A Spin-Dependent Model for Multi-Heme Bacterial Nanowires.

The electrical properties of conductive heme-based nanowires found in Geobacter sulfurreducens bacteria were investigated using spin-dependent density functional theory (DFT). Molecular orbitals were generated using a restricted open-shell model which was solved by applying constraints to the spin-separated unrestricted open-shell model. Charge transport was simulated at different length scales ranging from individual heme sites up to the monomer unit of the nanowire, looking at hopping and tunneling between neighboring heme porphyrins with different Fe oxidation states. The resulting spin-dependent DFT results indicate that tunneling rates between heme sites are highly dependent on oxidation state and transport pathway modeled. The model demonstrates the importance of spin dependence for electron hopping, oxidation state, and decoherence transport in cytochromes. Applying non-equilibrium's Green's function to the system confirmed a substantial decrease in decoherent charge transport for the oxidized molecule at lower Fermi energies. In addition, partial or full oxidation of the heme sites in the nanowire created conditions for spin-dependent transport that can be exploited for spin-filtering effects in nanodevices.