| Electron transfer in proteins is an issue of fundamental importance in biochemistry. This is especially true for proteins whose functionality relies on redox reactions, which by definition means that electron exchange must take place between the protein and another protein, a cofactor, or the external environment. An important example consists of proteins containing heme groups, where the iron ion in the protoporphyrin ring changes valence state depending on the whether an electron is accepted or given up by the protein. Much experimental work has been carried out since the late 1950s in order to develop an understanding of protein electron transfer mechanisms. This understanding is mainly based on the statistical average of protein ensembles measured by spectroscopic and electrochemical techniques.;Since the 1990s, electron transfer through individual molecules has been probed using electrochemical scanning tunneling microscopy (EC-STM) in aqueous solution at room temperature. Models that explain the behavior of electron transfer include the resonant tunneling model and two-step electron transfer model.;Recently, the fabrication of nanometer-gap electrodes by electromigration (break-junction) techniques has made it possible to study electron conductance through small single molecules that undergo redox reactions at cryogenic temperatures. By applying a bias voltage V between the electrodes, as well as a gate voltage VG to the sample, the molecular energy levels can be probed and characteristic single-electron transistor (SET) behavior, such as a Coulomb blockade or a Kondo resonance, can be observed under the right conditions. This has created the possibility of making devices that exploit the unique electronic properties of organic and biomolecular compounds, such as their small size, stability, and dependence on conformation.;Considering the successful results in molecular SETs, an interesting question is whether similar devices can be fabricated using proteins. In principle, protein-based SETs could be observed at moderate cryogenic temperatures for heme proteins. Ideally, proteins with localized, well-defined energy levels are necessary.;In this work, the mechanism of electron transfer by myoglobin using nanometer-gap platinum electrodes was investigated. The electrodes were fabricated by breaking a small junction by electromigration at cryogenic temperatures. Apomyoglobin (myoglobin without the heme group) was used as a reference. The experimental results suggest single electron transport behavior is mediated by resonance of the electronic levels of the heme group in the myoglobin protein. This demonstrates that myoglobin across nanometer-gap electrodes could be utilized to fabricate single electron transistors. The orientation and conformation of myoglobin in the gap of electrodes may significantly affect the conductance of these devices. |
