The origin of vibrational mode couplings in various secondary structural motifs of polypeptides

Electrostatic (through-space) and covalent (through-bond) contributions to couplings involving the C=O and C-N vibrational stretching modes of the amide group in the a-helix and the parallel and antiparallel beta-sheet structures of alanine polypeptides are analyzed. Coupling constants computed at the density functional theory level are compared with the transition dipole coupling model and the complete electrostatic interaction between transition densities. We find that the transition densities of C=O modes are localized, and the electrostatic mechanism then holds. In contrast, the C-N mode transition densities are delocalized, and covalent contributions to the coupling are significant.