AB-INITIO MOLECULAR-ORBITAL STUDY OF THE AMIDE-I VIBRATIONAL INTERACTIONS BETWEEN THE PEPTIDE GROUPS IN DIPEPTIDE AND TRIPEPTIDE AND CONSIDERATIONS ON THE CONFORMATION OF THE EXTENDED HELIX

Interrelationships among the coupling between the amide I vibrations o f peptide groups, the A-E-1 wavenumber difference of the amide I mode and the conformation of helical polypeptide chains were examined theor etically. Ab initio molecular orbital (MO) calculations were performed for a glycine dipeptide and a glycine tripeptide with various phi and psi angles to obtain the coupling constants between the amide I vibra tions of the nearest peptide groups and between those of the second ne arest peptide groups. It was found that the coupling constants between the second nearest peptide groups were reasonably well. explained by the transition dipole coupling mechanism, whereas the coupling constan ts between the nearest peptide groups contained other factors that mai nly depended on psi. The wavenumbers of the A and E-1 components of th e amide I mode were calculated for various helices on the basis of the se coupling constants. The A component is strong in the infrared (IR) spectrum when the transition dipole of the amide I vibration is nearly parallel to the helix axis, whereas the E-1 component has a strong IR intensity when they are nearly perpendicular to each other. The A com ponent has a strong Raman intensity in both cases. Therefore, the Rama n-infrared wavenumber difference of 10-20 cm(-1) observed for the amid e I bands of poly(L-glutamate) and poly(L-lysine) with charged side ch ains indicates that these polypeptides, in that so-called 'extended he lix' state, have conformations giving rise to strong IR bands of the E -1 component which are lower in wavenumber than the Raman bands of the A component by 10-20 cm(-1). The ranges of the phi and psi angles tha t are consistent with such spectral features are discussed on the basi s of the calculated structures and amide I wavenumbers. (C) 1998 John Wiley & Sons, Ltd.