The effect of image force on the efficiency of electron tunneling through macromolecular structures

Abstract

In electron tunneling (ET) along the boundary of two media with different high-frequency dielectric permittivities, the energy gap for the tunneling electron is affected by image forces. Macromolecules and macromolecular structures contain segments possessing different permittivities. Therefore, in any description of ET one must take image forces into account. Here, the spatial form of donor-acceptor ET through bridged molecular groups situated near macromolecular segments is investigated. It is shown that image forces can significantly reduce the energy levels of an electron on the bridge groups. As a result, the ET rate in the donor-bridge-acceptor system can increase by many orders of magnitude. the effect is more favorable for extended bridged chains. This fact may be used in explaining donor-acceptor conductivity of electron pathways in real macromolecular biological structures. Bridged groups in electron pathways may be provided by molecular groups whose affinity for the tunneling electron is higher than that of other groups. Furthermore, the electron overlap integral between the bridged groups should be large enough. That is, the role of bridge can be played by the polypeptide chain backbone, by side-chains of some amino acids (particularly those with aromatic rings), and by conjugated systems of the carotinoid type situated near macromolecular segments or near the surfaces of membranes or channels.