Coupling between amide I vibrations of neighboring peptide groups and conformation of an extended helix

Interrelationship among the coupling between the amide I vibrations of peptide groups, the A-E-1 wavenumber difference of the amide I mode, and the conformation of helical polypeptide chains is examined theoretically. Ab initio molecular orbital (MO) calculations are performed for a glycine dipeptide and a glycine tripeptide with various phi and psi angles to obtain the coupling constants between the amide I vibrations of neighboring peptide groups. It is found that the coupling constants between the second nearest peptide groups are reasonably well explained by the transition dipole coupling mechanism, whereas the coupling constants between the nearest peptide groups contain other factors that mainly depend on psi. The wavenumbers of the A and E-1 components of the amide I mode are calculated for various helices by using these coupling constants. The Raman-infrared wavenumber difference of 10-20 cm(-1) observed for the amide I bands of poly(L-glutamate) and poly(L-lysine) with charged side chains indicates that these polypeptides, in the so-called "extended helix" state, have conformations giving rise to the E-1 component with a strong IR intensity and the A component with a strong Raman intensity, with the wavenumber difference between them being 10-20 cm(-1). The ranges of the phi and psi angles that are consistent with such spectral features are discussed on the basis of the calculated structures and amide I wavenumbers.