Direct measurement of 15N chemical shift anisotropy in solution

The magnitude and orientation of the N-15 chemical shift anisotropy (CSA) tensors are determined for human ubiquitin in solution from N-15 relaxation data at 600, 500, and 360 MHz. The analysis uses the model-independent approach [Fushman, D.; Cowburn, D. J. Am. Chem. Soc. 1998, 120, 7109-10] based on a ratio, eta/R-2, of the cross correlation (eta) between N-15 CSA and N-15-H-1 dipolar interaction and of the rate (R-2) of N-15 transverse relaxation. Since the eta/R-2 ratio does not contain any direct dependence on protein dynamics, the present approach is free from assumptions about overall and local motions. The N-15 CSA values fall in the range -125 to -216 ppm, with the average value of -157 +/- 19 ppm; the average angle between the NH bond and the unique principal axis of the N-15 CSA tensor was 15.7 +/- 5.0 degrees (range 6-26 degrees). The results indicate the importance of residue-specific N-15 CSA for accurate analysis of dynamics from relaxation data, and provide access to the CSA in solution, which may be structurally useful.