  The use of relaxation interference in the methyl Transverse
Relaxation-Optimized SpectroscopY (TROSY) experiment has opened new avenues for
the study of large proteins and protein assemblies in nuclear magnetic
resonance. So far, the theoretical description of the methyl-TROSY experiment
has been limited to the slow-tumbling approximation, which is correct for large
proteins on high field spectrometers. In a recent paper, favorable relaxation
interference was observed in the methyl groups of a small protein at a magnetic
field as low as 0.33 T, well outside the slow-tumbling regime. Here, we present
a model to describe relaxation interference in methyl groups over a broad range
of magnetic fields, not limited to the slow-tumbling regime. We predict that
the type of multiple-quantum transitions that show favorable relaxation
properties change with the magnetic field. Under the condition of fast
methyl-group rotation, methyl-TROSY experiments can be recorded over the entire
range of magnetic fields from a fraction of 1 T up to 100 T.
