Optimizing data collection for structure determination.

The ultimate purpose of diffraction data collection is to produce a data set which will result in the required structural information about the molecule of interest. This usually entails collecting a complete and accurate set of reflection intensities to as high a resolution as possible. In practice, the characteristics of the crystal and properties of the X-ray source can be limiting factors to the data-set quality that can be achieved and a careful strategy has to be used to extract the maximum amount of information from the data within the experimental constraints. In the particular case of data intended for phasing using anomalous dispersion, the synchrotron beamline properties are relevant to determine how many wavelengths (one or more) should be used for the experiment and what the wavelength values should be. This will in turn affect the detailed strategy for data collection, including decisions about the data-collection sequence and how much data to collect at each wavelength. Collection of multiwavelength anomalous dispersion (MAD) data at three different wavelengths can provide very accurate experimental phases. Two-wavelength MAD experiments may offer the best compromise between phase quality and minimizing the effects of radiation damage to the sample. However, MAD experiments are demanding in terms of beamline wavelength range, easy tunability, stability and reproducibility. When the beamline cannot fulfill these demands, single-wavelength experiments may be a better option.