X-Ray Vision: Crystallography

Chemistry Activity
X-Ray Vision
Crystallography

We usually think of X-rays as allowing us to see through things. For example, we use X-rays to look through the human body to see what is happening inside without having to perform surgery. But X-rays can also help us see things that are very small. Scientists use X-ray crystallography to determine the molecular structures of compounds, especially very complicated molecules. In this method, X-rays are passed through a crystal of the substance in question. The X-rays are diffracted, or bent, by the electron clouds of the atoms in the molecule. X-rays are used because the wave length of the radiation used must be on the same order of magnitude as the bond lengths in the molecule. Using the pattern of diffracted radiation, scientists can map the arrangement of atoms in the molecule.

Every crystal is made up of repeating unit cells. A unit cell is the smallest arrangement of atoms that, when repeated, is representative of the entire crystal. By finding the structure of the unit cell, scientists can project the structure of the entire crystal. The technique was first developed by an Australian scientist named William Henry Bragg in the early 1900s, and the structures of many drugs were first determined using X-ray crystallography.

Postage stamp honoring
Dorothy Hodgkin.

Among those drugs was penicillin. By 1942, the race to produce the new antibiotic wonder drug was being run against the backdrop of World War II. By 1940, Alexander Fleming, Howard Florey, and Ernest Chain were cooperating on their research into the promising drug. Ernst Chain, an eminent British biochemist, had met a young crystallographer named Dorothy Crowfoot Hodgkin some years earlier in passing on the streets of Oxford. Hodgkin had been working on the structure of cholesterol in her Oxford lab, and Chain promised to supply her with samples of penicillin. It had become apparent that penicillin production by conventional means would yield far less than the growing war injuries demanded. If scientists could synthesize the drug in the lab, production might be increased dramatically. But synthesis would depend on knowing the structure of the drug, and by 1942, Hodgkin had embarked on determining its structure in her X-ray crystallography lab in Oxford. By 1945, she had found the structure. In 1964, Hodgkin was awarded the Nobel Prize in Chemistry for this discovery along with her discoveries of the structures of a number of other biologically important molecules, including vitamin B₁₂.

In this activity you will attempt to determine the arrangement of “atoms” in a “molecule” using evidence gathered indirectly from light passing through the molecule. The molecule will be a ball-and-stick model of a simple organic molecule. In X-ray crystallography, the molecule would be very complex and the radiation would be diffracted rather than simply casting shadows as it does in this activity. However, you will be able to see how the shadows can be mapped into a three-dimensional structure as you do this activity. Your teacher will direct you in this activity.
For more information, at other Web sites...

Dorothy Crowfoot Hodgkin — biography, part of Chemical Achievers from the Chemical Heritage Foundation.
The Nobel Prize in Chemistry 1964 — includes biographical information on Dorothy Crowfoot Hodgkin as well as descriptions of her prize-winning work.
William Henry Bragg and William Lawrence Bragg — biographies, part of Chemical Achievers from the Chemical Heritage Foundation.
The Nobel Prize in Physics 1915 — includes biographical information on William Henry Bragg and William Lawrence Bragg as well as descriptions of their prize-winning work.
The Nobel Prize in Physiology or Medicine 1945 — includes biographical information on Alexander Fleming, Howard Florey, and Ernest Chain as well as descriptions of their prize-winning work.

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