Comparison of phasing methods for sulfur‐SAD using in‐house chromium radiation: case studies for standard proteins and a 69 kDa protein

Phasing of the crystal structures of four standard proteins (lysozyme, trypsin, glucose isomerase and thaumatin) and a novel 69 kDa protein from Thermus thermophilus , TT0570, was performed using the single‐wavelength anomalous diffraction of S atoms intrinsically present within the native protein molecules. To utilize the sulfur anomalous diffraction, the data sets were collected using the loopless data‐collection method with chromium K α X‐rays of wavelength 2.29 Å. Three phasing methods, MLPHARE , SHARP and OASIS ‐2004, were tested in combination with the DM or SOLOMON density‐modification method. The results showed that the solvent contents are still an important factor for phasing with the S‐SAD method, even when longer wavelength Cr  K α radiation is used. Of the three procedures, the improved direct phasing of OASIS ‐2004 with its implemented fragment feedback to the direct‐method probability calculation gave the best results in determining the initial phases. For all five proteins, almost the entire models could be built automatically.