SAD phasing with triiodide, softer X-rays and some help from radiation damage

SAD data on a triiodide derivative of porcine pancreatic elastase have been recorded from a single sample using 2.0 Å wavelength X-rays. The large anomalous signal of iodine at this wavelength allowed the detection of heavy-atom sites and subsequent structure determination using low-redundancy data. Substantial radiation damage was observed during the measurements and this prevented the merging together of all data. However, a straightforward treatment of the radiation effects on the heavy-atom model during parameter refinement resulted in additional phase information being gleaned from the observed reduction in iodine occupancy factors, which in turn produced a significant improvement in the quality of the electron-density map.