And...reAction!

As Titanic captured the Oscar limelight, a much smaller, shorter action picture opened to its own glowing reviews. In the March 20 issue of Science, an international team led by Keith Moffat presented the first images of how light gets transformed into chemical energy—the first stage of activities as different, and fundamental, as photosynthesis and vision.

Moffat is a professor in biochemistry & molecular biology and executive director of the University’s Consortium for Advanced Radiation Sources (CARS). His research team included Benjamin Perman, SM’96, of biochemistry; Vukica Srajer, Zhong Ren, and Tsui-yi Teng from CARS; and researchers from the European Synchrotron Radiation Facility (ESRF) in Grenoble, France; Lund University; and the E.C. Slater Institute of the University of Amsterdam, Netherlands.

The team’s images were made possible by a recent, million-fold improvement in the time resolution of X-ray measurements at ESRF, letting researchers produce pictures of changes in the shape of a working protein that occur in billionths of seconds. The process, still being refined by several teams worldwide, involves cooling the “actors” to slow their activity while focusing extremely bright lights on them to increase the camera’s shutter speed.

Unlike Leonardo DiCaprio, however, the stars of Moffat’s pictures—in this case, a blue-light photoreactive protein of the xanthopsin family’s eubacterium Ectothiorhodospira halophila—have not become anything like a household name.

The bacterium—which is found only in a few high, arid lake beds in Oregon and salt depressions in the Egyptian desert—is “rather obscure,” admits Moffat. “On second thought, make that ‘incredibly obscure.’” But its status as an unknown may soon be a thing of the past. Its light-sensitive protein’s debut performance is bringing it to scientific center stage, attracting attention not just from biochemists but also from computer engineers who hope the protein may some day play a leading role in their projects, such as the development of optical computers.

The star protein has all the qualifications a casting director might select for an optical storage and transport mechanism. When exposed to a light, the protein immediately flips from one structure to a slightly different conformation. “In the dark, the system is cocked and ready for structural changes,” explains Moffat. A single photon provides enough energy to “pull the trigger,” he says.

The protein appealed to the research team because it is comparatively small, simple, and water-soluble. Its charm for optical computing, on the other hand, is that it is extremely robust. “If handled correctly it can tolerate intense, repetitive stimulation from lasers, X-rays, or light,” says Moffat.

Although the Science paper focused only on the first billionth of a second after light exposure, the research team has gathered information on a series of subsequent changes in the molecular structure after that first impulse. The end result will be a longer motion picture, lasting several billionths of a second, that follows the subsequent steps in this fundamental biological process.

The researchers are also eager to perform still faster and more detailed crystallography at the Advanced Photon Source at Argonne National Laboratory, permitting them to observe biomolecules in their true, dynamic state.

Meanwhile, the bacteria are responding in typical Hollywood fashion to all the enormously bright lights beamed at their delicate photoreceptors: Like starlets confronted by paparazzi, they swim the other way.—John Easton