Characterization of 15N Chemical Shift and 1H−15N Dipolar Coupling Interactions in a Peptide Bond of Uniaxially Oriented and Polycrystalline Samples by One-Dimensional Dipolar Chemical Shift Solid-State NMR Spectroscopy

The magnitudes and orientations of the principal elements of the 15 N chemical shift and 1 H− 15 N dipolar coupling interaction tensors pertaining to the glycine residue in 15 N -acetyl glycine (NAG) and [ 15 N-Gly]collagen were determined by the analysis of one-dimensional dipolar chemical shift powder patterns. A one-dimensional 1 H− 15 N dipolar 15 N chemical shift spectrum was obtained on a [ 15 N-Gly]collagen fiber sample with the fiber axis oriented parallel to the external magnetic field. The dipolar chemical shift spectrum enabled the orientation of the peptide plane to be determined relative to the direction of the applied magnetic field or the triple-helix axis of the collagen fiber. The magnitudes of the principal elements of the tensors and their orientations in the molecular frame for these two sites are quite different. The magnitudes of the chemical shift tensors are 42.3, 67, and 223.4 ppm for [ 15 N-Gly]collagen and 37, 82.8, and 220.4 ppm for NAG. The angle (β N ) between the least shielded 15 N chemical shift tensor element, σ 33N, and the N−H bond is 24.5° for [ 15 N-Gly]collagen and 25.5° (or 154.5°) for NAG. The angle (α N ) between the most shielded 15 N chemical shift tensor element, σ 11N, and the projection of the N−H bond on the σ 11N −σ 22N plane is 145° for [ 15 N-Gly]collagen and 25° (or 155°, 205°, or 335°) for NAG. Because of the identical dipolar chemical shift powder patterns for four different α N values (35°, 145°, 215°, and 325°) the correct value of the α N angle was determined as 145° using the dipolar chemical shift spectrum of the oriented [ 15 N-Gly]collagen sample.