A two-dimensional magic-angle decoupling and magic-angle turning solid-state NMR method: An application to study chemical shift tensors from peptides that are nonselectively labeled with ¹⁵N isotope

A two-dimensional solid-state NMR technique is presented that can be used to determine the ¹⁵N chemical shift and ¹H-¹⁵N dipolar coupling tensors in powder samples of polypeptides containing ¹⁵N isotopes at multiple sites. By combining the magic-angle rf decoupling in one time period and the magic-angle turning pulse sequence in another time period of a 2D experiment, we obtain 2D spectra in which the convoluted chemical shift anisotropy (CSA) and dipolar coupling line shapes appear along one axis and the normal MAS spectrum appears along the other axis. The magnitudes of the principal elements of the ¹⁵N CSA tensors and their orientations in the molecular frame for n-acetyl-¹⁵N-L-Val-¹⁵N-L-Leu (NAVL) and n-acetyl-¹⁵N-D,L-Val (NAV) powder samples are determined using this method. The magnitudes of the ¹⁵N CSA tensors are 60.2 ± 1, 87.1 ± 1, and 230.1 ± 1 ppm for the Val residue in NAVL; 58.7 ± 1, 93.7 ± 1, and 232.8 ± 1 ppm for the Leu residue in NAVL; 59.6 ± 1, 80.5 ± 1, and 235.3 ± 1 ppm for site I in NAV; and 57.5 ± 2, 81.0 ± 2, and 227.0 ± 2 ppm for site II in NAV. The experimental results also suggest that the most-shielded axis, σ_11N, and the σ_22N axis of the ¹⁵N CSA tensor are significantly tilted away from the peptide plane and the normal to the peptide plane, respectively. The tilt angles (α_N) are 34 ± 12° for Val and 36 ± 11° for Leu in NAVL, whereas it is 5 ± 22° in NAV. The angles (β_N) between the least-shielded axis of the ¹⁵N CSA tensor, σ_33N, in the peptide plane and the N-H bond are determined to be 20 ± 2° for Val and 18 ± 2° for Leu in NAVL and 21 ± 2° for NAV. The values are in good agreement with some of the recent solid-state NMR experimental studies on peptides. The values for the β_N angle reported in this study are also in agreement with the solution NMR studies on water-soluble proteins.