Nature's Electronic Ink

September 17, 1998

Anyone who has ever fallen on grass knows that nature has chemicals that are as permanent as ink. At least one of those chemicals holds promise as an "electronic ink" that can be used in improved computer displays.

The chemical is bacteriorhodopsin, a purple protein essential to the cell wall of Halobacterium halobium, a mysterious resident of salt-marshes and lakes. When nutrients get scarce, this bacteriorhodopsin becomes a light-converting enzyme that keeps the organism's life cycle going. It's a protein powerhouse that in times of famine flips back and forth between purple and yellow colors. If controllable, this could be valuable in computer display panels.

In the last 25 years, bacteriorhodopsin has excited a great deal of interest among biochemists, biophysicists, and most recently among companies seeking to build battery-conserving, long-life computer displays. The protein, sometimes called nature's "electronic ink" was grown in orbit on board the Space Shuttle for a scientific team from Justus-Liebig University in Glessen, Germany and the Institute for Physiological Chemistry in Hamburg.

Part of the attraction to understanding these light powerhouses is that natural materials often perform very complex functions that cannot be easily obtained from manufactured materials such as semiconductors. They have been optimized for these functions by billions of years of evolution and often perform them better than any human-designed material could.

For example, bacteriorhodopsin is an attractive material for all-optical 'light' computers because of its two stable protein forms, one purple and one yellow. Shining two lasers of different wavelengths alternately on the protein flips it back and forth between the two colors. Several research groups have already used bacteriorhodopsin as computer memory and as the light-sensitive element in artificial retinas.

According to their report, the space crystal was stabilized under microgravity conditions... Further experiments in microgravity, as a favorable environment of improved crystallogenesis, provide additional progress in the investigation of difficult membrane proteins such as bacteriorhodopsin.

In nature, this salt-loving, probably ancient, organism undergoes a light-stimulated cycle of protein rearrangements which can interact photochemically. This may be much how similar retinal proteins in the eye allow more evolved organisms to see.

Analyzing them on Earth has been difficult because these kinds of complex membrane proteins typically require detergents to make them compatible with biological analysis in water.

The cubic-shaped space crystals showed a nearly 20-fold larger volume compared to earth-grown counterparts. In comparing space grown crystals of the bacteriorhodopsin with similar crystals formed on earth, the team found that a favorable environment minimizing gravity may advance the search for new means to reveal the biological function of these complex molecules.

The large volume of the space-grown crystals will help scientists read the protein's blueprint and understand how it operates. From this, they hope to develop versions that could be used in future computers.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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