Probing Transient Conformational States of Proteins by Solid-State R 11 Relaxation-Dispersion NMR Spec- troscopy

International audienceSolution-state NMR spectroscopic techniques, and in par-ticular so-called relaxation-dispersion (RD) NMR approaches, have proven very successful for studying ms–ms motion and characterizing the exchanging short-lived con-formations. [1] RD NMR techniques exploit the effect of conformational-exchange processes on line broadening, that is, on the relaxation rates of nuclear spin coherence (R 2 , R 11). By quantifying spin relaxation rates in the presence of a variable radiofrequency (rf) field, RD approaches provide information about relative populations and exchange rates, as well as chemical shifts of short-lived conformational states, and thus about local structure. [1, 2] In the case of very large assemblies or insoluble aggregates, where solution-state NMR is severely challenged, magic-angle-spinning solid-state NMR (MAS ssNMR) is rapidly emerging as a tool for the study of structure and dynamics. However, the character-ization of conformational-exchange dynamics in the solid state remains challenging. Herein, we show a ssNMR approach in which amide-15 N R 11 RD data, that is, the rate of coherence decay under 15 N spin-lock fields of variable field strengths, are quantitatively analyzed and interpreted in terms of conformational dynamics, providing insight into short-lived states in terms of chemical shifts and bond vector orientations. We investigate the robustness of this approach by studying a conformational flip in crystalline ubiquitin. Conformational fluctuations between different states expose a given nuclear spin in a protein to different local environments, which are characterized by different bond geometries. A simple case, exchange between two states, is shown in Figure 1 a. As a result of conformational-exchange dynamics, a given spin will experience a fluctuation of its chemical shift (CS), as well as of its bond vector orientations and, thus, dipolar coupling interactions with neighboring spins and CS anisotropies (CSA). In solution-state NMR, dipolar coupling and CSA interactions are averaged to zero by Brownian movement (molecular tumbling). Consequently, only fluctuations of the isotropic CS are relevant when considering conformational dynamics on the microsecond to millisecond timescale. R 11 RD experiments in solution can thus only pick up conformational dynamics if they involve a change in the isotropic CS. The relevant theory is well established, and the effects of exchange can be described by the Bloch–McConnell equations. [3