Structural Basis for Metallic Conductivity in Bacterial Pili Protein Filaments*

Electron transfer in proteins typically occurs via quantum tunneling or hopping mechanism and metallic conductivity has been considered impossible. The quantum interference of partial electron waves scattered from a periodic structure can give rise to electron delocalization and therefore metallic conductivity. Our previous studies suggested that pili protein nanofilaments of Geobacter sulfurreducens possess a periodic structure that can confer metallic-like conductivity [1]. We aim to identify the structural and molecular mechanism of metallic conductivity. Here, using a suite of complementary methods such as X-ray diffraction, Circular Dichroism, Fluorescence Microscopy and Infrared Spectroscopy, we identify the molecular architecture responsible for metallic conductivity. Our studies suggest that aromatic amino acids in pili are closely packed from each other (< 4 Å), forming pi stacking, that can cause intermolecular electron delocalization, conferring metallic conductivity to pili. We observe large conformational changes in pili that accelerate electron transfer in pili. Furthermore, our studies show that improved metallic nature in the pili correlates with the improved pi stacking.
[1] Malvankar et al. Nature Nanotechnology, 6, 573-579 (2011)