Amide vibrations and their conformational dependences in β-peptide.

The characteristics of the amide-A and amide-I modes in a β-homoalanine dipeptide (β-HADP) have been examined as a function of backbone dihedral angles. The harmonic frequencies were obtained using the density functional theory. The anharmonic frequencies and diagonal anharmonicities were obtained by using the Morse potential. Local-mode frequencies and intermode couplings were obtained using the computed normal-mode frequencies and eigenvectors. It was found that the vibrational frequencies for the two types of amide modes are both conformational-dependent. The inter-amide-A and inter-amide-I couplings in the β-peptides were predicted to be generally weaker than those in the α-peptides. Structural bases of the amide-A and amide-I local modes in the β-peptides are discussed.