Structure solution with weak anomalous data

Abstract

A key challenge in the use of the SAD phasing method is the ability to carry out the process in cases where the anomalous signal-to-noise ratio is low. The magnitudes of the anomalous differences between Bijvoet pairs of reflections depends on the types and numbers of anomalously scattering atoms in the structure, the wavelength of data collection, and the details of data collection, particularly the number of times each X-ray reflection is measured. In experiments using small numbers of selenium atoms in SAD phasing of large structures, in sulfur SAD phasing, and in recent experiments using X-ray lasers with SAD phasing, exceptional efforts can be necessary in order to obtain sufficient signal-to-noise.