Gorgel, M.; Boggild, A.; Jensen Ulstrup, J.; Weiss, M.S.; Mueller, U.; Nissen, P.; Boesen, T.: Against the odds? De novo structure determination of a pilin with two cysteine residues by sulfur SAD. Acta Crystallographica Section D - Biological Crystallography 71 (2015), p. 1095-1101
Exploiting the anomalous signal of the intrinsic S atoms to phase a protein structure is advantageous, as ideally only a single well diffracting native crystal is required. However, sulfur is a weak anomalous scatterer at the typical wavelengths used for X-ray diffraction experiments, and therefore sulfur SAD data sets need to be recorded with a high multiplicity. In this study, the structure of a small pilin protein was determined by sulfur SAD despite several obstacles such as a low anomalous signal (a theoretical Bijvoet ratio of 0.9% at a wavelength of 1.8 A ° ), radiation damage-induced reduction of the cysteines and a multiplicity of only 5.5. The anomalous signal was improved by merging three data sets from different volumes of a single crystal, yielding a multiplicity of 17.5, and a sodium ion was added to the substructure of anomalous scatterers. In general, all data sets were balanced around the threshold values for a successful phasing strategy. In addition, a collection of statistics on structures from the PDB that were solved by sulfur SAD are presented and compared with the data. Looking at the quality indicator Ranom/Rp.i.m., an inconsistency in the documentation of the anomalous R factor is noted and reported.