title: Ab initio study of (13)C(alpha) chemical shift anisotropy tensors in peptides.
abstract: This study reports magnitudes and the orientation of the (13)C(alpha) chemical shift anisotropy (CSA) tensors of peptides obtained using quantum chemical calculations. The dependency of the CSA tensor parameters on the energy optimization of hydrogen atom positions and hydrogen bonding effects and the use of zwitterionic peptides in the calculations are examined. Our results indicate that the energy optimization of the hydrogen atom positions in crystal structures is necessary to obtain accurate CSA tensors. The inclusion of intermolecular effects such as hydrogen bonding in the calculations provided better agreement between the calculated and experimental values; however, the use of zwitterionic peptides in calculations, with or without the inclusion of hydrogen bonding, did not improve the results. In addition, our calculated values are in good agreement with tensor values obtained from solid-state NMR experiments on glycine-containing tripeptides. In the case of peptides containing an aromatic residue, calculations on an isolated peptide yielded more accurate isotropic shift values than the calculations on extended structures of the peptide. The calculations also suggested that the presence of an aromatic ring in the extended crystal peptide structure influences the magnitude of the delta(22) which the present level of ab initio calculations are unable to reproduce.
authors.affiliation: Biophysics Research Division and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
authors.forename: Jeff
authors.initials: J
authors.lastname: Birn
authors.forename: Alan
authors.initials: A
authors.lastname: Poon
authors.forename: Y
authors.initials: Y
authors.lastname: Mao
authors.forename: A
authors.initials: A
authors.lastname: Ramamoorthy
mesh_terms: D016880:Anisotropy; D002247:Carbon Isotopes; D004151:Dipeptides / Q000737:chemistry; D006860:Hydrogen Bonding; D008956:Models, Chemical; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods*; D009842:Oligopeptides / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D002247:Carbon Isotopes; D004151:Dipeptides; D009842:Oligopeptides