X-ray crystallography facts for kids

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An X-ray diffraction pattern of a crystallized enzyme. The pattern of spots (reflections) and the relative strength of each spot (intensities) is used to work out the structure of the enzyme

X-ray crystallography is a way to see the three-dimensional structure of a molecule. The electron cloud of an atom bends the X-rays slightly. This makes a "picture" of the molecule that can be seen on a screen. It can be used for both organic and inorganic molecules. The sample is not destroyed in the process.

The technique was jointly invented by Sir William Bragg (1862–1942) and his son Sir Lawrence Bragg (1890–1971). They won the Nobel Prize in Physics for 1915. Lawrence Bragg is the youngest to be made a Nobel Laureate. He was the Director of the Cavendish Laboratory, Cambridge University, when the discovery of the structure of DNA was made by James D. Watson and Francis Crick in February 1953.

The oldest method of X-ray crystallography is X-ray diffraction (XRD). X-rays are fired at a single crystal and the way they are scattered produces a pattern. These patterns are used to work out the arrangement of atoms inside the crystal.

X-ray analysis of crystals

The incoming beam (from upper left) causes each scatterer (e.g. electron) to re-radiate a part of its energy as a spherical wave.
If atoms are arranged symmetrically with a separation d, these spherical waves will add up only where their path-length difference 2d sin θ equals a multiple of the wavelength λ. In that case, a reflection spot occurs in the diffraction pattern

Crystals are regular arrays of atoms, and X-rays are waves of electromagnetic radiation. Atoms scatter X-ray waves, mainly through the atoms' electrons. An X-ray striking an electron produces secondary spherical waves emanating from the electron. The electron is known as the scatterer. A regular array of scatterers produces a regular array of spherical waves. Although these waves cancel one another out in most directions, they add up in a few specific directions, determined by Bragg's law:

$2d \sin \theta = n \lambda$

Here d is the spacing between diffracting planes, $\theta$ is the incident angle, n is any integer, and λ is the wavelength of the beam. These specific directions appear as spots on the diffraction pattern called reflections. Thus, X-ray diffraction results from an electromagnetic wave (the X-ray) hitting a regular array of scatterers (the repeating arrangement of atoms within the crystal).

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