SOLUTION NMR MEASUREMENT OF AMIDE PROTON CHEMICAL-SHIFT ANISOTROPY INN-15-ENRICHED PROTEINS - CORRELATION WITH HYDROGEN-BOND LENGTH

Cross-correlation between N-15-H-1 dipolar interactions and H-1(N) che mical shift anisotropy (CSA) gives rise to different relaxation rates of the doublet components of H-1-{N-15} peptide backbone amides. Two s chemes for quantitative measurement of this effect are described and d emonstrated for samples of uniformly N-15-enriched ubiquitin and perde uterated N-15-enriched HIV-1 protease. The degree of relaxation interf erence correlates with the isotropic H-1(N) chemical shift, and result s indicate that an upfield change of the most shielded principal compo nents of the CSA tensor is correlated with an approximately 2-fold lar ger downfield shift of the average of the other two components. The ma gnitude of the relaxation interference is large in beta-sheet and cons iderably smaller in alpha-helices. This correlation is not dominated b y the backbone geometry bur reflects the slightly longer hydrogen bond length in helices compared to beta-sheet. The smallest relaxation int erference 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 n ot highly mobile.