing agent at 3.35 T and 1.4 K. The main finding of this work is that not all C-13 spins in acetate are polarized with equal DNP efficiency using this relatively wide electron spin resonance linewidth free radical. In fact, the carbonyl C-13 spins have about twice the solid-state C-13 polarization level of methyl C-13 spins. Deuteration of the methyl group provides a DNP signal improvement of methyl C-13 spins on a par with that of carbonyl C-13 spins. On the other hand, both the double C-13-labeled [1,2-C-13(2)] acetate and [1,2-13C2, 2 H3] acetate have a relative solid-state C-13 polarization at the level of [2-C-13] acetate. Meanwhile, the solid-state C-13 Ti relaxation times at 3.35 T and 1.4 K were essentially the same for all six isotopomers of C-13 acetate. These results suggest that the intramolecular environment of C-13 spins plays a prominent role in determining the C-13 DNP efficiency, while the solid phase (CT1)-C-13 relaxation of these samples is dominated by the paramagnetic effect due to the relatively high concentration of free radicals. Published by AIP Publishing.