Dorothy Crowfoot Hodgkin: X-Ray Crystallography

Dorothy Crowfoot Hodgkin made invaluable contributions in the field of X-ray crystallography. Using this procedure, Hodgkin was able to "see" inside atoms and molecules to learn how they function. An X-ray is a form of electromagnetic radiation that can penetrate most substances. When a substance is exposed to X-rays, the areas with the highest atomic weight, or those that are most "solid," absorb most of the radiation, and therefore show up best when photographed. This is why bones are most visible in an X-ray photograph of the human body.

Unlike a human body, the chemical compounds that Hodgkin and her colleagues studied do not have a "skeleton" that would show up clearly in an X-ray photograph, and they do not exist in a consistent state. Instead, these microscopic combinations of elements can be liquids, solids or gases.

In order to "see" inside a compound, it has to be in a specific solid state known as a crystal. A molecule that is crystallized, or made into a crystal, looks like a diamond or other rock crystal: it has a defined geometric structure made up of edges and planes. The first step in X-ray crystallography is getting the desired compound to crystallize, a process that can be extremely difficult.

Once a compound has been crystallized, it is then placed in front of a beam of X-rays. When a human body is X-rayed, the radiation is absorbed by the bones and tissue. Crystals do not absorb radiation; instead, the X-rays "bounce off" of their edges. This process is known as diffraction.

Diffraction refers to the way that waves of energy spread out when they come into contact with a barrier. Diffraction occurs when ocean waves part around a rock, a boat or a jetty. The waves continue their motion, but the barrier "bends" them and changes their direction. Sound waves also diffract, which is why the music from a single speaker can be heard in several rooms in a house.

When a crystal is X-rayed, the X-rays follow the same pattern of diffraction. They bend and change direction when they encounter changes in the inner structure of the crystal. An electronic detector follows the path of the X-rays through the crystal, and forms a "picture" of the crystal from the inside out.

Once the electronic detector has produced a two-dimensional model of the crystal, a series of mathematical calculations is required to create a full three-dimensional model. In Hodgkin's time, this was a lengthy process, requiring a team of scientists using computers. Modern X-ray crystallographers use advanced computer programs to achieve these results.

X-ray crystallography is an important tool for scientists to understand the way chemical compounds work. This information can help them know how a compound reacts inside an organism, such as the human body. It can also help them produce compounds synthetically, in a laboratory. This has allowed scientists to make large quantities of important drugs such as penicillin.

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By Jessica Hannon

Source: Dorothy Crowfoot Hodgkin, 2006, p2, 1p