Higher magnetic fields lead to higher sensitivity and higher resolution. Reaching higher fields is key to study biomolecular systems of increasing complexity. Yet, higher fields are not optimal for all applications of NMR, neither for all nuclei. For instance, the chemical shift anisotropies of carbon-13 nuclei in many chemical entities, or that of fluorine-19 lead to transverse relaxation rates incompatible with efficient NMR experiments of large biomolecules and assemblies at the highest magnetic fields. On the other hand, low magnetic fields provide rich information on molecular dynamics, as demonstrated by relaxometry, but are generally associated with sensitivity and resolution incompatible with site-specific studies of biomolecules. A dilemma for biomolecular NMR is: how can we benefit from the highest magnetic fields available while optimizing the field-dependent sensitivity, resolution and information of most NMR experiments?
The solution to this dilemma is to couple high-field NMR with low- or variable-field NMR in a single spectrometer. We use a sample shuttle to displace the NMR sample in the stray field of a high-field magnet to explore low magnetic fields in the course of an NMR experiment, while keeping polarization and detection at high field for sensitivity and resolution. In addition, the sample shuttle couples a magnetic center at 0.33 T with a magnetic center at 14.1 T in a two-field NMR spectrometer. This system allows us to perform pulse sequences, where each part is performed at the most optimal of the two fields. We will show a series of applications to the determination of site-specific protein dynamics on nanosecond timescales as well as a series of examples of two-field NMR experiments that provide more efficiency or information than the equivalent high-field-only experiment. Two-field NMR spectroscopy opens a route to boost the potential of high-resolution biomolecular NMR.