Multidimensional
Nuclear Magnetic Resonance (NMR) is based on the 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 Ultra-Low Field (ZULF). Exploiting mixing under ZULF-NMR
conditions, heteronuclear TOtal Correlation SpectroscopY (TOCSY) experiments were
developed to highlight coupled spin networks. We demonstrate <sup>1</sup>H-<sup>13</sup>C
and <sup>1</sup>H-<sup>15</sup>N correlations in ZULF-TOCSY spectra of labelled
amino acids, which allow one to obtain cross-peaks among all hetero-nuclei
belonging to the same coupled network, even when the direct interaction between
them is negligible. We also demonstrate the interest of ZULF-TOCSY to analyze
complex mixtures on the supernatant of ISOGRO, a growth medium of isotope-labelled
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.
Total Correlation Spectroscopy across All NMR-Active Nuclei by Mixing at Zero Field