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What is X-ray Crystallography ? X-ray crystallography is an experimental technique that exploits the fact that X-rays are diffracted by crystals. It is not an imaging technique. X-rays have the proper wavelength (in th Å t ö (i the Ångström range, ~10-10 m) t be scattered by the electron cloud of an atom of 10 10 ) to b tt d b th l t l d f t f comparable size. Based on the diffraction pattern obtained from X-ray scattering off the periodic assembly of molecules or atoms in the crystal, the electron density can be reconstructed. Additional phase information must be extracted either from the diffraction data or from supplementing diffraction experiments to complete the reconstruction (the p phase problem in crystallography). A model is then progressively built into the p y g p y) p g y experimental electron density, refined against the data and the result is a quite accurate molecular structure. Why Crystallography ? The knowledge of accurate molecular structures is a prerequisite for rational drug design and for structure based functional studies to aid the development of effective therapeutic agents and drugs. Crystallography can reliably provide the answer to many structure related questions, from global folds to atomic details of bonding. In contrast to NMR, which is an indirect spectroscopic method, no size limitation exists for the molecule or complex to be studied. The price for the high accuracy of crystallographic structures is that a good crystal must be found, and that only limited information about the molecule's dynamic behavior is available from one single diffraction experiment.

Outline of the experiment
In a macromolecular X-ray diffraction experiment a small protein crystal is placed into an intense X-ray beam and the diffracted X-rays are collected with an area detector (it is advantageous to cool the crystals to low temperatures, primarily to prevent radiation damage). damage) The diffraction pattern consists of reflections of different