Direct low-resolution phasing from electron-density histograms in protein crystallography

An approach to direct phasing of low-resolution reflections is proposed. It is based on the generation of a large number of phase sets and selection of those variants whose electron-density-synthesis histograms are close to a prescribed standard. Classifying them into clusters and averaging them inside every cluster restricts their number to one to three usually, in which a phase set close to the standard is contained. The best variant can be recognized by the properties of its cluster. Test phasing of 29 low-resolution reflections has resulted in a correlation coefficient of 0.94 and a mean phase difference of 40° compared with the true phases.