Towards a molecular-level understanding of metal-like conductivity in bacterial protein nanowires

Protein appendages of Geobacter sulfurreducens exhibit metal-like conductivity and transport electrons at rates rivaling to those of metallic carbon nanotubes. Our recent experiments have revealed an increase in π-stacking upon lowering the pH of nanowires leading to a 100-fold increase in conductivity and carrier density. We employed molecular dynamics simulations to obtain a molecular-level understanding of the enhanced π-stacking obtained under highly acidic pH. Electronic structure calculations were also performed using QM/MM technique to elucidate the frontier orbitals and the energy gap between them. Our MD simulations suggest that under highly acidic pH significant structural rearrangement of the protein structure leads to the enhanced π-stacking. Electronic structure calculation reveals that the enhanced π-stacking results in higher coupling between the carrier states. Hence, conductivity is increases by several fold at highly acidic pH. Our results suggest that significant orbital overlap between adjacent charge carriers is pivotal in the observed conductivity of these nanowires.