Thermodynamic interpretation of NMR relaxation parameters in proteins in the presence of motional correlations

It is shown by quasiharmonic analysis that the conformational partition function of a globular protein sampled on the ns time scale can be factorized in good approximation into a purely reorientational part, which determines heteronuclear NMR spin relaxation, and a remaining part that includes other types of intramolecular motions. This factorization provides a statistical mechanical basis for the thermodynamic interpretation of NMR relaxation parameters of proteins where atomic motions can be significantly correlated. Reorientational entropy differences between different backbone parts of the globular protein ubiquitin are found to be remarkably insensitive to motional correlation effects, which in some cases may significantly facilitate the entropic interpretation of changes of NMR order parameters.