Proton Off-Resonance R1ρ Near-Rotary-Resonance Relaxation Dispersion under Fast Magic-Angle Spinning as a Versatile Access to Conformational Exchange.

Relaxation dispersion techniques allow access to μs time-scale protein motion both in solution and in solid-state NMR. For solids under fast magic-angle spinning, R1ρ relaxation dispersion for spin-lock fields around any rotary-resonance conditions (NERRD) adds highly versatile observables absent in solution that can report on site-specific μs time-scale fluctuations of anisotropic interactions. Proton NERRD, as opposed to 15N or 13C relaxation dispersion, can report on changes within the regional proton dipolar-coupling network around a given site. It is associated with distinct technical opportunities and, in principle, avoids the necessity for heteronuclear isotope labeling. Here we show that one of the current limitations─the presence of strong spin diffusion effects during proton R1ρ measurements, which can cause a deindividualization of the site-specific NERRD profiles and hence a blurring of their motional information─can effectively be ameliorated by off-resonance spin locks. Even though the fundamental limitations of proton relaxation compared to relaxation of more isolated nuclei cannot fully be overcome by technical innovations, the reduction of proton-proton crosstalk facilitates qualitative access to μs time-scale motion exclusively through protons in biology and materials.