Chemical Shift Tensors in Peptides: A Quantum Mechanical Study

DFT-IGLO shielding tensor calculations for the non-hydrogen nuclei in model dipeptides are presented. The behavior of the principal components, their magnitude and orientation, is investigated as a function of the dihedral angles φ and ψ. For both C° and amide N sites, the shielding tensor follows the molecular framework, while for C α the tensor does not. Shielding tensor surfaces that span the entire Ramachandran surface (φ = −180° to 180°, ψ = −180° to 180°) have been constructed for both carbonyl carbon and amide nitrogen shielding tensors in a model glycyl-glycine dipeptide. The principal tensor element σ 22 of the carbonyl shielding tensor always lies close to the C O bond (7°−9°) while the σ 11 component of the amide nitrogen shielding tensor is always slightly tilted (18°−22°) from the N−H bond. The effects of hydrogen bonding on the orientation and magnitude of the shielding tensor components are also investigated at selected conformations: extended, φ = 180° and ψ = 180°; sheet, φ = −120° and ψ = 120°; and helix, φ = −60° and ψ = −60°. For C α, calculations indicate that the shielding value along the C α −H α vector is very sensitive to secondary structure.