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The Dayside : Happy first century, crystallography!

By: Charles Day
21 February 2014

In January this year, Nature published a series of articles in celebration of the 100th anniversary of crystallography—or, to be precise, the 100th anniversary of the award of the Nobel Prize in Physics to the originator of the field, Max von Laue.

Two years earlier, in 1912, Laue was skiing when an idea popped into his head: If you could send x rays through a crystal, they would be deflected by the crystal's atoms, interfere with each other, and form a diffraction pattern that embodied the crystal's structure. By the summer of that year, Laue and two of his colleagues at Munich's Ludwig-Maximilians University, Walter Friedrich and Paul Knipping, had observed the predicted pattern in the lab.

If your physics education was like mine, you first encountered crystallography in a course on condensed matter. Understanding how atomic structure influences the electronic, optical, magnetic, mechanical, and thermal properties of solids is, of course, a major theme of condensed matter physics. My favorite textbook on the subject, Michael Marder's Condensed Matter Physics (2000), starts with three chapters about crystals.

But it wasn't until I began to write about crystallography for Physics Today in the late 1990s that I appreciated the field's diversity, utility, and importance.

The first crystallography story I wrote appeared in March 1999. "Electron cryomicroscopy comes of age" briefly reviewed the growing application of electron diffraction to determine the structure of large biomolecules.

This photograph accompanied a news story in the November 2002 issue of Physics Today about how lobsters change color when cooked. The impressively blue crustacean was a candidate for the cover of the issue, but it lost out to a photo of spurting lava.

In reporting the story, I learned from Richard Henderson of Cambridge University a key aspect of crystallography that I hadn't appreciated before. The short wavelengths needed to probe atomic structure—whether they belong to electrons, neutrons, or x rays—mean that the probing particles have such high energies that a sustained barrage of them will destroy a biomolecule. In x-ray crystallography, the damage is shared among myriad copies of the molecule arrayed identically in a crystal. In electron cryomicroscopy, the damage is also shared among copies, but they are typically arranged in a random pattern encased in ice (to help immobilize fragments dislodged by the bombardment).

The notion of apportioning probing particles to optimize science cropped up again in a story I wrote for the May 2001 issue entitled "Ultrafast x-ray diffraction tracks molecular shape-shifting". I opened with a quote from the co-discoverer of DNA's physical and chemical structure:

"If you want to understand function, study structure,” exhorted Francis Crick. But knowing a molecule’s static structure is often only the first step toward unraveling how the molecule works, especially in the world of biology. The molecular grappling hook that HIV uses to latch onto its cellular prey, for example, depends on a change of structural shape to initiate infection.

The story's subject was an experiment conducted by Simone Techert, Friedrich Schotte, and Michael Wulff at the European Synchrotron Radiation Facility in Grenoble, France. They had set themselves the goal of determining how an organic molecule of modest size flexes in response to light. Given that the time scale of the flexing is on the order of picoseconds, their experiment needed not only picosecond control but also enough x rays packed into a narrow time window to form clear diffraction patterns.

The beamlines at ESRF, where Techert and her colleagues conducted the experiment, are among the most intense in the world. She and colleagues could chop the beam into 150-ps pulses and still have enough x rays to determine how their molecule flexed, how long it took to flex, and how long it took to relax to its ground state.

In subsequent years I revisited crystallography (or, more broadly, diffraction studies) 13 more times. I wrote, for example, about the structural origin of the change in color that lobsters undergo when they're cooked and about a study that combined three different techniques—small-angle x-ray scattering, electron cryomicroscopy, and NMR spectroscopy—to determine the mutated and unmutated structures of p53, a protein that helps protect us from cancer.

Zap and snap

But my favorite story continued the theme of apportioning photons to optimize science. For the January 2007 issue, I wrote about an experiment that demonstrated the feasibility of what is perhaps the ne plus ultra of diffraction studies: If you concentrate, in time and space, enough photons and direct them at a single molecule, you can extract a diffraction pattern just before the photons destroy the molecule.

The feasibility demonstration, conducted by a team led by Henry Chapman of Lawrence Livermore National Laboratory in California and Janos Hajdu of Uppsala University in Sweden, used a micronscale drawing of two stick figures as its target, rather than a biomolecule. And the photons were in the extreme UV, not the x-ray waveband. But the principle of taking a diffraction pattern of a single object—what I call "zap and snap"—was vindicated.

The principle has been put into practice at SLAC's Linac Coherent Light Source, a free-electron laser that produces intense, 50-femtosecond pulses of hard x rays. Last year, for example, Thomas Barends of the Max Planck Institute for Medical Research in Heidelberg, Germany, and his collaborators applied zap and snap to determine the structure of an enzyme, a lysozyme, about which the team had no prior structural knowledge.

What scientific riches will crystallography's second century bring? I can't be sure, but I hope that the next generation of free-electron lasers will create beams powerful enough to explain just how a process that I can see now from my office window—melting—plays out on the atomic scale.

The 100th anniversary of Laue's Nobel is not the only crystallographical milestone of 2014. This January saw the debut of Structural Dynamics, a new journal co-published by the American Crystallographic Association and AIP Publishing. Since zap and snap is very much within the journal's editorial range, I expect to follow the journal with interest.

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