15NH backbone dynamics of protein GB1:: Comparison of order parameters and correlation times derived using various "model-free" approaches

A new model-free approach that allows visualization of the distribution of motional correlation times is presented and is used to analyze N-15 NMR relaxation data [T-1, T-2, and {H-1}-N-15 NOE], acquired at 500, 600, and 800 MHz, on the uniformly N-15-enriched, 56 residue B1 domain from immunoglubulin protein G (GB l). Nanosecond time scale internal motions are found for all NHs of residues in protein GB1, a finding which is consistent with the concept of hierarchical internal motions in proteins first forwarded by Frauenfelder. Order parameters and overall tumbling correlation times derived using this approach are not influenced by the number of internal motional modes. Comparatively, use of the Lipari-Szabo, Clore et al., or LeMaster "model-free" methods yields underestimated order parameters and overestimated overall tumbling correlation times due to the presence of nanosecond time scale internal motions. This new model-free approach provides a straightforward way to derive more accurate order parameters and correlation times for overall tumbling and internal motions.