Direct Measurement of 15N Chemical Shift Anisotropy in Solution

Abstract

The magnitude and orientation of the 15N chemical shift anisotropy (CSA) tensors are determined for human ubiquitin in solution from 15N 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, η/R2, of the cross correlation (η) between 15N CSA and 15N−1H dipolar interaction and of the rate (R2) of 15N transverse relaxation. Since the η/R2 ratio does not contain any direct dependence on protein dynamics, the present approach is free from assumptions about overall and local motions. The 15N 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 15N CSA tensor was 15.7 ± 5.0° (range 6−26°). The results indicate the importance of residue-specific 15N CSA for accurate analysis of dynamics from relaxation data, and provide access to the CSA in solution, which may be structurally useful.