Some Recent Developments in NMR Approaches for Studying Liquid Molecular Dynamics and Their Biological Applications

Two aspects of NMR relaxation are reviewed and their use in studying molecular dynamics in the liquid state and their application to biological systems are discussed. The first aspect concerns the use of ‘null‐point spectra’ for observing the effects of higher rank multipoles (tensors) in the relaxation of quadrupolar nuclei. The second aspect concerns the effects of cross‐correlation in homo‐ and heteronuclear spin systems. The application of null point spectral analysis to study cross‐correlation in homonuclear spin systems is also discussed. Analysis of the effects of higher rank multipoles and of cross‐correlation effects are shown to provide substantially more structural and motional information about spin systems than that available from traditional relaxation measurements. While similar in information content to multiple quantum techniques, the null point technique is experimentally less demanding. Over the past several years, work in this laboratory has been heavily involved in the theoretical formulation and the application of these techniques to biological systems. Illustrations of the use of these techniques to biological systems are drawn from our recent work.