Characterization of protein motions in the intermediate timescale via MAS NMR

Abstract: The rotating frame relaxation (R1?) experiment is a powerful probe of microsecond to millisecond timescale motions, and has been developed and applied for solution NMR studies of biopolymers. Recent applications of rotating frame relaxation in magic angle spinning solid-state NMR studies highlight an analogous, possibly more powerful opportunity to study dynamics of biopolymers. However, if anisotropic interactions are large, confounding coherent evolution processes make it difficult to accurately determine the exchange effects. While amide 15N measurements have been performed and interpreted, sites with larger anisotropic interactions such as the carbonyl or aromatic groups with their large chemical shift anisotropy (CSA) have been impossible. We recently developed a pulse sequence that nearly eliminates coherent evolution during the spin lock, while the exchange effects can still be observed (called ?refocused CSA rotating frame relaxation? or RECRR). We plan to characterize the ability of the RECRR experiment to analyze 13C carbonyl sites, focusing on slow intermediate exchange motions (k < ~ 103 s-1). Coherent contributions due to dipolar couplings during the RECRR experiment appear to be small, but will be characterized in detail using numerical simulations and experimental data using phenylalanine?HCl as a model system, and with this information in mind we will develop isotopic enrichment protocols for these experiments. We will perform relaxation dispersion measurements on the protein ubiquitin, contrasting RECRR and the traditional spin lock, and characterizing the ability of these two experiments to obtain quantitative descriptions of the motion. Since RECRR is able to probe both 13C carbonyls and 15N amide sites, this experiment should uncover significantly more insights into the mechanism of motion for protein backbone sites. Motion models derived from simulations of these data will be evaluated using subsequent data at additional field strengths. Finally, we plan to apply these methods to study activation and inactivation of the potassium ion channel, KcsA. We contrast the dynamics of the Activated state in exchange with the Deactivated state, vs. the Activated state in exchange with the Deactivated state, both of which are expected to involve slow to intermediate motions. Previous work suggests that clusters of amino acids are associated with the allosteric coupling between activation and inactivation process. The proposed dynamics measurements will clarify the molecular processes involved.