Site-Specific Backbone Amide 15N Chemical Shift Anisotropy Tensors in a Small Protein from Liquid Crystal and Cross-Correlated Relaxation Measurements

Site-specific N-15 chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide N-15 nuclei in the B3 domain of protein G (GB3) from residual chemical shift anisotropy (RCSA) measured in six different mutants that retain the native structure but align differently relative to the static magnetic field when dissolved in a liquid crystalline Pf1 suspension. This information is complemented by measurement of cross-correlated relaxation rates between the N-15 CSA tensor and either the N-15-H-1 or N-15-C-13' dipolar interaction. In agreement with recent solid state NMR measurements, the N-15 CSA tensors exhibit only a moderate degree of variation from averaged values, but have larger magnitudes in a-helical (-173 +/- 7 ppm) than in beta-sheet (-162 +/- 6 ppm) residues, a finding also confirmed by quantum computations. The orientations of the least shielded tensor component cluster tightly around an in-peptide-plane vector that makes an angle of 19.6 +/- 2.5 degrees with the N-H bond, with the asymmetry of the N-15 CSA tensor being slightly smaller in alpha-helix (eta = 0.23 +/- 0.17) than in beta-sheet (eta = 0.31 +/- 0.11). The residue-specific N-15 CSA values are validated by improved agreement between computed and experimental N-15 R-1p relaxation rates measured for N-15-{H-2} sites in GB3, which are dominated by the CSA mechanism. Use of residue-specific N-15 CSA values also results in more uniform generalized order parameters, S-2, and predicts considerable residue-by-residue variations in the magnetic field strengths where TROSY line narrowing is most effective.