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

Hyperpolarization of C-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 DNP of single or double C-labeled sodium acetate with or without H-enrichment of the methyl group and using a 4-oxo-TEMPO 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 spins in acetate are polarized with equal DNP efficiency using this relatively wide electron spin resonance linewidth free radical. In fact, the carbonyl C spins have about twice the solid-state C polarization level of methyl C spins. Deuteration of the methyl group provides a DNP signal improvement of methyl C spins on a par with that of carbonyl C spins. On the other hand, both the double C-labeled [1,2-C] acetate and [1,2-C, H] acetate have a relative solid-state C polarization at the level of [2-C] acetate. Meanwhile, the solid-state C T relaxation times at 3.35 T and 1.4 K were essentially the same for all six isotopomers of C acetate. These results suggest that the intramolecular environment of C spins plays a prominent role in determining the C DNP efficiency, while the solid phase C T relaxation of these samples is dominated by the paramagnetic effect due to the relatively high concentration of free radicals.