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| - Experimental and Theoretical Investigation of the 13C and 15NChemical Shift Tensors in MelanostatinExploring theChemical Shift Tensor as a Structural Probe
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| - The determination of backbone conformations in powdered peptides using 13C and 15N shifttensor information is explored. The 13C and 15N principal shift values in natural abundance 13C and 15Nmelanostatin (l-Pro-l-Leu-Gly amide) are measured using the FIREMAT technique. Furthermore, theorientation of the C−N bond in the 13C shift principal axis system for the backbone carbons is obtainedfrom the presence of the 13C−14N dipolar coupling. The Ramachandran angles for the title compound areobtained from solid-state NMR data by comparing the experimentally determined shift tensor informationto systematic theoretical shielding calculations on N-formyl-l-amino acid-amide models. The effects ofgeometry optimization and neglect of intermolecular interactions on the theoretical shielding values in themodel compounds are investigated. The sets of NMR derived Ramachandran angles are assembled in aset of test structures that are compared to the available single-crystal X-ray structure. Shift tensor calculationson the test structures and the X-ray structure are used to further assess the importance of intermolecularinteractions when the shift tensor is used as a structural probe in powdered peptides.
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