The 3-D structure of a molecule provides a unique understanding of how the molecule functions: X-ray crystallography allows the structure determination at near-atomic resolution.

Using the three-dimensional structure of biological macromolecules to infer how they function is one of the most important fields of modern biology. The availability of atomic resolution structures provides a deep and unique understanding of protein function, and helps to unravel the inner workings of the living cell.

To date, the majority of the Protein Data Bank (rcsb-PDB) entries are macromolecular structures that were determined using X-ray crystallography.

The requisite for a successful X-ray crystallographic application is to obtain single crystals of the target protein. Data is then collected by diffracting X-ray from the single crystal that has an ordered pattern of atomic orientation. The assembly of atoms and molecules in the crystal can be deduced from the measurement of X-ray scattering.

To obtain crystals suitable for crystallographic studies, the macromolecule (e.g. protein, nucleic acid, protein-protein complex or protein-nucleic acid complex) must be purified to homogeneity, or as close as possible to homogeneity. The homogeneity of the preparation is a key factor in obtaining crystals that diffract to high resolution.