DETERMINATION OF THE C-13 CHEMICAL-SHIFT AND N-14 ELECTRIC-FIELD GRADIENT TENSOR ORIENTATIONS WITH RESPECT TO THE MOLECULAR FRAME IN A POLYPEPTIDE

With the determination of the nuclear spin interaction tensor orientation for the site of interest in a macromolecule it is now possible to achieve very high resolution structural and dynamic information from uniformly aligned samples of a macromolecule. Here the orientation of both the carbonyl C-13 chemical shift and N-14 electric field gradient tensors are determined with respect to the molecular frame for backbone sites in polypeptides from solid state NMR spectroscopy. N-15 and N-14 dipolar coupled C-13 spectra were analyzed. In both cases at a 50 MHz C-13 frequency the presence of the dipolar coupling was plainly seen in powder pattern spectra, and for N-14 coupled spectra the effect of the quadrupole interaction was pronounced. Such in situ tensor characterizations are shown to substantially enhance the interpretation of structurally derived data. To make meaningful comparisons among the previously reported tensor orientations it was necessary to define the tensor elements based on their general orientation with respect to the molecular frame rather than the typical frequency based assignments. This is necessitated by the variation in tensor element magnitudes among a set of closely related compounds that makes the frequency based assignments meaningless.