Histogram matching as a new density modification technique for phase refinement and extension of protein molecules

Abstract
A new density modification technique - histogram matching - is being developed with encouraging results. Its application to the known structure of pig 2Zn insulin refines the 6500 1.9 Å MIR phases from a mean error of 60° to one of 46°. With these refined phases as a starting point for phase extension to 1.5 Å, the mean error for the final 13 000 phases is 46°. The original 1.9 Å phases continue to refine during the phase extension to a final mean error of 40°. A comparison is made with similar calculations already published.