For diamagnetic molecules at moderate field strengths, molecules have little preference in orientation, the tumbling samples a nearly isotropic distribution, and average dipolar couplings goes to zero. Actually, most molecules have preferred orientations in the presence of a magnetic field, because most have anisotropic magnetic susceptibility tensors, Χ.
The method is most suitable for systems with large values for magnetic susceptibility tensor. This includes: Protein-nucleic acid complex, nucleic acids, proteins with large number of aromatic residues, porphyrin containing proteins and metal binding proteins (metal may be replaced by lanthanides).
For a fully oriented molecule, the dipolar coupling for an H-N amide group would be over 20 kHz, and a pair of protons separated by 5 Å would have up to ~1 kHz coupling. However the degree of alignment achieved by applying magnetic field is so low that the largest H-N or H-C dipolar couplings are <5 Hz. Therefore, many different alignment media have been designed:
*Lipid bicelles (with large magnetic susceptibility): measured RDCs were of the order of hundreds of Hz.
*Liquid crystalline bicelles: measured RDCs were between -40 and +20 Hz.
*Rod-shaped viruses, including filamentous bacteriophage (large anisotropic magnetic susceptibility).
*DNA nanotubes (compatible with detergents used to solubilize membrane proteins)