BioMolecular Crystallography From phases determination to final structure

The process of determining the structure of a biological macromolecule by crystal X-ray diffraction is generally divided schematically into six stages: obtaining the macromolecule in its pure state (or macromolecules in the case of assemblies), crystallization, collection of diffraction data, phasing, construction of the crystallographic structure by interpretation of electron density maps, refinement and validation of the structure. Purifying the macromolecule(s) and obtaining quality crystals (diffraction limit better than 3 Å) are the first two limiting steps in a structural project. These steps are described in the article [P 1 110] .

This article focuses on phase determination, the third major problem in biocrystallography, and quality control methods for the resulting structures. These steps are currently characterized by the use of sophisticated mathematical methods, implemented in increasingly automated and easy-to-use programs. The routine use of anomalous diffusion has revolutionized the problem of phases. In recent years, major methodological advances, accompanied by increasingly powerful computing resources now accessible on a personal computer, have made it possible, in simple cases, to rapidly solve a 3D structure from a very limited number of crystals, sometimes of very small sizes (a few micrometres), with a minimum of human intervention. All these advances enable structuralists to continually push back the limits of problems that can be solved using biocrystallography.