Amide Vibrations and Their Conformational Dependences in β-Peptide

The characteristics of the amide-A and amide-I modes in a beta-homoalanine dipeptide (beta-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 beta-peptides were predicted to be generally weaker than those in the alpha-peptides Structural bases of the amide-A and amide-I local modes in the beta-peptides are discussed