What can be done with a good crystal and an accurate beamline?
X-ray single-wavelength anomalous diffraction (SAD) data from a crystal of proteinase K were collected using synchrotron radiation of 0.98 A wavelength at SER-CAT 22-ID beamline, Advanced Photon Source, Argonne National Laboratory. At this wavelength, the expected Bijvoet ratio resulting from the presence of one calcium, one chloride and ten S atoms in the 279-residue protein is extremely small at approximately 0.46%. The direct-methods program SHELXD located 11 anomalous sites using data truncated to 2 A resolution. SHELXE was used to produce an easily interpretable electron-density map. This study shows that an accurate beamline and a good-quality crystal provide the possibility of successfully using a very weak anomalous signal of sulfur measured at a short wavelength for phasing a protein structure, even if a small degree of radiation damage is present.