X-Rays Used to Drive Formation of New Crystals that Resemble Some Biologic Structures

A team of researchers has discovered that X-rays can trigger the formation of a new type of crystal: charged cylindrical filaments structured similar to a bundle of pencils experiencing repulsive forces, which is unknown in crystals. Similar phenomena may occur naturally in biology, such as in the cytoskeleton filaments of cells, which control cell division and migration in cancer metastasis and many other processes.

The study‘s findings, which were published in the January 29, 2010, issue of the journal Science, expand scientific knowledge of crystals, whether from nature, technologic devices, or the lab, and also create a way to use X-rays to control the structure of materials or to develop novel biomedical therapies.

Crystal formation is typically based on attractive forces between atoms or molecules, making the discovery, made by investigators from Northwestern University (Evanston, IL, USA) completely unexpected. "This is a very intriguing and astonishing result,” said Dr. Samuel I. Stupp, the article's senior author and professor of chemistry, materials science and engineering, and medicine. "The filaments are charged so one would expect them to repel each other, not to organize into a crystal. Even though they are repelling each other, we believe the hundreds of thousands of filaments in the bundles are trapped within a network and form a crystal to become more stable.”

The discovery of the new crystals was unexpected. Very early one morning at Argonne National Laboratory (Argonne, IL, USA), the members of Dr. Stupp's research team applied synchrotron X-ray radiation to a solution of peptide nanofibers they were studying. (The peptides are small synthetic molecules that can be used to create new materials.) The researchers saw the solution go from clear to opaque. "There was a dramatic change in the way filaments scattered the radiation,” said first author Honggang Cui, a postdoctoral fellow in Dr. Stupp's lab. "The X-rays turned a disordered structure into something ordered--a crystal.”

The X-rays increase the charge of the filaments, and thus a repulsive electrostatic force triggers the crystallization--a hexagonal stacking of filaments. Trapped in a three-dimensional network, the charged bundled filaments are unable to escape from each other. The crystals disappear when the X-rays are switched off, and the material is not significantly damaged by the radiation.

Because of repulsive forces, the filaments are positioned far apart from each other, with as much as 320 angstroms separating the filaments. This remarkable feature is not found in ordinary crystals where molecules are less than five angstroms apart. "There are oceans of water inside the crystal,” Dr. Stupp said. "More than 99% of the structure is water.”

The researchers also observed that when the concentration of the charged filaments in solution was higher, the same crystals formed spontaneously without the need to expose them to X-rays.

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