Studies of dynamic nuclear polarization with nitroxides in aqueous solution

The dynamic nuclear polarization effect between the nitroxide radical TEMPO and the H-1 protons of water solution was investigated at an electron pumping frequency of 9.7 GHz for different experimental conditions. In particular, we compared N-14-4-hydroxy-TEMPO (TEMPOL) with 4-oxo-TEMPO and evaluated the effect of H-2 and N-15 isotope labelling. Furthermore, we tested the effect of concomitant irradiation on both electron paramagnetic resonance (EPR) lines with the N-15-labelled compound. Our results show that the reduction in the EPR line width given by the H-1 and N-15 labelling and the collapse in the number of hyperfine lines due to N-15 substitution leads to the highest enhancement of epsilon = -140 ever reported in the literature. Concomitant pumping on two hyperfine lines does not give higher enhancements at our experimental conditions.