In-house sulfur SAD phasing: a case study of the effects of data quality and resolution cutoffs

Single-wavelength anomalous diffraction (SAD) utilizing the weak signal of inherently present S atoms can be successfully used to solve macromolecular structures, although this is mostly performed with data from a synchrotron rather than a laboratory source. Using high redundancy, sufficiently accurate anomalous data may now often be collected in the laboratory using Cu Kalpha X-ray radiation. Systematic analyses of a laboratory-derived data set illuminate the effects of data quality, redundancy and resolution cutoffs on the ability to locate the S atoms and phase the structure of Ptr ToxA, a 13.2 kDa toxin secreted by the fungus Pyrenophora tritici-repentis. Three sulfurs contributed to the successful phasing of the structure and were located using the program SHELXD. It is observed that data quality improves with increasing redundancy, but after a certain point becomes worse owing to crystal decay, so that there is an optimal amount of data to include for the sulfur substructure solution. Further, the success rate in locating S atoms is dramatically improved at lower resolutions and in a manner similar to data quality, there exists an optimal resolution at which the likelihood of solving the substructure is maximized. Based on these observations, a strategy for SAD data collection and substructure solution is suggested.