Protein single-crystal diffraction with 5 Å synchrotron X-rays at the sulfur K-absorption edge

Sulfur atoms, an integral part of many proteins, are possible candidates for anomalous scattering in phase determination by multiple-wavelength methods. The main difficulty encountered is that a wavelength of about 5 Å is required to obtain a large anomalous signal from these atoms, leading to very large absorption effects. Initial experiments have been carried out using a synchrotron X-ray source, evacuated beam 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