X-Ray Studies
X-ray crystallography is a process by which the extremely fine atomic structure of many crystals can be examined and recorded. It was first developed not as a research tool but as a means of determining the nature of x rays themselves.
X rays were discovered—quite accidentally—in 1895 by Wilhelm Röntgen (1845-1923). Although his intensive research revealed much about the properties of these new rays, such as their ability to penetrate certain substances, Röntgen could not ascertain whether x rays consisted of particles or longitudinal waves. This question puzzled scientists until 1912, when German physicist Max von Laue (1879-1960) directed an x ray beam through a crystal. As the X-ray struck the lattice-like pattern of atoms within the crystal, an interference (or diffraction) pattern was formed--an effect that could only occur if x rays were waves, like light.
Laue's experiment proved to his fellow scientists the longitudinal nature of x rays. However, it was an Australian professor, William Henry Bragg, and his son William Lawrence Bragg who realized the significance of Laue's discovery. They surmised that the structure of the crystal on a molecular level could be deduced from a study of the interference pattern. In order to prove their theory they also designed an x-ray spectrometer to measure the specific wavelengths of x rays, and devised a mathematical system for analyzing the information. In 1915 the father-son team shared the Nobel Prize for Physics for the establishment of a new scientific method, x-ray crystallography.
In crystallography, x rays are used to probe the structure of a variety of crystals. The pattern of diffracted x rays is analogous to an atomic "shadow"--by examining where the x rays are blocked by the crystal's atoms, scientists can define the structure of those atoms. This was first seen by the Braggs, who found that crystals consist not of molecules, but rather of groups of layered ions; for example, a sodium chloride crystal is formed from sodium ions and chlorine ions. X-ray crystallography quickly became an important tool for validating many of Danish physicist Niels Bohr's (1885-1962) theories of atomic structure.
Perhaps the most important application of x-ray crystallography is its use in synthesizing substances, particularly in medicine. Many of the medicinal chemicals that have been discovered by scientists are very difficult to produce naturally in large amounts. In this case, it becomes necessary to create the chemicals in the laboratory through synthesis. However, before a chemist can synthesize a substance, a very specific map of its atomic structure must be obtained, a map that can only be drawn by using x-ray crystallography. Few scientists have been more successful at this than the British chemist Dorothy Hodgkin (1910-). During World War II, Hodgkin and her colleagues determined the structure of penicillin, whose synthesis was necessary to supply army hospitals. Since then, Hodgkin's team has worked on the crystallographic cartography of vitamin B12 (prescribed to prevent pernicious anemia) and insulin (used in the treatment of diabetes). Other researchers have used x-ray technologies to record the structures of proteins, hemoglobin, and the now-familiar double-helix of DNA (deoxyribonucleic acid).
The development of x-ray crystallography also created the science of mineralogy. Once they were able to examine in detail the inner structure of many minerals, mineralogists were able to define the major mineral groups. The understanding that stems from crystallography has also allowed scientists to construct the man-made minerals used in industry.
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