Model-free approach beyond the borders of its applicability.

Model calculations presented in this article show that commonly used methodology of 15N relaxation data analysis completely fails in detecting nanosecond time scale motions if the major part of the molecule is involved in these motions. New criteria are introduced for the detection of such cases, based on the dependence of the apparent overall correlation time, derived from the T1/T2 ratio, on the spectrometer frequency. Correctly estimating the overall rotation correlation time tauR was shown to play the key role in model-free data analysis. It is found, however, that in cases of slow internal motions with characteristic times of more than 3-4 ns, the effective tauR provided by the T1/T2 ratio for individual amide nitrogens can be used for the characterization of the fast picosecond internal dynamics.