In-house measurement of the anomalous signal of sulfur and its use for phasing

Recent developments in X-ray instrumentation, like high intensity sources, cryo devices and new detectors now make it possible to measure reflection intensities precisely enough so that even the anomalous signal of weak scatterers such as sulfur, present in most proteins, can be employed for phase determination.This eliminates the need to prepare selenomethionine or other derivatives.The anomalous signal of sulfur at Cu Kα wavelength is 0.5 electrons, which, although on the verge of measurability, under optimal conditions can be successfully exploited to give sufficient phase information.In-house sources, especially if equipped with a goniometer (rather than a single rotation axis) enabling a much higher redundancy to be obtained, even have advantages over synchrotron sources for phasing: it is possible to measure for a longer time without significant radiation damage.For phasing purposes the highest possible resolution is not required, often a high-resolution synchrotron dataset at shorter wavelength may be used for phase extension and structure refinement.Using the knowledge accumulated from several test cases, the key steps in data acquisition will be presented and analyzed from the point of view of success in subsequent phasing.Selected examples will illustrate the solution of test and unknown structures using SHELXD and SHELXE.In favorable cases i.e. very high resolution or high solvent content, SAD phasing based on the anomalous scattering of sulfur alone can give very high quality maps.