Screening for phasing atoms in protein crystallography.

Introduction Determining crystallographic phases remains a key step in the solution of macromolecular structures. For most novel structures, phases must be determined experimentally using methods that rely on finding the positions of a few special (either electron-dense ‘heavy’, or anomalously scattering) atoms. The scattering contribution of these atoms, in combination with knowledge of their positions, can be used to calculate phases for the total crystal contents. Whether these calculations rely on multiple wavelength anomalous dispersion (MAD), multiple isomorphous replacement (MIR), single isomorphous replacement with anomalous scattering (SIRAS) or other methods will depend entirely on the ordered presence of these special atoms. Thus, the ability to efficiently prepare samples containing such sites is of fundamental importance in macromolecular crystallography.