How Fast is Your Camera? Timescales for Molecular Motion and their Role in Restraining Molecular Dynamics

High-resolution structural information is routinely available for soluble proteins, largely from x-ray crystallography and solution nuclear magnetic resonance (NMR). However, these techniques are far harder to apply to membrane-bound proteins; membrane proteins are generally reluctant to crystallize, and the need for a surrounding lipid matrix generally means that NMR must be performed under solid-state as opposed to solution conditions. The latter imposes some restrictions on the kinds of information that can be readily extracted from experiments, and one often must rely on experimental methods such as residual dipolar coupling and chemical shift anisotropy, which yield information about the orientations of specific moieties relative to the magnetic field.