The application of multivariate statistical techniques improves single-wavelength anomalous diffraction phasing

Recently, there has been a resurgence in phasing using the single-wavelength anomalous diffraction (SAD) experiment; data from a single wavelength in combination with techniques such as density modification have been used to solve macromolecular structures, even with a very small anomalous signal. Here, a formulation for SAD phasing and refinement employing multivariate statistical techniques is presented. The equation developed accounts explicitly for the correlations among the observed and calculated Friedel mates in a SAD experiment. The correlated SAD equation has been implemented and test cases performed on real diffraction data have revealed better results compared with currently used programs in terms of correlation with the final map and obtaining more reliable phase probability statistics.