Total Correlation Spectroscopy across All NMR-Active Nuclei by Mixing at Zero Field

Multidimensional nuclear magnetic resonance (NMR) is based on a combination of well-established building blocks for polarization transfer. These blocks are used to design correlation experiments through one or a few chemical bonds or through space. Here, we introduce a building block that enables polarization transfer across all NMR-active nuclei in a coupled network of spins: isotropic mixing at zero and ultralow field (ZULF). Exploiting mixing under ZULF-NMR conditions, heteronuclear TOtal Correlation SpectroscopY (TOCSY) experiments were developed to highlight coupled spin networks. We demonstrate ¹H-¹³C and ¹H-¹⁵N correlations in ZULF-TOCSY spectra of labeled amino acids, which allow one to obtain cross-peaks among all heteronuclei belonging to the same coupled network, even when the direct interaction between them is negligible. We also demonstrate the potential of ZULF-TOCSY to analyze complex mixtures on a growth medium of isotope-labeled biomolecules. ZULF-TOCSY enables the quick identification of individual compounds in the mixture by their coupled spin networks. The ZULF-TOCSY method will lead to the development of a new toolbox of experiments to analyze complex mixtures by NMR.