Combined analysis of ¹⁵N relaxation data from solid- and solution-state NMR spectroscopy

It is well-known that structures of globular proteins in liquid and in crystalline solid are essentially identical. Many lines of evidence suggest that internal dynamics are also similar (assuming that the solid sample is well-hydrated and the measurements are conducted at the same temperature). On the basis of this premise, we undertake a combined analysis of solid- and liquid-state ¹⁵N relaxation data from a small globular protein, α-spectrin SH3 domain. The interpretation using the extended Lipari-Szabo model demonstrates that liquid R₁, R₂, NOE, and solid R₁ data measured at multiple fields are mutually consistent. To validate these results, we prepared a series of samples where the protein is dissolved in a water-glycerol solvent. The presence of glycerol ensures that the overall protein tumbling is slowed, thus increasing the visibility of nanosecond time-scale internal motions. When additional data are included in the fitting procedure, a credible picture of protein dynamics is obtained. In particular, the analysis suggests that ns time-scale motions with very low amplitude, S²≈ 0.95, are present throughout the protein. It is envisaged that combined analyses of liquid- and solid-state data can provide an efficient method for detailed characterization of internal dynamics in proteins at multiple time scales.