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Ultrafast first step for light into sight.

Researchers have caught a glimpse of the first step in the chemical process leading to vision. Using extremely short pulses of laser light, two separate groups obtained evidence suggesting that the initial chemical change--the twisting of a chemical bond in the protein rhodopsin in response to the absorption of a photon -- occurs in 200 femtoseconds. But the two groups offer conflicting interpretations of what happens during that brief interval.

"This is the first time that anybody has been able to resolve this transition," says physicist Robert H. Callender of City College of City University in New York, who heads one of the research teams. "It's really fast. There are a lot of things happening even at 200 femtoseconds."

"We are getting to the point where we can really understand molecularly why a photochemical process occurs," says chemist Richard A. Mathies of the University of California, Berkeley, a member of the other group. "Once we understand enough about what controls these things, we can essentially design molecules to do the kind of photochemistry we want. This is one of the first experiments getting us toward that."

A report detailing the City College group's findings will appear in the Nov. 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. Researchers from the University of California, Berkeley, and the Lawrence Berkeley Laboratory present their results in the Oct. 18 SCIENCE.

Both teams probed the pace of photon-induced changes in the structure of a light-sensitive portion of rhodopsin, the chief component of the rod and cone cells in the retina. And both identified two crucial stages in the process: one occurring about 200 femtoseconds after the initial absorption of a photon, and a longer stage lasting about 3 picoseconds.

According to the City College researchers, the first stage represents the formation of an intermediate chemical structure, in which a particular chemical bond in rhodopsin has only half twisted into its final position; the second stage completes the process. The Berkeley team, however, contends that the reaction is complete after 200 femtoseconds and the second stage reflects the time needed for the product to rid itself of excess energy.

"The experimental results do not contradict each other. It's the interpretation that is very different," Mathies says. Both teams acknowledge that the difference can be attributed in part to the fact that they studied the reaction using somewhat different wavelengths of light and laser pulses of different duration.
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Title Annotation:the chemical processes leading to vision
Author:Peterson, Ivars
Publication:Science News
Date:Oct 26, 1991
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