Molecular dynamics as an efficient process to predict ¹⁵N chemical shift anisotropy at very high NMR magnetic fields

The emergence of very high NMR magnetic fields will certainly encourage the study of larger biological systems with their dynamics and interactions. NMR spin relaxation allows probing the dynamical properties of proteins where the ¹⁵N longitudinal (R₁) and transverse (R₂) relaxation rates in addition to the ¹H-¹⁵N heteronuclear NOE describe the ps-ns time scale. Their analytical representation involves the chemical shift anisotropy (CSA) effect that represents the major contribution at a very high magnetic field above 18.8 T. An accurate analysis of the latter parameters in terms of model free (MF) requires considering its effect. Until now, a uniform value of -160 ppm for the CSA has been widely used to derive the backbone order parameters (S²), giving rise to a large fluctuation of its value at very high magnetic fields. Conversely, the use of a site-specific CSA improves the accurate analysis of protein dynamics but requires a cost-effective experimental multi-field approach. In the present paper, we show how the CSA mainly contributes to the relaxation parameters at 28.2 T compared to lower magnetic fields and may bias the determination of S². We propose to replace the time-consuming measurement of spin relaxation at multiple fields by a combination of molecular dynamics (MD) and the measurement of spin relaxation at one very high magnetic field only. We applied this strategy to three well-folded proteins (ubiquitin, GB3 and ribonuclease H) to show that the determined order parameters are in good agreement with the ones obtained by means of experimental data only.