Microsecond Dynamics in Ubiquitin Probed by Solid-State ¹⁵ N NMR Spectroscopy R_1ρ Relaxation Experiments under Fast...

¹⁵ N R_1ρ relaxation experiments in solid-state NMR spectroscopy are sensitive to timescales and amplitudes of internal protein motions in the hundreds of nano- to microsecond time window, which is difficult to probe by solution-state NMR spectroscopy. By using ¹⁵ N R_1ρ relaxation experiments, a simplified approach to detect low microsecond protein dynamics is described and residue-specific correlation times are determined from the ratio of ¹⁵ N R_1ρ rate constants at different magic angle spinning frequencies. Microcrystalline ubiquitin exhibits small-amplitude dynamics on a timescale of about 1 μs across the entire protein, and larger amplitude motions, also on the 1 μs timescale, for several sites, including the β₁ -β₂ turn and the N terminus of the α helix. According to the analysis, the microsecond protein backbone dynamics are of lower amplitude than that concluded in previous solid-state NMR spectroscopy studies, but persist across the entire protein with a rather uniform timescale of 1 μs.