Density functional calculations of oxygen, nitrogen and hydrogen electric field gradient and chemical shielding tensors to study hydrogen bonding properties of peptide group (O=C-NH) in crystalline acetamide

A density functional theory (DFT) study was carried out to investigate hydrogen bonding (HB) properties of peptide group (O=C-NH) in crystalline acetamide. Since the peptide group in acetarnicle contributes to N-H center dot center dot center dot O and C-H center dot center dot center dot O types of HB interactions, acetamide is considered as the simplest form of peptide linkage in proteins. The evaluated NMR parameters including quadrupole coupling constants and asymmetry parameters from the calculated electric field gradient (EFG) tensors at the sites of O17, N14 and H2 nuclei and isotropic chemical shieldings from the calculated chemical shielding (CS) tensors at the sites of O17, N15 and H1 nuclei reveal the major contribution of O=C-NH group to HB interactions. Although N-H center dot center dot center dot O type of HB interaction play the major role in the HB properties of peptide group in lattice form of crystalline acetamide, however, the role of weaker C-H center dot center dot center dot O type of HB interaction cannot be neglected. (C) 2007 Elsevier Inc. All rights reserved.