PROTEIN SINGLE-CRYSTAL DIFFRACTION WITH 5 ANGSTROM SYNCHROTRON X-RAYSAT THE SULFUR K-ABSORPTION EDGE

Sulfur atoms, an integral part of many proteins, are possible candidat es for anomalous scattering in phase determination by multiple-wavelen gth methods. The main difficulty encountered is that a wavelength of a bout 5 angstrom is required to obtain a large anomalous signal from th ese atoms, leading to very large absorption effects. Initial experimen ts have been carried out using a synchrotron X-ray source, evacuated b eam tubes, a diffractometer inside a vacuum chamber, a special sample holder and a suitable scattering geometry. The results are encouraging , showing that Bragg reflections can be measured, and that changes in their intensities around the absorption edge are observable.