Theoretical approaches are being developed to extract information concerning the nature of internal motions in a variety of biopolymers from nuclear magnetic relaxation and time-resolved fluorescence depolarization experiments. Nuclear magnetic resonance experiments also provide information on the structure of molecules since the dipole-dipole interaction between two nuclei is proportional to the inverse third power of the distance between them. The influence of vibrational motion on bond lengths and quadrupole constants obtained solid state powder patterns was examined. It was shown that such motions average both the magnitude and orientation of the intrinsic interaction tensor and that the effect of vibrations on relaxation can be rigorously incorporated into an effective coupling constant that is identical to the one that determines the line shape. The influence of vibrational averaging on the magnitude of the dipolar interaction tensor was shown to be independent of the molecule and to be small. On the other hand, averaging of the orientation of the dipolar tensor depends on the size of the molecule because of the predominant role played by low frequency tortional modes. These results have important implications for the value of the effective internuclear distance that should be used for the interpretation of dipolar relaxation experiments. The unique information content of solid state NMR powder patterns on the nature of the orientational distribution function which describes amplitudes of molecular motions has been established. The line shape was shown to be completely specified by its second and third moments. The second moment was shown to be proportional to the generalized order parameter, thus establishing the connection between solution and solid state experiments. Finally, the Lipari-Szabo model-free approach to the interpretation of relaxation experiments has been successfully applied to a multifield C13 NMR study of the molecular dynamics of selectively C13-enriched ribonuclease complexes.
