  Nuclear magnetic resonance (NMR) is sensitive to dynamics on a wide range of
correlation times. Recently, we have shown that analysis of relaxation rates
via fitting to a correlation function with a small number of exponential terms
could yield a biased characterization of molecular motion in solid-state NMR,
due to limited sensitivity of experimental data to certain ranges of
correlation times. We introduced an alternative approach based on 'detectors'
in solid-state NMR, for which detector responses characterize motion for a
range of correlation times, and reduce potential bias resulting from the use of
simple models for the motional correlation functions. Here, we show that
similar bias can occur in the analysis of solution-state NMR relaxation data.
We have thus adapted the detector approach to solution-state NMR, specifically
separating overall tumbling motion from internal motions and accounting for
contributions of chemical exchange to transverse relaxation. We demonstrate
that internal protein motions can be described with detectors when the overall
motion and the internal motions are statistically independent. We illustrate
the detector analysis on ubiquitin with typical relaxation data sets recorded
at a single or at multiple high magnetic fields, and compare with results of
model-free analysis. We also compare our methodology to LeMaster's method of
dynamics analysis.
