Exponential Distance Decay of Electron Transfer Rates without Tunneling: A Flickering Resonance Model for Transport

Abstract

Biological electron transfer (ET) reactions are usually described in the framework of coherent two-state electron tunneling or multi-step hopping. Yet, many ET reactions involve multiple redox cofactors with vibronic broadenings on the same scale as the energy gaps among the states and the interstate electronic interactions. In this regime, excursions to electronic resonances among states are found to support coherent (ballistic) charge transfer through the structures. Importantly, ET rates arising from flickering resonance (FR) decay exponentially with distance: the probability of matching multiple energies is multiplicative. Transport arising from FR thus mimics the exponential distance decay that is well known for electron tunneling, but the rapid decay with distance is of an entirely different origin. Likely candidates for FR transport are macromolecules with ET groups in van der Waals contact: DNA, bacterial nanowires, multi-heme proteins, strongly coupled porphyrin arrays, and proteins with closely packed redox-active residues. I will develop the theory of FR transport and apply this model to problems in nucleic acid charge transfer.