Celia Harrison, Ph.D., Principal Scientist
To see the individual atoms within a molecule, one must use light with a very short wavelength. This light falls in the X-ray region of the electromagnetic spectrum, far outside of the visible light region to which our eyes are sensitive. The electron beams used for electron microscopy also have short wavelengths, but they interact so strongly with molecular samples that the object of interest is usually destroyed before complete data can be collected. X-rays are much gentler, and in fact they tend to go through objects rather than interacting with them at all. This property makes X-rays very useful for medical imaging, but causes experimental hurdles when we use them to see biological molecules.
When X-rays do interact with an object, they are scattered by it. A mathematical formula allows us to convert the pattern of scattered X-rays into an image of the object. To image a sample with X-rays, we must present the X-ray beam with a crystal composed of the molecule we are studying. The crystal (with millions of similarly oriented copies of the molecular sample) scatters far more X-rays than a single molecule would, and allows us to take pictures of the scattering pattern. By rotating the crystal while taking a series of these pictures, we can develop a complete 3-D model of the molecule, with precise coordinates for every atom.
This process requires a great deal of specialized machinery and computer power. Producing defect-free crystals of a given sample often takes years, and since only a small fraction of the X-rays that strike the crystal are scattered by it, we need to use an X-ray generator that is much more powerful than those used for medical imaging. The intensity of this X-ray beam requires that we cool the sample using a liquid nitrogen system during data collection. We also use a very sensitive X-ray detector to record the patterns produced by the scattered X-rays. BBRI’s X-ray diffraction facility was established five years ago and is currently used by four laboratories.
The crystal scatters far more X-rays than a single molecule would, and allows us to take pictures of the scattering pattern…to develop a complete 3-D model of the molecule.