Figure compares J-spectra of both forms of urea in dimethylsulfoxide (DMSO) and water. The experimental spectrum in DMSO agrees well with the simulated spectrum, especially in terms of peak positions. In contrast, lines become substantially broader (∼ 3 times) when water is used as a solvent (Fig. ). This is expected since amide protons are known to undergo chemical exchange with water protons and this process contributes to nuclear spin relaxation rate . However, the mere fact of being able to observe these multiplets in J-spectra indicates that the proton chemical-exchange rate in urea at neutral pH level is slow enough for ZULF NMR measurements compared to the spin evolution due to J-couplings. Also, this is confirmed by a chemical-exchange rate for neutral pH being equal to approximately 1.9 s -1 , which is significantly slower than dominant interaction in the system ( 1 J NH ) .