Niedbalski, Peter, Christopher Parish, Andhika Kiswandhi, and Lloyd Lumata. “13C Dynamic Nuclear Polarization Using Isotopically Enriched 4-Oxo-TEMPO Free Radicals.” Magnetic Resonance in Chemistry 54, no. 12 (2016): 962–67.
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 15N and/or 2H isotopic labeling of 4-oxo-TEMPO free radical on 13C DNP of 3 M [1-13C] 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 13C DNP: 4-oxo-TEMPO, 4-oxo-TEMPO-15N, 4-oxo-TEMPO-d16 and 4-oxo-TEMPO-15N,d16. Our results indicate that, despite the striking differences seen in the electron spin resonance (ESR) spectral features, the 13C DNP efficiency of these 15N and/or 2H-enriched 4-oxo-TEMPO free radicals are relatively the same compared with 13C DNP performance of the regular 4-oxo-TEMPO. Furthermore, when fully deuterated glassing solvents were used, the 13C DNP signals of these samples all doubled in the same manner, and the 13C 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 13C 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.