Yongdong Jin, at the University of Washington, Seattle, US, and colleagues explain why bacteriorhodopsin is a leading candidate for biomolecular electronics

Bacteriorhodopsin changes colour from purple to yellow following light absorption

Evolution and natural selection have optimised biological molecules for certain tasks, some of which are of interest to scientists who try to mimic biological behaviour in electronic devices. To make biomolecular electronics possible, scientists need to find ways to use biomolecules in solid-state electronics. Because ionic rather than electronic currents carry electricity between components of living organisms, one of the biggest challenges in biomolecular electronics is finding biomolecules that can carry electronic currents.

Bacteriorhodopsin has emerged as an attractive biomolecule for biomolecular electronics. It is a membrane protein, isolated from a salt-water microorganism that, like plants, uses sunlight for energy. Bacteriorhodopsin absorbs sunlight and pumps protons across the cell membrane, converting the solar energy to chemical energy that is stored inside the cell. Human eyes contain similar proteins, rhodopsins, which convert absorbed light to optic nerve signals rather than stored chemical energy. Both proteins contain a segment called retinal, a vitamin A derivative that plays an important role in electron transport through the proteins.

So far, much is known about bacteriorhodopsin's structure and function and a variety of biochemically and chemically modified mutants are available. This knowledge and availability, together with bacteriorhodopsin's photoactivity - it changes colour from purple to yellow following light absorption - and unusual stability, make it a leading candidate for biomolecular electronics.

The most important concern when interfacing biomolecules with solid supports for electronic devices is to ensure that immobilisation on to the solid support as a film does not affect the biological function of interest. Also, the photoelectric conversion efficiency of a bacteriorhodopsin film depends strongly on the orientation of the protein and, to allow the film to carry sufficient charge, it must be at least a micrometre thick. Producing high quality bacteriorhodopsin films with good orientation represents a technical challenge.

Bacteriorhodopsin has been shown to remain functional after integration as a monolayer into a solid state electronic junction. The exact mechanism for current transport is not known but it is clear that it is mediated by the retinal segment. It is likely that the transport benefits from the existence of the proton-pumping pathway in bacteriorhodopsin. Remarkably, when no current is passed through the junction, no photovoltage flows, indicating that bacteriorhodopsin is photoconductive (its conductivity increases during light absorption) rather than photovoltaic (it converts solar energy directly into electricity) when in a solid-state electronic device. Under ambient conditions, the light-driven proton-pumping activity of the sandwiched bacteriorhodopsin monolayer contributes negligibly to the photocurrent.

It can be concluded that the solid-state electronic behaviour of bacteriorhodopsin is derived from its chemical structure - its ability to conduct - rather than from its original, proton-pumping biological function. Its ability to carry electronic current allows some wilder speculation on evolution: why would nature create and maintain a relatively efficient system for electronic conduction and then not use it? Is this simply an accident of biology or did evolution forego electron transport early on in favour of other kinds of energy? And if so, why?

Read more in 'Bacteriorhodopsin as an electronic conduction medium for biomolecular electronics' in issue 11 of  Chemical Society Reviews

 

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