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

Hyperpolarization of 13 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 13 C DNP of single or double 13 C-labeled sodium acetate with or without 2 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 13 C spins in acetate are polarized with equal DNP efficiency using this relatively wide electron spin resonance linewidth free radical. In fact, the carbonyl 13 C spins have about twice the solid-state 13 C polarization level of methyl 13 C spins. Deuteration of the methyl group provides a DNP signal improvement of methyl 13 C spins on a par with that of carbonyl 13 C spins. On the other hand, both the double 13 C-labeled [1,2-13 C2 ] acetate and [1,2-13 C2 , 2 H3 ] acetate have a relative solid-state 13 C polarization at the level of [2-13 C] acetate. Meanwhile, the solid-state 13 C T1 relaxation times at 3.35 T and 1.4 K were essentially the same for all six isotopomers of 13 C acetate. These results suggest that the intramolecular environment of 13 C spins plays a prominent role in determining the 13 C DNP efficiency, while the solid phase 13 C T1 relaxation of these samples is dominated by the paramagnetic effect due to the relatively high concentration of free radicals.