Lab Source Anomalous Scattering Using Cr Kα Radiation

Abstract

High-throughput crystallography requires a method by which the structures of proteins can be determined quickly and easily. Experimental phasing is an essential technique in determining the three-dimensional protein structures using single-crystal X-ray diffraction. In macromolecular crystallography, the phases are derived either by Molecular Replacement (MR) method using the atomic coordinates of a structurally similar protein or by locating the positions of heavy atoms that are intrinsic to the protein or that have been added (MIR, MIRAS, SIR, SIRAS, MAD and SAD). Availability of in-house lab data collection sources (Cu Kα and Cr Kα radiation), cryo-crystallography and improved software for heavy atom location and density modification have increased the ability to solve protein structures using SAD. SAD phasing using intrinsic anomalous scatterers like sulfur, chlorine, calcium, manganese and zinc, which are already present in the protein becomes increasingly attractive owing to the advanced phasing methods. An analysis of successful SAD phasing on three proteins, lysozyme, glucose isomerase and thermolysin based on the signal of weak anomalous scatterers such as sulfur atom and chloride ion have been carried out. This analysis also proves that even the anomalous signal provided or present naturally in a macromolecule is good enough to solve crystal structures successfully using lab source chromium-generated X-ray radiation.