Analysis of the amide 15N chemical shift tensor of the Cα tetrasubstituted constituent of membrane-active peptaibols, the α-aminoisobutyric acid residue, compared to those of di- and tri-substituted proteinogenic amino acid residues

In protein NMR spectroscopy the chemical shift provides important information for the assignment of residues and a first structural evaluation of dihedral angles. Furthermore, angular restraints are obtained from oriented samples by solution and solid-state NMR spectroscopic approaches. Whereas the anisotropy of chemical shifts, quadrupolar couplings and dipolar interactions have been used to determine the structure, dynamics and topology of oriented membrane polypeptides using solid-state NMR spectroscopy similar concepts have been introduced to solution NMR through the measurements of residual dipolar couplings. The analysis of N-15 chemical shift spectra depends on the accuracy of the chemical shift tensors. When investigating alamethicin and other peptaibols, i.e. polypeptides rich in alpha-aminoisobutyric acid (Aib), the N-15 chemical shift tensor of this C-alpha-tetrasubstituted amino acid exhibits pronounced differences when compared to glycine, alanine and other proteinogenic residues. Here we present an experimental investigation on the N-15 amide Aib tensor of N-acetyl-Aib-OH and for the Aib residues within peptaibols. Furthermore, a statistical analysis of the tensors published for di- (glycine) and tri-substituted residues has been performed, where for the first time the published data sets are compiled using a common reference. The size of the isotropic chemical shift and main tensor elements follows the order di- < tri- < tetra-substituted amino acids. A N-15 chemical shift-H-1-N-15 dipolar coupling correlation NMR spectrum of alamethicin is used to evaluate the consequences of variations in the main tensor elements for the structural analysis of this membrane peptide.