Quantum Transport in Conductive Bacterial Nanowires

The electrical properties of conductive heme-based nanowires found in the pili in Geobacter sulfurreducens bacteria were investigated using a density functional theory (DFT) model. Green's function methods were used to calculate quantum transmission and single molecule conductance in both the low temperature (coherent) and room temperature (decoherent) regimes. Several approaches were attempted for modeling the energy levels of the heme-sites, including semi-empirical methods, and quantum transmission was calculated at several different length scales. This result was compared to experimental findings as well as other modeling results for similar cytochrome structures, such as electron hopping models applied to neighboring heme sites. The results show that coordinated hemes prefer a low spin state with electron delocalization over the porphyrin rings and coordinating histidine groups from the protein scaffold. Orbital overlap between heme centers was shown to have a significant impact on quantum transport, with perpendicular heme centers having a rate limiting effect on transport. Semi-empirical models such as the extended Hückel method were found to be inaccurate for modeling transport, showing the importance of electron-electron repulsion and a more detailed model for the organometallic bonding.