X-ray crystallography is a technique to
generate the three-dimensional structures of proteins at an atomic level and
understand their functions from its crystallised form. There are three main components
needed to complete an x-ray crystallography analysis – a purified sample at
high concentration which is crystallised, an X-ray source and a detector where
the resulting diffraction patterns are processed.

The
resolution of an image in any form of microscopy depends on the wavelength of
electromagnetic radiation used. Light microscopy (where the range of
wavelengths is 380-750nm) is used to see individual cells and sub-cellular
organelles. Electron microscopy (where the wavelength is approximately 10nm) is
used to see cellular architecture and the shapes of large protein molecules. Therefore,
to see proteins at an atomic level, electromagnetic radiation of around 0.1 nm
should be used – this corresponds to the wavelength of X-rays. 1

The diffraction
from a single molecule is too weak to be measured therefore the proteins are
crystallised. The diffraction of a crystal is measurable as there are multiple
copies within them. Typical protein cells crystals are about 0.2mm in size but
usable crystals have been reported from tens of microns to a few millimetres
2.

The crystallisation
of proteins cause the protein atoms to be orientated in a fixed way with
respect to one another while still maintaining their biologically active
conformations (however this may not always be the case x-ray crystallography
cannot be used).