Smile and prepare to be vaporized! In what you might call the most violent snap-shot ever, physicists have used a blast of x-rays to determine the structure of a tiny object, even as the x-rays blew it apart. The "single-shot" technique marks an important step toward deciphering the structure of proteins by zapping just a single molecule--a potentially revolutionary technique that physicists hope to perfect with the world's first x-ray lasers, currently under construction in the U.S., Germany, and Japan.
A protein's shape and structure largely determine what it does. Researchers usually get that information by shining x-rays on the proteins. Ordinarily, they first crystallize the proteins so that many copies fit into an orderly array. The crystal then scatters x-rays in certain directions to produce a "diffraction pattern" that encodes the protein structure. In recent years, scientists have used x-rays from particle accelerators called synchrotrons to determine the structures of thousands of crystallized proteins.
But some proteins refuse to crystallize, so scientists are searching for other ways to determine their structures. In principle, a single molecule will produce a decodable diffraction pattern if it's blasted with x-rays about a billion times more intense than can be produced with today's brightest synchrotron sources. But there's a catch: All those x-rays will heat up the molecule and vaporize it. So researchers must deliver the x-rays in an ultra-short burst and glimpse the diffraction pattern before the molecule explodes.
Henry Chapman of Lawrence Livermore National Laboratory in California and colleagues have shown they can do that with a larger object and longer-wavelength, less-energetic "soft" x-rays. Using the FLASH soft x-ray laser at the German Electron Synchrotron (DESY) laboratory in Hamburg, the team zapped a test image (two micron-sized stick figures carved into a film of silicon nitride) with a trillion soft x-rays. The shot lasted a mere 25 millionth-billionths of a second, or femtoseconds, and turned the film into 60,000 degree plasma--but not before the researchers captured the scattered x-rays to reconstruct the picture. The key to the technique was a specially designed mirror that redirected the diffracted x-rays onto a photo detector while filtering out x-rays from the glowing plasma, says Chapman, whose team reported its findings online 12 November in Nature Physics.
"This is fabulous," says Phil Bucksbaum, a physicist at Stanford University in Palo Alto, California, and the Stanford Synchrotron Radiation Laboratory in Menlo Park. "We went from imagining that something like this might be possible to demonstrating it with the highest intensity x-ray source we have." Individual proteins will vaporize even faster, so researchers will have to figure out how to catch the diffraction pattern in just a few femtoseconds. They'll also have to figure out how to guide a single molecule into the beam, says Roger Falcone, a physicist at the University of California, Berkeley, and Lawrence Berkeley National Laboratory. "No one sees any show-stoppers," Falcone says, "but those concepts haven't been demonstrated yet."