Amide I modes in the N-methylacetamide dimer and glycine dipeptide analog: Diagonal force constants

The local amide I mode frequency of a peptide has been found to be strongly affected by the interpeptide interaction, because the electronic and molecular structures of the peptide bond change due to the electrostatic interaction with surrounding peptides. Ab initio vibrational analyses of three different series of N-methylacetamide dimers and glycine dipeptide analog in α-helical and β-sheet conformations have been performed. It is found that the diagonal force constant shift originates from the electronic structure change of a given peptide in combination with the cubic anharmonicity of the local amide I mode.