The bacterium Geobacter sulfurreducens grows electrically-conductive pili, which act as protein nanowires, in order to transfer electrons from the cell to electron acceptors in its environment when direct charge transfer through the cell membrane is not feasible. Understanding the electronic structure of the pili can provide insight into fundamental processes of electron transfer in biological systems.This study investigated the electronic structure of these protein nanowires using the toolbox of scanning probe microscopy, specifically scanning tunneling microscopy and point tunneling spectroscopy. These measurements were performed at 77 K and at room temperature. The measured data are compared to theoretical calculations.Density of states measurements using tunneling spectroscopy show that these pili act as narrow-gap biological semiconductors at 77 K. The onset of nonzero density of states remains within the metabolically-relevant voltage range. At room temperature, spectroscopy of the pili retains a gap-like structure, but this pseudogap is raised to a nonzero density of states at even the smallest applied voltages. These pilus nanowires also exhibit a distinct spatial dependence of the density of states across the breadth of the pili.
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