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

A two-dimensional solid-state NMR technique is presented that can be used t o determine the N-15 chemical shift and H-1-N-15 dipolar coupling tensors i n powder samples of polypeptides containing 15N isotopes at multiple sites. By combining the magic-angle rf decoupling in one time period and the magi c-angle turning pulse sequence in another time period of a 2D experiment, w e 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 elem ents of the 15N CSA tensors and their orientations in the molecular frame f or n-acetyl-N-15-L-Val-N-15-L-Leu (NAVL) and n-acetyl-N-15-D,L-Val (NAV) po wder samples are determined using this method. The magnitudes of the N-15 C SA tensors are 60.2 +/- 1,87.1 +/- 1, and 230.1 +/- 1 ppm for the Val resid ue 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; an d 57.5 +/- 2, 81.0 +/- 2, and 227.0 +/- 2 ppm for site II in NAV. The exper imental results also suggest that the most-shielded axis, sigma (11N), and the sigma (22N) axis of the N-15 CSA tenser are significantly tilted away f rom the peptide plane and the normal to the peptide plane, respectively. Th e tilt angles (alpha (N)) are 34 +/- 12 degrees for Val and 36 +/- 11 degre es for Leu in NAVL, whereas it is 5 +/- 22 degrees in NAV. The angles (beta (N)) between the least-shielded axis of the N-15 CSA tensor, sigma (33N), in the peptide plane and-the N-H bond are determined to be 20 +/- 2 degrees for Val and 18 +/- 2 degrees for Leu in NAVL and 21 +/- 2 degrees for NAV. The values are in good agreement with some of the recent solid-state NMR e xperimental studies on peptides. The values for the PN angle reported in th is study are also, in agreement with the solution NMR studies on water-solu ble proteins.