BioMolecular Crystallography Crystallisation and data collection

Single-crystal X-ray diffraction is the method par excellence for studying biological macromolecules at the atomic scale. The process of determining a biological macromolecule structure by X-ray diffraction on crystals is schematically divided into six steps:

• obtaining the macromolecule in its pure state (or macromolecules in the case of assemblies);

• crystallization ;

• diffraction data collection ;

• phasing ;

• construction of the crystallographic structure by interpretation of electron density maps ;

• structure refinement and validation.

The last three steps are described in [P 1 111] .

This article deals with the first steps in this process, right through to obtaining diffraction data. These stages are characterized by miniaturization and extensive automation, with increasingly reduced human intervention. The intrinsic physico-chemical properties of biological macromolecules give rise to crystals with large crystal lattice parameters and generally limited diffraction power compared with the standard for small organic molecules. As of December 2018, 57% of the crystallographic structures deposited in the RCSB PDB database have a diffraction limit of less than 2 Å, and only 13% have a diffraction limit better than 1.6 Å. This calls for appropriate methods and techniques throughout the crystallographic process. This methodology, specific to biological macromolecules, will be presented in this article.