Full Correlations across Broad NMR Spectra by Two-Field Total Correlation Spectroscopy

Total correlation spectroscopy (TOCSY) is a key experiment to assign nuclear magnetic resonance (NMR) spectra of complex molecules. Carbon-13 TOCSY experiments are essential to assign signals of protein side chains. However, the performance of carbon-13 TOCSY deteriorates at high magnetic fields since the necessarily limited radiofrequency irradiation fails to cover the broad range of carbon-13 frequencies. Here, we introduce a new concept to overcome the limitations of TOCSY by using two-field NMR spectroscopy. In two-field TOCSY experiments, chemical shifts are labelled at high field but isotropic mixing is performed at a much lower magnetic field, where the frequency range of the spectrum is drastically reduced. We obtain complete correlations between all carbon-13 nuclei belonging to amino acids across the entire spectrum: aromatic, aliphatic and carboxylic. Two-field TOCSY should be a robust and general approach for the assignment of uniformly carbon-13 labelled molecules in high-field and ultra-high field NMR spectrometers beyond 1000 MHz.