The Proton Pump

Despite their progress in developing solar cells, giant reflectors and other devices, scientists still lag far behind nature in their ability to harness solar energy. No man-made device can match the performance of the green pigment chlorophyll: through the process of photosynthesis, it converts some 30% of the sunlight striking it into the chemical energy that plants use to create their own food. Even more frustrating, chlorophyll has defied the efforts of scientists to use it directly to produce energy for man; the pigment is highly unstable.

But man may yet employ a natural converter to get energy directly from sunlight. Last week Cell Biologist Walther Stoeckenius, 54, with colleagues at the University of California at San Francisco and a team from NASA'S Ames Research Center, announced that a purple pigment found in red bacteria from the Dead Sea and salt flats round the world also directly converts sunlight into energy. While the pigment is less efficient than chlorophyll--only an estimated 10% of light energy is converted--it is more stable and easily extracted from the bacteria.

Visual Purple. Stoeckenius began the work that led to his discovery in 1965 while serving as an associate professor at New York City's Rockefeller University. Studying the structure of a microbe called Halobacterium halobium --the organism that gives red herring their distinctive color--he found a purple pigment that was chemically similar to "visual purple," a pigment in the retinas of animal and human eyes. The similarity led Stoeckenius and his co-workers to suspect that the pigment helped the organism use light for its life processes.

Continuing his research after transferring to California in 1967, Stoeckenius found that the pigment, called bacteriorhodopsin, functioned as a sort of pump, converting sunlight directly into electrochemical energy. Light striking a pigment molecule causes it to eject a hydrogen ion--or proton--that passes through the cell's membrane. The movement of the positively charged protons through the membrane leaves an excess of negative charge on one side of the membrane. That produces a voltage gradient and results in an electrical current flowing through the membrane. In the process, which involves at least five separate steps, each bacteriorhodopsin molecule pumps out a proton every 250th of a second and provides the energy the organism needs to synthesize adenosinetri-phosphate (ATP), the energy-storing molecule common to all living cells.

Stoeckenius reported that researchers are already experimenting with bacteriorhodopsin in efforts to build a photoelectric cell. Stoeckenius also believes bacteriorhodopsin's chemical similarity to visual purple could help scientists better understand the basic processes of vision and could offer new insights into cell biology. "All living cells need to pump ions across their cell membranes," says Stoeckenius. "It seems to me that we are close to discovering certain basic cellular functions."

Because the pigment helps the bacteria to push salt through their membranes to the outside, enabling them to survive in salt water, researchers believe it may be useful in desalination projects. Their scheme is to shine light on pigment-coated membranes that separate pools of salt water; the pigment would presumably pump minerals from one side of the membrane to the other, leaving one pool relatively salt-free. An understanding of how the membrane does this may be useful in building large-scale desalination plants. Man's supplies of fresh water are rapidly dwindling. A pump based on these principles may enable him to replenish them by extracting fresh water from the sea.

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