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

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 ¹H nuclei and varying the composition of the D₂O/H₂O mixture used as a solvent. Specifically, we demonstrate that by increasing the proton exchange rate in the [¹³C,¹⁵N₂]-urea solution, the spin system simplifies, manifesting through a single narrow spectral peak. Additionally, we show that the spectra of ¹H/D isotopologues of [¹⁵N₂]-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.