Toward imaging single biomolecules

By Davide Castelvecchi
From Science News, April 21st, 2007; Vol.171 #16 (p. 253)

free electron laserLawrence Livermore National Laboratory
CATCH IT WHILE YOU CAN. X rays from the left scatter off molecules (multicolored clusters) and reach a detector screen in this artist’s impression. Scientists can then reconstruct the molecules’ structure.

Blowing stuff up can sometimes be good for science. A new generation of lasers is expected to image complex molecules, such as proteins, even though the molecules themselves explode in the process.

Scientists can deduce a molecule’s form from the way in which X rays scatter off it. This technique, called X-ray diffraction, normally requires coaxing large numbers of the molecule into crystal form. But some molecules—including many of the proteins that shuttle ions in and out of cells—are difficult or impossible to crystallize. In a few years, new machines called free-electron lasers may image single molecules with X-ray pulses that are up to 10 billion times as concentrated as the pulses that current X-ray sources can deliver.

But some scientists have worried that these machines would be too powerful for X-ray diffraction, destroying molecules before any meaningful data can be extracted.

Using a prototype X-ray free-electron laser in Hamburg, Germany, an international team of physicists directed the beam onto nanoscale samples of reflective material. During pulses lasting 25 millionths of a billionth of a second, or femtoseconds, the sample still reflected the laser light precisely—meaning that the deformation and ultimate explosion of the material took longer than the pulse duration.

The explosion mechanism will be different in the more energetic future machines, says Stefan Hau-Riege, of Lawrence Livermore (Calif.) National Laboratory. But the research suggests that X rays will have enough time to scatter, carrying structural information before molecules blow apart.

The results appear in the April 6 Physical Review Letters.

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