X-ray crystallography is a costly and challenging method of visualizing the structure of molecules in a crystal lattice. It has been used to determine the structure of various materials, including DNA, proteins, and viruses. Obtaining a pure crystal is difficult, and the sample is subjected to an intense beam of X-rays to produce a diffraction pattern. By rotating the crystal and recording the patterns, an electron density map is derived, and a hypothesis is formed about the atomic structure. The result is then uploaded to central databases.
X-ray crystallography is a highly accurate, but also difficult and expensive means of visualizing the exact structure of a given molecule or macromolecule in a crystal lattice. Because a diverse set of materials produce crystals, including salts, metals, minerals, semiconductors, and various inorganic, organic, and biological molecules, X-ray crystallography is essential in many scientific fields. A crystal is any regularly repeating arrangement of unit cells ranging in size from fewer than 100 atoms (small-molecule crystallography) to tens of thousands (macromolecular crystallography).
X-ray crystallography is famous for being the first tool used to discover the structure of DNA, but it has also been used to determine the structure of diamond, table salt, penicillin, numerous proteins and entire viruses. In all, over 400,000 structures have been described using X-ray crystallography. These can be found in the Cambridge structure database.
To analyze a sample by X-ray crystallography, it is first necessary to obtain a highly pure crystal of the material to be studied with a very regular structure. This is often the hardest part as many crystals have nanoscale defects that make X-ray crystallography difficult.
Subsequently, the sample is subjected to an intense beam of X-rays of uniform wavelength. These X-rays produce a diffraction pattern as they reflect off the sample. This diffraction pattern is somewhat similar to what is seen when multiple stones are thrown into a pond – where the waves cross are peaks that make up the diffraction pattern.
By slowly rotating the crystal, hammering it with X-rays, and meticulously recording the diffraction patterns at each orientation, a map of the electron density can be derived. This electron density map is then used to form a hypothesis about the atomic structure to which it corresponds. The diffraction patterns are then analyzed in light of the hypothesized structure and, if it seems plausible that the given structure could produce the observed diffraction pattern, a conclusion is drawn. The result is then uploaded to central databases of the type mentioned earlier.
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