What Can We Learn from Protein-Based Electron Transport Junctions?

To explain what drives us to study electron transport (ETp) through electrode/protein/electrode solid-state junctions (cf. Figure 1) we present some of the reasons, mostly in the form of the following questions: 1. Scientific curiosity: How can electron transport take place through nonconjugated, flexible, polyelectrolytic macromolecules? Answering this question is also driven by intense current interest to understand ETp via so-called bacterial nanowires. (1−3) 2. Biological implications and relevance:Can we learn from understanding ETp via proteins also about their role in biological electron transfer (ET)? 3. Physico-chemical insights: Which constituting elements and properties of proteins are involved in effective electron transport? The following can be singled out: a. primary, secondary, and tertiary structure; b. π-electron content and H-bonding character of amino-acid residues; c. cofactors and their redox properties; alternatively, these can be described in terms of: i. the (electronic) energy levels of a cofactor’s HOMO and LUMO; ii. the energy difference between these levels, and between each of these levels and the electrode Fermi level; (51) iii. the difference between the electrochemical potentials of the electrodes (= Fermi level) and of the protein (≈ redox potential (51)). 4. Potential applications: Can proteins serve as components of electronic devices as part of true bioelectronics?