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Electronic transport is predominantly the domain of man-made materials and devices. Biology, on the other hand, tends to manage charge conduction via transport of ions. Consequently, interfacing biological and synthetic systems is an imperfect and often crude endeavor. New materials to interface with specific cellular and enzymatic processes are required to address challenges in integrating biology with electronic systems. Nature provides inspiration for exactly such biointerface materials. Many microbes in anoxic soils and sediment respire using extracellular electron transfer. Some of these species, specifically of the Geobacter genus, synthesize fiber-like appendages, called pili, which conduct charge over distances of microns to millimeters to reach remote electron acceptors. Our studies show that the conductivity in Geobacter pili is inherent to the protein fibers themselves, and that they exhibit band-like electronic transport characteristics. Understanding how extended protein fibers can support band-like transport is impractical in Geobacter pili, in part due to the lack of an appropriate crystal structure. Based on sequence and structure motifs from native pili fibers, we instead developed a new class of self-assembling de novo peptides with well-defined solid state X-ray crystallographic structure and solution behavior. These peptides self-assemble through a novel coiled-coil interaction, a Phe-Ile zipper, to form unique, antiparallel hexamers (ACC-Hex) and fibers. The Phe-Ile zipper motif is general, allowing for the incorporation of various natural and non-natural amino acid mutations. These sequence variants were used to determine the assembly mechanism of ACC-Hex and create coiled coils with uncommonly high stability to denaturation. Fibers assembled from these peptides are electrically conductive and exhibit characteristics of band-like electronic transport, similar to Geobacter pili, making them ideal for device applications. These self-assembling peptides potentially expand the synthetic biology toolkit to include autonomously-generated bioelectronics interfaces, and their well-defined structure suggests them as an experimental platform to study structure-property relationships of long-range electronic conduction in proteins and other amino acid biomaterials.
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