Coherent & incoherent electron transfer in biological systems

Intermolecular electron transport is vital to the life of all respiring organisms (i.e., the vast majority of Earth's biomass). During electron transfer, any structures between the donor and acceptor molecules are collectively referred to as the "bridge." Interferences between multiple tunneling pathways through the bridge can enhance or reduce electronic coupling. A clear picture of decoherence due to dynamic bridge effects is therefore necessary to understand electron transport in biological systems. In respiring organisms, adenosine-5'-triphosphate (ATP) is recycled at a membrane-bound complex called the "electron transport chain," which in turn is powered by electricity produced from the oxidation of food (chemosynthesis) or the absorption of sunlight (photosynthesis). In this work, we examine a single bridging step in the electron transport chain of Paracoccus denitrificans. Recent experiments have shown this process to be extremely sensitive to the change of just a few atoms in the surrounding protein structure. We hypothesize that this reaction is mediated by a bridge of water molecules stabilized by nearby amino acid residues, effectively suppressing the decoherence of the through-water coupling term. We test this hypothesis using molecular mechanics and density functional theory to calculate the electronic coupling matrix elements for a variety of configurations. We believe the mathematical tools and intuition of quantum information theory will help provide deeper insight into the role of decoherence in this intriguing phenomenon.