Model-free approach to the dynamic interpretation of residual dipolar couplings in globular proteins

The effects of internal motions on residual dipolar NMR couplings of protei ns partially aligned in a liquid-crystalline environment are analyzed using a 10 ns molecular dynamics (MD) computer simulation of ubiquitin, For a se t of alignment tensors with different orientations and rhombicities, MD-ave raged dipolar couplings are determined and subsequently interpreted for dif ferent scenarios in terms of effective alignment tensors, average orientati ons of dipolar vectors, and intramolecular reorientational vector distribut ions. Analytical relationships are derived that reflect similarities and di fferences between motional scaling of dipolar couplings and scaling of dipo lar relaxation data (NMR order parameters). Application of the self-consist ent procedure presented here to dipolar coupling measurements of biomolecul es aligned in different liquid-crystalline media should allow one to extrac t in a "model-free" way average orientations of dipolar vectors and specifi c aspects of their motions.