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

摘要

Site-specific ~(15)N chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide ~(15)N 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 Pf11 suspension. This information is complemented by measurement of cross-correlated relaxation rates between the ~(15)N CSA tensor and either the ~(15)N-~1H or ~(15)N-~(13)C' dipolar interaction. In agreement with recent solid state NMR measurements, the ~(15)N CSA tensors exhibit only a moderate degree of variation from averaged values, but have larger magnitudes in α-helical (-173 ± 7 ppm) than in β-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° with the N-H bond, with the asymmetry of the ~(15)N CSA tensor being slightly smaller in a-helix (η = 0.23 ± 0.17) than in β-sheet (η = 0.31 ± 0.11). The residue-specific ~(15)N CSA values are validated by improved agreement between computed and experimental ~(15)N R_(1p) relaxation rates measured for ~(15)N-{~2H} sites in GB3, which are dominated by the CSA mechanism. Use of residue-specific ~(15)N 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.