Comparison of solid-state dipolar couplings and solution relaxation data provides insight into protein backbone dynamics

Analyses of solution ¹⁵N relaxation data and solid-state ¹H^N?¹⁵N dipolar couplings from a small globular protein, α-spectrin SH3 domain, produce a surprisingly similar pattern of order parameters. This result suggests that there is little or no ns-μs dynamics throughout most of the sequence and, in particular, in the structured portion of the backbone. At the same time, evidence of ns-μs motions is found in the flexible loops and termini. These findings, corroborated by the MD simulations of α-spectrin SH3 in a hydrated crystalline environment and in solution, are consistent with the picture of protein dynamics that has recently emerged from the solution studies employing residual dipolar couplings.