Magnitudes and Orientations of the Principal Elements of the 1H Chemical Shift, 1H−15N Dipolar Coupling, and 15N Chemical Shift Interaction Tensors in 15Nε1-Tryptophan and 15Nπ-Histidine Side Chains Determined by Three-Dimensional Solid-State NMR Spectroscopy of Polycrystalline Samples

The magnitudes and orientations of the principal elements of the 1 H chemical shift, 1 H− 15 N dipolar coupling, and 15 N chemical shift interaction tensors in 15 N ε1 -tryptophan and 15 N π -histidine nitrogen sites were determined by the analysis of three-dimensional powder patterns obtained from 15 N-labeled powder samples of the amino acids. Although the magnitudes of the principal elements of the 1 H and 15 N chemical shift tensors for these two sites are quite different, their molecular orientations in the molecular frame are very similar. The least shielded 15 N chemical shift tensor element, σ 33N, and the most shielded 1 H chemical shift tensor element, σ 11H, are approximately colinear with the N−H bond in both cases. The principal elements, σ 22H and σ 22N, are in the plane of the indole ring for tryptophan and in the plane of the imidazole ring for histidine but oppose each other. σ 11N and σ 33H are perpendicular to the planes of these heterocyclic rings. The chemical shift tensors of the 1 H and 15 N nuclei in these two side chain nitrogen sites are distinctly different from those of backbone amide nitrogen sites.