Figure shows the spectrum of N-MP dissolved in H 2 O. The spectrum consists of a strong peak at about 80 Hz, arising from water protons, and two broader satellite humps, symmetric around the proton peak (75-85 Hz). The latter are groups of overlapping peaks, originating from the precession of N-MP protons coupled to N. When the storage time (in a storage field of 10 µT) was increased, a water-proton peak decayed faster than the N-MP peaks (Fig. ). This reveals a shorter longitudinal relaxation time of the water protons than for the N-MP protons. To calculate the T 1 lifetimes of the protons, data points, corresponding to the normalized integration values of proton peaks in the 75-85 Hz frequency range, were plotted as a function of the storage time (Fig. ). Due to the overlap between the water-and MP-proton peaks, the data (denoted by green triangles) was fit to bi-exponential decay. This allowed us to determine the N-MP and water-proton relaxation times as 4.8(5) and 1.24(3) s, respectively. A similar study was performed for N-MP dissolved in D 2 O, where only the N-MP peaks were observed. Figure shows the amplitude of the peak measured at 81 Hz versus the storage time. This data was fit with a single exponential decay and the longitudinal relaxation time T 1 of 2.38(3) s was determined. These results reveal an interesting effect of the shorter relaxation time of N-MP protons in water compared to that in the deuterated solution. A similar solvent effect on the T 1 relaxation of both N and 1 H in N-MP was also observed in high-field NMR (see Supplementary Information). This result might be explained by the isotope effect in which the solvents differ in their hydrogen bonding characteristics and the formation of the solvation spheres . In addition, lower mobility of D in D 2 O compared to H in H 2 O may result in slower dipole reorientation, hence a higher relaxation rate is expected in D 2 O, especially of small ionic compounds . Despite these speculations, better understanding of relaxation mechanisms of coupled spin systems at ZULF conditions needs further studies.