Proteins exist not as singular structures with precise coordinates, but rather, as fluctuating bodies that move rapidly through an enormous number of conformational substates. These dynamics have important implications for understanding protein function and for structure-based drug design. NMR spectroscopy is particularly well-suited to characterize dynamics of proteins and other molecules in solution at atomic resolution. Here, NMR relaxation methods for characterizing thermal motions on the picosecond-nanosecond (ps-ns) timescale are reviewed. Motion on this timescale can be conveniently captured by the Lipari-Szabo order parameter, S2, a bond-specific measure of restriction of motion. Approaches for determining order parameters are discussed, as are recent examples from the literature that link ps-ns dynamics with conformational entropy, allostery, and protein function in general.
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