Theory of Non-rigid Rotational Motion Applied to NMR Relaxation in RNA

摘要

Solution NMR spectroscopy can elucidate many features of the structure and dynamics of macromolecules, yet relaxation measurements, the most common source of experimental information on dynamics, can only sample certain ranges of dynamic rates. A complete characterization of motion of a macromolecule therefore requires the introduction of complementary experimental approaches. Solid-state NMR successfully probes the nanoseconds to microseconds (ns to μs) time scale, a dynamic window where solution NMR results have been deficient, and probes conditions where the averaging effects of rotational diffusion of the molecule are absent. Combining the results of the two distinct techniques in a single framework provides greater insight into dynamics, but this task requires the common interpretation of results recorded under very different experimental conditions. Here we provide a unified description of dynamics that is robust to the presence of large-scale conformational exchange, where the diffusion tensor of the molecule varies on a time scale comparable to rotational diffusion in solution. We apply this methodology to the HIV-I TAR RNA molecule, where conformational rearrangements are both substantial and functionally important. The formalism described herein is of greater generality than earlier combined solid state/solution NMR interpretations, if detailed molecular structures are available, and can offer a more complete description of RNA dynamics then either solution or solid state NMR alone.