Dear Ms. Holmes,

I was wondering: When you were explaining the purpose of the crystals that we "made" in your station in Huntsville, I didn't really understand. Was that just to see what they'd look like or was there another scientific thing we were helping with? Also, how can I get more information on the x-ray crystallography stuff? It sounded really interesting and I was wondering how they take the "dots" from the x-rays and find the atomic (?) structure of the protein. It makes sense that they would be able to, I'd just like to really know how. Thank you!

Sincerely, Elise Humbert

Elise,

The sessile drop crystallization screens the students prepared are part of the "ground based" crystal screens that are compared with crystals formed in microgravity. You as well as Andrew, Stacy, Katie and Leanne, prepared lysozyme crystals that usually obtain monoclinic crystals. We will be posting an x-ray diffraction image on Katie's page, in the near future, that was obtained from x-ray diffraction conducted on one of her crystals.

We do not have a direct method to view these molecules. By x-ray diffraction however we can obtain a series of images (like the one shown below) that can be analyzed by computer computational methods and mathematics to generate an electron density map. From this map and the linear sequence of the amino acids in the protein, a crystallographer may determine the structure of the protein. The "dots" are called "reflections" and each reflection represents a particular position in reciprocal space and has an "intensity" (darker <---> lighter) with which it is associated. Several computer programs are available today to help the crystallographer solve the structure of these proteins. These programs for the most part have taken over the laborious task of solving the atomic structure factor, the structure factor and the electron density equations used sequentially to generate the electron density map.

In the sketch above, a long monoclinic lysozyme crystal is mounted in a glass capillary. The x-ray beam is diffracted through the crystal and the detector behind the crystal records the reflections as shown in the shaded area.

A good beginning reference text is Crystallography Made Crystal Clear by Gale Rhodes. ISBN 0-12-587075-2 (first edition, although a second is now available).