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J Phys Chem B. 2015 Sep 10;119(36):11978-87. doi: 10.1021/acs.jpcb.5b07181. Epub 2015 Aug 28.

The Quest for Simplicity: Remarks on the Free-Approach Models.

Author information

  • 1Department for NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry , Am Fassberg 11, 37077 Göttingen, Germany.
  • 2Deutsches Zentrum für Neurodegenerative Erkrankungen (DZNE) , Am Fassberg 11, 37077 Göttingen, Germany.
  • 3Centre of New Technologies, University of Warsaw , Banacha 2C, 02-097 Warsaw, Poland.
  • 4Institute of Biochemistry and Biophysics, Polish Academy of Sciences , Pawinskiego 5A, 02-106 Warsaw, Poland.


Nuclear magnetic relaxation provides a powerful method giving insight into molecular motions at atomic resolution on a broad time scale. Dynamics of biological macromolecules has been widely exploited by measuring (15)N and (13)C relaxation data. Interpretation of these data relies almost exclusively on the use of the model-free approach (MFA) and its extended version (EMFA) which requires no particular physical model of motion and a small number of parameters. It is shown that EMFA is often unable to cope with three different time scales and fails to describe slow internal motions properly. In contrast to EMFA, genuine MFA with two time scales can reproduce internal motions slower than the overall tumbling. It is also shown that MFA and simplified EMFA are equivalent with respect to the values of the N-H bond length and chemical shift anisotropy. Therefore, the vast majority of (15)N relaxation data for proteins can be satisfactorily interpreted solely with MFA.

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