C dynamic nuclear polarization using isotopically enriched 4-oxo-TEMPO free radicals.

The nitroxide-based free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) is a widely used polarizing agent in NMR signal amplification via dissolution dynamic nuclear polarization (DNP). In this study, we have thoroughly investigated the effects of N and/or H isotopic labeling of 4-oxo-TEMPO free radical on C DNP of 3 M [1- C] sodium acetate samples in 1 : 1 v/v glycerol : water at 3.35 T and 1.2 K. Four variants of this free radical were used for C DNP: 4-oxo-TEMPO, 4-oxo-TEMPO- N, 4-oxo-TEMPO-d and 4-oxo-TEMPO- N,d . Our results indicate that, despite the striking differences seen in the electron spin resonance (ESR) spectral features, the C DNP efficiency of these N and/or H-enriched 4-oxo-TEMPO free radicals are relatively the same compared with C DNP performance of the regular 4-oxo-TEMPO. Furthermore, when fully deuterated glassing solvents were used, the C DNP signals of these samples all doubled in the same manner, and the C polarization buildup was faster by a factor of 2 for all samples. The data here suggest that the hyperfine coupling contributions of these isotopically enriched 4-oxo-TEMPO free radicals have negligible effects on the C DNP efficiency at 3.35 T and 1.2 K. These results are discussed in light of the spin temperature model of DNP. Copyright © 2016 John Wiley & Sons, Ltd.