MAGNITUDES AND ORIENTATIONS OF THE PRINCIPAL ELEMENTS OF THE H-1 CHEMICAL-SHIFT, H-1-N-15 DIPOLAR COUPLING, AND N-15 CHEMICAL-SHIFT INTERACTION TENSORS IN N-15(EPSILON-1)-TRYPTOPHAN AND N-15(PI)-HISTIDINE SIDE-CHAINS DETERMINED BY 3-DIMENSIONAL SOLID-STATE NMR-SPECTROSCOPY OF POLYCRYSTALLINE SAMPLES

The magnitudes and orientations of the principal elements of the H-1 c hemical shift, H-1-N-15 dipolar coupling, and N-15 chemical shift inte raction tensors in N-15(epsilon l)-tryptophan and N-15(pi)-histidine n itrogen sites were determined by. the analysis of three-dimensional po wder patterns obtained from N-15-labeled powder samples of the amino a cids. Although the magnitudes of the principal elements of the H-1 and N-15 chemical shift tensors for these two sites are quite different, their molecular orientations in the molecular frame are very similar. The least shielded N-15 chemical shift tensor element, sigma(33N), and the most shielded H-1 chemical shift tensor element, sigma(11H), are approximately colinear with the N-H bond in both cases. The principal elements, sigma(22H) and sigma(22N), are in the plane of the indole ri ng for tryptophan and in the plane of the imidazole ring for histidine but oppose each other. sigma(11N) and sigma(33H) are perpendicular to the planes of these heterocyclic rings, The chemical shift tensors of the H-1 and N-15 nuclei in these two side chain nitrogen sites are di stinctly different from those of backbone amide nitrogen sites.