Similar proteins demonstrate different charge transport characteristics offering a route to biological electronic devices.
Long-range electron transfer in proteins is an important biological process, driving functions such as cellular respiration. If the charge transport properties of these biological materials can be fully understood and controlled, opportunities open up for cheap, flexible, biocompatible electronics. Yuichi Tokita and colleagues from Sony and Nagoya University in Japan have now revealed the electronic properties of two structurally similar proteins that take part in charge transport in cellular membranes.
Zinc-substituted cytochrome b562 (right) immobilized on a gold electrode acts as an n-type photodiode, whereas zinc-cytochrome c (left) acts as a p-type photoconductor.© 2011 Wiley-VCH
The researchers deposited the two proteins with similar active centers but oppositely charged molecular surfaces onto self-assembled monolayers chosen so as to present the most electronically active part of the molecule to a light source overhead. On illumination, zinc-substituted versions of the two proteins, cytochrome c (Zn-cyt-c) and cytochrome b562 (Zn-cyt-b) generated photocurrents with different characteristics — the former displayed hole conduction via the valence band while the latter showed electronic conduction (see image).
Tokita and his colleagues then carried out a series of calculations to investigate the mechanisms underlying these different photoinduced responses. They identified the electronic orbitals responsible for photoexcitation and attributed charge transfer to orbital coupling in a four-orbital model. In the electron-conducting Zn-cyt-b, electrons traverse unoccupied orbitals in the protein’s conduction band, whereas in the hole-conducting Zn-cyt-c, holes traverse the valence band of occupied orbitals. The orbitals involved are located appropriately in the proteins’ band structures to achieve a semiconductor state.
In studying the ground state electronic structure of the two proteins, the researchers found that the energies of the molecular orbitals of Zn-cyt-b was higher than that of Zn-cyt-c, reflecting a difference in charge. They found that it was the difference in charge that ultimately decided whether the protein was hole- or electron-conducting, as the charge controls the electron density in the molecular orbitals.
“The most challenging part of the project as a whole is to apply our findings to develop practical technologies,” says Tokita. “The next step will be to make functioning devices, such as a p–n diode using p and n proteins.” The researchers are optimistic about the future of the field. “We think protein-based electronic devices could be on the market within ten years,” says Tokita