Single-Protein Electronics: Effect of Surface Mutagenesis and Multiheme Redox Cofactors

<i>Bioelectronics</i> is a rapidly evolving field moving towards designing nanoscale electronic platforms that allow <i>in vivo</i> sensing, fuel cell powering and chemical biosynthesis. Such devices typically require interfacing a complex biomolecular moiety as the active component to an electronic platform for signal transduction and/or electron source wiring. Inevitably, a true systematic design goes through a bottom-up understanding of the structurally related electrical signatures of such hybrid biomolecular circuits, which will ultimately lead us to tailor its electrical properties and exploit them as high performance bioelectronic devices with a wide variety of applications in organic electronics, sensing, biomanufacturing, <i>etc</i>.<br/>In this contribution, we will present our latest efforts on understanding and control charge transport in a single-protein junction. Our approach relies on trapping individual redox proteins in a tunneling junction under electrochemical control to characterize their main electrical signatures. The method can capture very fine details of the charge transport mechanisms across proteins in an aqueous environment [1,2]. Our studies start with a benchmark redox protein model such as a bacterial blue Cu-Azurin. We will show first the main observed electrical signatures of these systems that make them particularly efficient in transporting charge. We then bioengineer the outer protein surface using point-site mutagenesis as a mean to get a more detailed picture of possible electron pathways through the protein backbone [2,3]. Our results suggest that the protein might not use distinct physical electron pathways across its structure, but transport mechanism can be switched upon quenching of particular motions in the protein structure via surface mutations. We then compare the above redox protein model to a natural multiheme molecular wire. Multiheme proteins have been recently discovered as the building blocks in highly conducting pili and transmembrane structures of certain species of the so called electrical bacteria [4]. Such large supramolecular ensembles of multiheme protein complexes support long-range charge transport in record distances over micrometres [5]. We have trapped individual small tetraheme proteins (STC) in our tunnelling gap and measured conductance. STC constitutes a well-known repeating arrangement of <i>heme </i>cofactors along larger multiheme complex structures. Our first results suggest a single STC unit can transport electrons via efficient electron tunnelling process along the ~4 nm long protein axis with very shallow electron decay constant. We believe these results contribute to the observed long-range charge transport behaviour observed in these multiheme-based molecular wires.<br/>Summing up, the above work shows the potential of electrochemically controlled nanoscale protein junctions to both elucidating charge transport mechanisms in biological systems as well as in enabling a bottom-up design of electrode/protein interfaces for the future generations of bioelectronic devices.