Protein Backbone Dynamics and 15N Chemical Shift Anisotropy from Quantitative Measurement of Relaxation Interference Effects

Cross-correlation between 15 N− 1 H dipolar interactions and 15 N chemical shift anisotropy (CSA) gives rise to different relaxation rates of the doublet components of 15 N−{ 1 H} peptide backbone amides. A simple scheme for quantitative measurement of this effect is described which yields information on the magnitude of the CSA from the relative intensities of 1 H− 15 N correlations obtained with two slightly different pulse schemes. The method is applied to a sample of uniformly 15 N-enriched ubiquitin and measurements conducted at two field strengths (8.5 and 14 T) yield identical results. The degree of relaxation interference correlates with the isotropic 15 N chemical shift and results indicate that the sum of the most shielded principal components of the CSA tensor is nearly invariant to structural differences in the polypeptide backbone. The relaxation interference is directly proportional to the generalized order parameter, S 2, of the peptide backbone amides, and this relation can be utilized to obtain approximate values for these order parameters.