X-rays are used to probe the atomic or molecular structure of matter because the wavelength of the radiation is of approximately the same dimension as an atom. Similarly longer wavelength visible light is appropriate for studying larger structures, e.g. cell organelles. However, since there is no known X-ray lens the equivalent function of a glass lens for visible light in a conventional microscope has to be performed by computational transformation of X-ray diffraction patterns.

The basic steps in a macromolecular crystal structure analysis involve:

1. (i) crystallisation;

2. (ii) space group and cell parameter determination;

3. (iii) data collection;

4. (iv) phase determination;

5. (v) electron density map interpretation;

6. (vi) refinement of the molecular model.

Figure 2.1 (a)—(f) illustrates some of these steps showing, as an example, the structure determination of human erythrocyte purine nucleoside phosphorylase (PNP) (Ealick et al (1990)). A list of general texts on crystallography is given in the bibliography, section 1.

CRYSTALLISATION, CRYSTALS AND CRYSTAL PERFECTION, SYMMETRY

Crystallisation is a process involving precipitation of the dissolved protein from solution. This is achieved by decreasing the protein solubility, decreasing any repulsive forces between individual protein molecules and/or increasing the attractive forces. The crystals that might be produced need to be of ‘X-ray diffraction quality’.