Mechanism of the Secondary Structure Dependence of the Infrared Intensity of the Amide II Mode of Peptide Chains

The mechanism of the secondary structure dependence of the infrared (IR) intensity of the amide II mode of peptide chains is examined theoretically. It is shown that a large interpeptide flux of electrons (charge flux) is induced by the amide H mode of peptide chain in the C-5 (beta-strand) conformation through the H center dot center dot center dot O interaction, giving rise to large IR intensity enhancement. Taking into account also that this IR intensity enhancement does not occur in the case of the C-7 conformation (with intrastrand hydrogen bonding), it is concluded that strong secondary structure dependence of the amide II IR intensity is related to the amplitude of the charge flux and the intrachain relative configuration of peptide groups. These results provide deeper insight into the structural information contained in vibrational spectroscopic features and will be useful, for example, for analyzing the structural variations induced by some changes in the thermodynamic state.