W0332

Single-wavelength anomalous diffraction (SAD) on native proteins has enormous potential for sulfur phasing. In recent years, with developments in cryocrystallography and detector technology, cases are being reported of in-house (Cu Kα) sulfur-SAD phasing on low resolution data¹. These techniques depend on the measurement of very small difference in anomalous signal (typically 1-1.5%) and hence requires extremely precise data. In order to optimize data quality, the signal to noise ratio (S/N) has to be improved. There are various empirical ways of increasing S/N, amongst which using a sensitive and low noise detector, increasing the X-ray flux or integration time and reducing background scatter due to the crystal mount. S/N is a function of the square root of the number of observations of a given measurement and hence collecting a more redundant dataset will also give more accurate data. Merely using multiple measurements of the same reflection improves the counting statistics, but does not necessarily yield the most accurate dataset. In recent years various data collection strategies have been suggested to improve the quality of data. A systematic evaluation of the effect of data collection strategy on the data quality and hence the anomalous signal is presented.