Phasing on Rapidly Soaked Ions

Abstract

Publisher Summary There are three basic ways of solving macromolecular crystal structures: (1) the molecular replacement method, (2) direct methods, and (3) the heavy-atom method. Molecular replacement involves the use of a known search model, closely similar to the macromolecule being investigated. Direct methods are routinely used to solve structures of small molecules, where the diffraction data extend to atomic resolution. As a consequence, these two methods cannot be used to solve novel structures with crystals diffracting to lower than atomic resolution. The heavy atoms, providing the initial phase information, can be present in the original macromolecule, such as certain transition metals in metalloproteins, sulfur in proteins, and phosphorus in nucleic acids. However, most general is the derivatization before or after crystallization. The classic derivatization approach involves prolonged soaking of the native crystals in diluted solutions of various heavy-metal salts and coordination compounds. Such soaking procedures are time consuming and are often unsuccessful owing to the lack of heavy-atom binding or the deterioration of crystal quality. The rapid soak approach uses this property of protein crystals that allows small ions to diffuse within a short time to the solvent regions surrounding the protein molecules and adopt ordered sites at their surfaces.