World's first hard X-ray free-electron laser images intact viruses
By Ben Coxworth
February 7, 2011
In a paper published in the current edition of Nature, an international team of scientists describe how they obtained the world’s first single-shot images of intact viruses – a technology that could ultimately lead to moving video of molecules, viruses and live microbes. Another paper by the same team describes how they were also able to successfully utilize a new shortcut for determining the 3D structures of proteins. Both advances were achieved using the world’s first hard X-ray free-electron laser – the Linac Coherent Light Source (LCLS) – which scientists hope could revolutionize the study of life.
Located at the U.S. Department of Energy’s SLAC National Accelerator Laboratory run by Stanford University, the laser beam of the LCLS is a billion times brighter than any previous X-ray source, with an intensity sufficient to cut through steel. The duration of its individual pulses is incredibly short – a few millionths of a billionth of a second. That’s still long enough to cause its subjects to vaporize, but that doesn’t happen until after their pictures have been snapped.
Scientists from Arizona State University, Lawrence Livermore National Laboratory, SLAC and Sweden’s Uppsala University developed specialized equipment for injecting samples into its beam. Data was captured with an ultra-sensitive X-ray camera that was part of CAMP, a 10-US ton (9-metric ton), US$7 million device created by Germany’s Max Planck Advanced Study Group.
The two experiments were actually conducted back in December 2009, just two months after the LCLS became available for research use.
Imaging molecular structures of proteins
In the first experiment, nanocrystals containing copies of the protein Photosystem I were sprayed across the laser beam. Photosystem I is found in plant cells, where it converts sunlight to energy during photosynthesis. It’s a member of the membrane class of proteins, which occur in cell membranes and “control traffic in and out of the cell and serve as docking points for infectious agents and disease-fighting drugs.” To date, scientists know the structure of only six of an estimated 30,000 membrane proteins found in the human body, as it has proven difficult to convert them into crystals large enough to be imaged by conventional X-ray technology.
Using the LCLS, the team obtained approximately 3 million snapshots of Photosystem I-bearing nanocrystals from a multitude of angles, as they passed through a series of pulses of the laser beam. Ten thousand of those shots were then combined to form one image, which depicted a molecular structure that was “a good match” for the protein’s known structure.
The team plans to return to the facility later this month, subjecting more Photosystem I particles to a beam that is now much faster, and four times more intense. It is hoped that they will be able to obtain images of the protein’s structure in atom-by-atom detail.
Getting snapshots of viruses
In the second experiment, the team used no nanocrystals at all, instead spraying mimivirus particles through the beam – mimivirus is the world’s largest known virus, and it infects amoebas. While hundreds of viruses were hit by the beam, only two of them provided enough data for reconstitution of their images. In the images, the 20-sided structure of the virus’ outer coat is visible, as is an area of denser interior material, that might be their genetic material. The team returned to SLAC last month and used wavelengths that should maximize the contrast and detail in their images, which they will now be analyzing.
Typically, scientists have to freeze, slice, or otherwise disturb viruses in order to image them.
“This first data and these first papers are really just the first view of a new research frontier,” said SLAC Director Persis Drell. “They represent a turning point for the LCLS, demonstrating new technologies that will be great steps forward.”
Over 80 researchers from 21 institutions around the world were involved in the experiments.
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