Water H-1 relaxation dispersion analysis on a nitroxide radical provides information on the maximal signal enhancement in Overhauser dynamic nuclear polarization experiments

Water H-1 relaxation rate measurements of N-15-H-2-TEMPONE solutions at temperatures ranging from 298 to 328 K have been performed as a function of magnetic field from 0.00023 to 9.4 T, corresponding to H-1 Larmor frequencies of 0.01 to 400 MHz. The relaxation profiles were analyzed according to the full theory for dipolar and contact relaxation, and used to estimate the coupling factor responsible for observed solution DNP effects. The experimental DNP enhancement at H-1 Larmor frequency of 15 MHz obtained by saturating one of the lines of the N-15 doublet is only ca. 20% lower than the limiting value predicted from the relaxation data, indicating that the experimental DNP setup is nearly optimal, the residual discrepancy arising from incomplete saturation of the other line.