Probing Transient Conformational States of Proteins by Solid-State R(1ρ) Relaxation-Dispersion NMR Spectroscopy**

The function of proteins depends on their ability to sample a variety of states differing in structure and free energy. Deciphering how the various thermally accessible conformations are connected, and understanding their structures and relative energies is crucial in rationalizing protein function. Many biomolecular reactions take place within microseconds to milliseconds, and this timescale is therefore of central functional importance. Here we show that R(1ρ) relaxation dispersion experiments in magic-angle-spinning solid-state NMR spectroscopy make it possible to investigate the thermodynamics and kinetics of such exchange process, and gain insight into structural features of short-lived states.