An improved phase-extension procedure for isomorphous replacement phases

Abstract

A new phase-extension procedure has been applied to isomorphous replacement data and shown to yield improved phases and maps compared with standard solvent¯attening operating on a full set of centroid phases. In this procedure, a starting subset of core phases is selected based on the sharpness of the phase-probability curves. Phase extension using solvent¯attening as the density-modi®cation procedure is then carried out, gradually adding additional phases. In tests with known protein structures, the mean phase errors for the output expanded phase sets were reduced by 3±9 and the corresponding map correlation coef®cients were increased by 0.05±0.18 relative to phase sets from standard solvent-¯attening procedures. With SIR data, the lowest ®nal mean phase errors were approximately 58 and the corresponding map correlation coef®cients were in the range 0.53±0.68.