The Origin of Protein Sidechain Order Parameter Distributions

Previous work by Wand et al. (2001) showed that the NMR order parameters characterizing the amplitude of motion of protein side chains seemed to form a multimodal distribution.  At the time, no detailed explanation of this at the mol. level was offered, yet three classes of motion were inferred.  We have analyzed a larger published data set and found that, although the distribution is multimodal, the evidence for three classes is weak.  More significantly, we have been able to provide a simple phys. explanation for the distributions based on the results of mol. dynamics simulations.  This result will aid in the interpretation of data from NMR dynamics expts,