Fast X-ray flash produces results.

Fast X-Ray Flash Produces Results

Cornell University scientists have successfully snapped X-ray diffraction pictures of biological molecules 1 million times faster than ever before, opening the way for studies of how molecules change shape in the instant they perform important functions in the body. The test also proves the design of the X-ray-producing device that is the heart of a laboratory being built to produce X-ray beams 10,000 times brighter than previously possible, a capability that will enable scientists to push forward the study of metals and other materials.

X-ray diffraction is one of the oldest and best methods scientists have for looking at the structure of biological molecules; it was X-ray diffraction that gave clues to the structure of DNA, hemoglobin and other molecules. The benefits of the technique's high resolution have been somewhat offset, however, by the long exposure times necessary to get a good picture. In classical X-ray crystallography, samples in crystalline form must be exposed to X-rays for hours or days, yielding a static view of molecules that have sometimes been damaged by the X-rays themselves. Recently, scientists at the Massachusetts Institute of Technology succeeded in making millisecond X-ray diffraction photographs of proteins (SN: 9/19/87, p.182), but that was still not fast enough to capture the protein's changes in form as they happen.

The Cornell scientists used the new device, called an undulator, to produce a bright flash of X-rays, which enabled them to make X-ray diffraction photographs in one-tenth of a billionth of a second. The researchers think this will allow them to capture changes in such molecules as hemoglobin as it binds to oxygen or the visual pigment rhodopsin when it is struck by light. "There are many important biological processes that occur on this time scale," says Cornell biochemist Keith Moffat.

The new technique still requires the molecules to be in crystalline form. This makes the chemical reaction difficult but not impossible to induce, Moffat says. The crystals have a lot of water in them, allowing molecules to interact freely with each other, and it might be possible to start the chemical reaction with light just before the X-rays are turned on, he says.

The fast exposure time also causes less degradation of the sample because the molecule-destroying free radicals produced by the X-rays don't have time to do much damage, says Wilfried Schildkamp, another researcher on the team.

The device that made all this possible, the undulator, was developed by scientists at Cornell and the Argonne (Ill.) National Laboratory. It will later be used as the principal component of an X-ray study facility being built at Argonne, called the Advanced Photon Source. The undulator won't fit in most biology laboratories, because it requires an electron storage ring to function, such as the half-mile-diameter ring used at Cornell.

Scientists have long used the X-rays produced by such rings when fast-moving charged particles (in this case electrons) are turned by powerful magnets. This "synchrotron radiation" can be intensified by the undulator, which uses many magnets to make the electrons wiggle back and forth 61 times instead of making just a single turn as they do at each magnet in Cornell's electron ring. Also, unlike the broad-spectrum X-rays produced by single magnets, the X-rays emanating from the undulator are "pseudo-monochromatic" and range over only a few wavelengths, says Gopal Shenoy of Argonne.

When the Advanced Photon Source is completed in 1995 it will have 35 undulators, each much more powerful than the experimental model at Cornell. Scientists will use such intense X-rays to probe the structure of many materials, such as metals, meteorites and superconducting ceramics, say the Cornell researchers. "[The undulator] delivered everything it was supposed to and more," says Boris Batterman, director of the Cornell High Energy Synchrotron Source. "It shows that the Advanced Photon Source will be . . . the most versatile source of synchrotron radiation in the world."