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 flattening 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 flattening as the density-modification 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 degrees and the corresponding map correlation coefficients were increased by 0.05-0.18 relative to phase sets from standard solvent-flattening procedures. With SIR data, the lowest final mean phase errors were approximately 58 degrees and the corresponding map correlation coefficients were in the range 0.53-0.68.