Solution NMR measurement of amide proton chemical shift anisotropy in N-15-enriched proteins. Correlation with hydrogen bond length

Cross-correlation between N-15-H-1 dipolar interactions and H-1(N) chemical shift anisotropy (CSA) gives rise to different relaxation rates of the doublet components of H-1-{N-15} peptide backbone amides. Two schemes for quantitative measurement of this effect are described and demonstrated for samples of uniformly N-15-enriched ubiquitin and perdeuterated N-15-enriched HIV-1 protease. The degree of relaxation interference correlates with the isotropic H-1(N) chemical shift, and results indicate that an upfield change of the most shielded principal components of the CSA tensor is correlated with an approximately 2-fold larger downfield shift of the average of the other two components. The magnitude of the relaxation interference is large in beta-sheet and considerably smaller in alpha-helices. This correlation is not dominated by the backbone geometry bur reflects the slightly longer hydrogen bond length in helices compared to beta-sheet. The smallest relaxation interference effect in ubiquitin is observed for Ser(20)-H-N and Ile(36)-H-N, which are the only two amide protons that are not hydrogen bonded in the crystal structure of ubiquitin, inaccessible to solvent, and not highly mobile.