Transition dipole coupling in Amide I modes of betapolypeptides.

Our previous introduction of transition dipole coupling helped to explain the splittings in the Amide I modes of antiparallel chain pleated sheet polyglycine I. This mechanism has now been applied to the more likely rippled sheet structure of this polypeptide as well as to the pleated sheet structure of poly(L-alanine). A satisfactorily consistent explanation of the splittings in both polypeptides is obtained. Since a previously incorporated interaction constant has not been used in the present treatment, these results show that transition dipole coupling alone can provide the physical basis for understanding these splittings. It is therefore now possible to predict with confidence the hitherto unidentified v(pi,pi) frequency of the antiparallel chain pleated sheet as well as the characteristic frequencies of the parallel chain pleated sheet.