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Nature's "electronic ink"

Retinal protein crystallized on space mission

Owens Lake, CA, showing halobacteria bloomSept 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.

At right, above: The now almost-dry lake bed of Owens Lake, California, shows the pink bloom of halophilic ("salt-loving") bacteria in the muddy brine. Click on the picture to see several more views of the halophilic inhabitants of Owens Lake. (photo courtesy Tony Phillips/Bishopwebworks)

Artist's concept of halophyllic organisms

Above: Artist's concept of microorganisms in the Owens Lake - the image represents a magnification of approximately 2000x. The red coloration of the brine is caused primarlity by rod-shaped, salt-loving bacteria (Halobacterium). There are also two or more species of halophilic green algae, one of which, the Dunaliella, has an intense red form that adds to the reddish color of the lake bed. All are shown swimming among cuboidal crystals of sodium chloride (salt).

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.

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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.

 click for full picture
Above: a computer-generated slice of a larger image (632x490 pixels; 74KB) of the bacteriorhodopsin protein from the Brookhaven National Labs Protein Database.

Information

Principal investigator: Torsten Rothaermel, Gottgried Wagner, Justus-Liebig University, Dept. Biology, Senckenbergstrasse 17, 35390 Giessen, Germany

Co-investigators: Christian Betzel, Markus Perbandt, Institute of Physiological Chemistry, c/o DESY, Geb. 22a, Notkestrasse 85, 22603 Hamburg, Germany

References

Life and Microgravity Sciences (LMS) Space: Final Report, February 1998, NASA Marshall Space Flight Center, Huntsville, AL. compiled, J. P. Downey. NASA CP-1998-206960

Further readings

  • Altmuller, D., Grolig, F., Lindhardt, R., and Wagner, G.: Bacteriorhodopsin - A Membrane Protein to Convert Light Energy. In Proceedings of the Norderney Symposium on Scientific Results of the German Spacelab Mission D1, Norderney, Germany, August 27-29, 1986, pp. 294-296. (post-flight)
  • Cowen, R. Juggling at the speed of light. Science News 143(Jan. 23, 1993):63.
  • de Lucas, Larry, et al. Protein crystal growth in microgravity, Science, 246: 651 (1989)
  • Lewis, A., et al. Optical computation with negative light intensity with a plastic bacteriorhodopsin film. Science275(March 7, 1997):1462.
  • Lipkin, R. The eye's photochemistry. Science News146(Oct. 29, 1994):279.
  • _____. Lighting the way to speedier circuits. Science News 139(June 22, 1991):389.
go to LMS site on Liftoff!

One of several stories summarizing results from the 16-day Life and Microgravity Spacelab (LMS), which flew June 20-July 7, 1996, aboard Space Shuttle Columbia (STS-78; left). It featured 40 scientific investigations from 10 countries. Its record development and cost - each experiment cost about half of most Spacelab experiments - make LMS an example of how future space station missions can control experiments remotely from locations around the globe. LMS results were recently published by NASA (see below). The investigation in this story used the European Space Agency's Advanced Protein Crystallization Facility.

Other LMS stories:

  • Nature's sugar high - Spacelab successfully crystallizes an intensely sweet protein from the African Serendipity Berry that has 3000 times the kick of table sugar - and no calories.
  • Great Bugs of Fire Spacelab crystallizes a protein from a very weird, and surprisingly common, volcano-loving bug. Scientists hope to discover how these organisms can survive in such extreme conditions.
  • Nature's "electronic ink" Another extremophile - a bacterium which thrives in high-salt conditions - produces a fascinating protein which changes color extremely efficiently. Crystals from Spacelab make scientists hopeful that they can understand the biological function and apply it to, for example, artificial retinas for people. (this story)

Biotechnology in space

Some estimates suggest that human biology depends on the action of nearly half a million different enzymes and proteins. In fewer than 1 case in 100, we have a three-dimensional picture of shape and function of these complex chemicals. Since 1984, the Space Shuttle has carried experiments to determine the structures of large, biologically important molecules. This research has compiled results for a host of human diseases ranging from insulin (for the control of diabetes) to one enzyme called reverse transcriptase that can be blocked to inhibit HIV infection.

In comparing more than 33 such different biological molecules crystallized on the Shuttle and also in similar conditions on earth, space produced larger space crystals in 45% of the cases and new structures in nearly 20% of the cases. As many as half the space crystals had a 10% or better improvement in the x-ray brightness or the crystallographic resolution. Both are important to determining these large molecules' shape and exact atomic positions.

Web links

www.microgravity.com General information about science in low-gravity including protein crystal growth. Protein Crystal Growth Tutorial Describes how nearly perfect crystals are grown and analysed.
Microgravity News, Summer 1996 periodic newsletter; this on on LMS
Microgravity Research Program Officeat Marshall has a wealth of information and background on various microgravity projects
Life and Microgravity programmatic information from NASA headquarters.