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An X-ray for the cell's machinery: New laser method can probe single molecules in action.

An X-ray laser so bright and fast that it puts a paparazzi zoom lens to shame has allowed researchers to snap pictures of celebrity molecules that typically avoid scientists' prying eyes. The method should prove useful for investigating the structure and activity of drugs, molecules for fuels and other materials.

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Scientists have used the technique to image an important photosynthesis protein and avirus. Eventually, researchers may be able to use it to make movies of molecules interacting with each other.

"This will be extremely interesting in just about all biological systems," says physicist Henry Chapman of the Center for Free-Electron Laser Science in Hamburg. Chapman is a member of two international teams that report the technique's success February 3 in separate papers in Nature.

Scientists already use X-rays to image protein molecules; by collecting the diffraction patterns made when X-rays strike a molecule, researchers can piece together its three-dimensional structure. But current techniques require hefty, pure samples that must be isolated and crystallized before they can be looked at.

The X-ray laser used in the new work is so much brighter and faster than its predecessors that researchers don't need to grow their molecule of interest into a big, sturdy crystal. Researchers hope that one day the laser may reveal molecules interacting in their native habitats, such as within a cell or its membrane.

"The biggest problem has been membrane-bound proteins--they are very hard to get a detailed view of," says biophysicist Sebastian Doniach of Stanford University. "But these are the proteins that are really important for understanding how things enter the cell, how cells such as nerves signal, how drugs interact with a target cell."

The new method uses the Linac Coherent Light Source, which came online in 2009 at the SLAC National Accelerator Laboratory in Menlo Park, Calif. This free-electron laser produces pulses of hard X-rays a billion times brighter than the synchrotron X-rays used in traditional protein crystallography. The laser light's wavelength is close to the width of an atom, allowing resolution on an atomic scale. And its pulses are so short that it can capture images with a "shutter speed" on the femtosecond scale, quicker than a trillionth of a second.

By feeding a stream of molecules or another microscopic sample into the X-ray beam, scientists can take snapshot after snapshot, capturing meaningful structural information moments before each particle explodes into oblivion.

"The molecule in the beam doesn't know what hit it," says Chapman. "It just disappears in a flash of light."

Measurements suggest that the samples, be they proteins or virus particles, get hotter than the surface of the sun.

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Title Annotation:Matter & Energy
Author:Ehrenberg, Rachel
Publication:Science News
Geographic Code:1USA
Date:Feb 26, 2011
Words:437
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