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 the
Ångström range, ~10-8 cm) to be scattered by the electron cloud of an
atom of 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 phase problem in
crystallography). A model is
then progressively built into the 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.