Charge transfer and induced polarization in model peptide–ion complexes

Abstract

AbstractAb initio supermolecule computations of systems consisting of a peptide model (N‐methyl‐acetamide or N‐methyl‐propionamide) and monoatomic ions (Li+, Na+, F−, Cl−) have been analyzed and discussed in an attempt to throw more light on the possible role of charge transfer and related effects in biopolymers containing peptide links. Juxtaposition of ions to a peptide model appears to involve considerable overall charge transfer especially in the case of anions. This supports the assignment to charge transfer of long range electronic effects (like conduction) in polypeptides. On the other hand, induced polarization of the peptide system, which accompanies charge transfer suggests that weakening of hydrogen bonds (especially by cations) may activate long range transmission of perturbations via the concerted weakening of hydrogen bonds. A detailed analysis of the individual molecular orbitals in terms of valence–orbital weight and of hybridization has also been carried out. It is shown in particular that only a limited number of molecular orbitals are involved in the ion–peptide interaction, and that the changes at individual atoms are of different types and affect differently different regions of the molecule. Comparisons between minimal basis and 4‐31G calculations as well as ad hoc auxiliary computations on water–ion complexes have been made to check that conclusions based on the supermolecule approach are at least qualitatively reliable.