The role of 15N CSA and CSA/dipole cross-correlation in 15N relaxation in solid proteins

The influence of the 15N CSA on 15N longitudinal relaxation is investigated for an amide group in solid proteins in powder form under MAS. This contribution is determined to be typically 20-33% of the overall longitudinal relaxation rate, at 11.74 and 16.45 T, respectively. The improved treatment is used to analyze the internal dynamics in the protein Crh, in the frame of a motional model of diffusion in a cone, using the explicit average sum approach. Significant variations with respect to the determined dynamics parameters are observed when properly accounting for the contribution of 15N CSA fluctuations. In general, the fit of experimental data including CSA led to the determination of diffusion times (τw) which are longer than when considering only an 15N-1H dipolar relaxation mechanism. CSA-Dipole cross-correlation is shown to play little or no role in protonated solids, in direct contrast to the liquid state case.