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X-ray crystallography

Why can't you see an atom or molecule with visible light?

It never will be possible to see atoms or molecules using visible light, even with the most powerful of microscopes. In order to see an object, its size has to be at least half the wavelength of the light being used to see it. But the wavelength of visible light, though small, is much bigger than an atom, making it invisible. X-rays, however, have a wavelength short enough that they can be used to "see" atoms.

How does X-ray crystallography work?

The precise position of each atom in a molecule can only be determined if the molecule will form crystals, which researchers grow in the lab through a variety of methods. When X-rays hit a crystallized molecule, the electrons surrounding each atom bend, or diffract, the X-ray beam, which then forms a pattern as it exits -- an X-ray diffraction pattern. Crystals are used because the diffraction pattern from one single molecule could be insignificant, but the many individual, identical molecules in a crystal amplify the pattern. It takes a computer to mathematically interpret this pattern and reconstruct the positions of the atoms.

Purdue University researcher Michael G. Rossmann was a pioneer in using these techniques to study viruses. He has developed advanced techniques that rely on powerful computers to determine the structure of complex biological structures.

What's it good for?

Scientists have used X-ray diffraction patterns since the early part of this century to aid their studies of molecules. In 1953, when Watson and Crick looked at X-ray diffraction patterns from crystallized DNA, they were able to determine for the first time that DNA molecules exhibit a double-helical structure. But it was only in the late '50s, with the advent of computers, that scientists were able to determine the precise three-dimensional atomic structure of large molecules, such as proteins and enzymes.

Knowing the structure of biological molecules allows scientists to better understand how they work and can lead to better drugs and treatments for disease. For example, Rossmann and his colleagues have determined the structure of a receptor on a human cell that binds to the common cold virus. This work, and that of other laboratories, may help scientists determine how a cold virus and other viruses enter and infect human cells.

CONTACT: Susan Gaidos, Sr. Science Writer, Purdue News Service (765) 494-2081;

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