Because the Light is Better Here: Correlation‐Time Analysis by NMR Spectroscopy

Relaxation data in NMR spectra are often used for dynamics analysis, by modeling motion in the sample with a correlation function consisting of one or more decaying exponential terms, each described by an order parameter, and a correlation time. This method has its origins in the Lipari–Szabo model‐free approach, which originally considered overall tumbling plus one internal motion and was later expanded to several internal motions. Considering several of these cases in the solid state it is found that if the real motion is more complex than the assumed model, model fitting is biased towards correlation times where the relaxation data are most sensitive. This leads to unexpected distortions in the resulting dynamics description. Therefore dynamics detectors should be used, which characterize different ranges of correlation times and can help in the analysis of protein motion without assuming a specific model of the correlation function. Fighting bias: NMR Dynamics data are more sensitive to some correlation times than to others. Models of the correlation function tend to be biased towards where the light is better, that is, where the experiment is more sensitive, thereby yielding an unreliable characterization of the motion. Replacing modeling by detectors that are sensitive to different ranges of correlation times could help to overcome this bias.