Abstract
The optimization of the initial phasing has been a decisive factor in the success of the subsequent electron density modification, model building and structure determination of biological macromolecules with the single-wavelength anomalous dispersion (SAD) method. Two possible phase solutions (phi1 and phi2) generated from two symmetric phase triangles in the Harker construction according the SAD method cause the well-known phase ambiguity. We have developed a novel Direct phase selection method utilizing the thetaDS list as a criterion to select optimized phases phiam from phi1 or phi2 of the partial reflections with the high percentage of correct phases to replace the corresponding initial SAD phases phiSAD. Based on our work, the diffractions with angle thetaDS in a range 35 – 145° are selected for an optimized improvement; thetaDS is the angle between the initial phiSAD phase and a preliminary density-modification (DM) phase phiDM_NHL. The results show that our method improves significantly the final phases after the subsequent run of DM, compared to conventional phasing approaches, in terms of the increased correlation coefficient of electron-density maps and diminished mean phase errors. With the improved phases and density maps from our developed method of direct phase selection, the completeness of built residues with main chains and side chains of protein molecules is enhanced for efficient structure determination.