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Well-resolved and information-rich J -spectra are the foundation for chemical detection in zero-field NMR. However, even for relatively small molecules, spectra exhibit complexity, hindering the analysis. To address this problem, we investigate an example biomolecule with a complex J -coupling network─urea, a key metabolite in protein catabolism─and demonstrate ways of simplifying its zero-field spectra by modifying spin topology. This goal is achieved by controlling pH-dependent chemical exchange rates of 1 H nuclei and varying the composition of the D 2 O/H 2 O mixture used as a solvent. Specifically, we demonstrate that by increasing the proton exchange rate in the [ 13 C, 15 N 2 ]-urea solution, the spin system simplifies, manifesting through a single narrow spectral peak. Additionally, we show that the spectra of 1 H/D isotopologues of [ 15 N 2 ]-urea can be understood easily by analyzing isolated spin subsystems. This study paves the way for zero-field NMR detection of complex biomolecules, particularly in biofluids with a high concentration of water.

Zero-Field NMR of Urea: Spin-Topology Engineering by Chemical Exchange