In-house phasing with iodide soaks

Traditionally one assumes there is no significant anomalous signal for the native data (Friedel symmetry) and changes other than the overall decay are ignored in the analysis.However, one may often use the deviations from these two approximations for phasing.The analysis of the deviation from Friedel symmetry results in the SAD method for structure solution, that has recently become practical for very well diffracting crystals or for ones that contain an heavy atom (calcium or heavier).It has been established that the exposure of a crystal to x-rays produces defined changes in the protein structure such as the disruption of disulfide bonds, deamidation and decarboxylation.This information can be used for phasing if one can estimate the electron density of such changes.Our goal is to solve protein structures from a single native data set.We expect that adding the analysis of x-ray induced changes to SAD will allow solving the majority of native protein crystal structures.The practical application of this method requires the development of an optimal experimental protocol and improvement of the phasing analysis.The current methods are sub-optimal in their treatment of experimental errors that are about the same magnitude as the phase-sensitive signal.We are developing novel approaches to error-treatment and phasing using the combined SAD and radiation induced changes method and the results will be presented.