Experimental and theoretical investigation of the 13C and 15N chemical shift tensors in melanostatin-exploring the chemical shift tensor as a structural probe

The determination of backbone conformations in powdered peptides using C-13 and N-15 shift tensor information is explored. The C-13 and N-15 principal shift values in natural abundance C-13 and N-15 melanostatin (L-Pro-L-Leu-Gly amide) are measured using the FIREMAT technique. Furthermore, the orientation of the C-N bond in the C-13 shift principal axis system for the backbone carbons is obtained from the presence of the C-13-N-14 dipolar coupling. The Ramachandran angles for the title compound are obtained from solid-state NMR data by comparing the experimentally determined shift tensor information to systematic theoretical shielding calculations on N-formyl-L-amino acid-amide models. The effects of geometry optimization and neglect of intermolecular interactions on the theoretical shielding values in the model compounds are investigated. The sets of NMR derived Ramachandran angles are assembled in a set of test structures that are compared to the available single-crystal X-ray structure. Shift tensor calculations on the test structures and the X-ray structure are used to further assess the importance of intermolecular interactions when the shift tensor is used as a structural probe in powdered peptides.