Slow motions in microcrystalline proteins as observed by MAS-dependent ¹⁵N rotating-frame NMR relaxation

¹⁵N NMR relaxation rate R_1ρ measurements reveal that a substantial fraction of residues in the microcrystalline chicken alpha-spectrin SH3 domain protein undergoes dynamics in the μs-ms timescale range. On the basis of a comparison of 2D site-resolved with 1D integrated ¹⁵N spectral intensities, we demonstrate that the significant fraction of broad signals in the 2D spectrum exhibits the most pronounced slow mobility. We show that ¹⁵N R_1ρ's in proton-diluted protein samples are practically free from the coherent spin-spin contribution even at low MAS rates, and thus can be analysed quantitatively. Moderate MAS rates (10-30 kHz) can be more advantageous in comparison with the rates >50-60 kHz when slow dynamics are to be identified and quantified by means of R_1ρ experiments.