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[double bond]O and C[bond]N vibrational stretching modes of the amide group in the alpha-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[double bond]O modes are localized, and the electrostatic mechanism then holds. In contrast, the C[bond]N mode transition densities are delocalized, and covalent contributions to the coupling are significant.