How Does an Amide-N Chemical Shift Tensor Vary in Peptides?

This study addresses a void in the existing literature on the amide-(15)N chemical shift anisotropy (CSA) tensor of peptides: a systematic investigation of how the tensor varies in different peptides. Amide-(15)N CSA tensors for several dipeptides are obtained using quantum chemical calculations, as well as for a series of model Ala-X and X-Ala sequences in both α-helical and β-sheet conformations (where X is one of the naturally occurring amino acids). The calculated values show a significant variation in both isolated and extended peptide structures. Hydrogen bonding at both the carbonyl group and the N-H bond of the peptide plane is shown to affect the principal values of the tensor. Calculations on model peptides indicate that the amide-(15)N CSA tensor is dependent on atoms located within a distance of five bonds. Consequently, the tensor of a given peptide residue is unaffected by residues other than those adjacent to it, which implies that the amide-(15)N CSA tensor should be considered in the context of tripeptide sequences. This further suggests that the amide-(15)N CSA tensor of the second residue of a given tripeptide sequence may be extrapolated to the same sequence in any other polypeptide or protein, given the same backbone conformation and intermolecular environment. These conclusions will facilitate future NMR structural studies of proteins.