  Nuclear Magnetic Resonance (NMR) is a tool of choice to characterize
molecular motions. In biological macromolecules, pico- to nano-second motions,
in particular, can be probed by nuclear spin relaxation rates which depend on
the time fluctuations of the orientations of spin interaction frames. For the
past 40 years, relaxation rates have been successfully analyzed using the Model
Free (MF) approach which makes no assumption on the nature of motions and
reports on the effective amplitude and time-scale of the motions. However,
obtaining a mechanistic picture of motions from this type of analysis is
difficult at best, unless complemented with molecular dynamics (MD)
simulations. In spite of their limited accuracy, such simulations can be used
to obtain the information necessary to build explicit models of motions
designed to analyze NMR relaxation data. Here, we present how to build such
models, suited in particular to describe motions of methyl-bearing protein
side-chains and compare them with the MF approach. We show on synthetic data
that explicit models of motions are more robust in the presence of rotamer
jumps which dominate the relaxation in methyl groups of protein side-chains. We
expect this work to motivate the use of explicit models of motion to analyze MD
and NMR data.
