Relay Stations for Electron Hole Migration in Peptides: Possibility for Formation of Three-Electron Bonds along Peptide Chains

Our calculations found that the O therefore O three-electron (3e) bonds (2.16 similar to 2.27 angstrom) can be formed not only between two neighboring peptide units in a main chain but also between two adjacent peptide units in two different main chains in proteins. This finding may address electron hole migration from one peptide unit to the next in proteins. Evidently, stability of the O therefore O 3e bonded species is strongly dependent on the component of the oligopeptides and is reduced owing to the steric hindrance of the side chains when the big chains present in oligopeptides. Besides, formation of the O therefore O 3e bonds competes with the formation of the other forms of three-electron bonds depending on the component of the polypeptides. Formation of the O therefore S 3e bond is thermodynamically more favorable than that of the O therefore O 3e bond for the oligopeptides containing sulfur atom in their side chains. Similarly, formation of the O therefore pi 3e bond between aromatic ring of the side chain and the neighboring peptide unit is more stable than that of the O therefore O 3e bond when the aromatic amino acids present in the oligopeptides. We infer that a series of three-electron bonds may be formed during the electron hole migration along the peptide backbone in proteins and assist electron hole transport as relay stations, supporting the peptide chain as a conduction wire. The ab initio molecular dynamics simulations of the polypeptides also support this conclusion.