DIRECT-SPACE METHODS IN PHASE EXTENSION AND PHASE REFINEMENT .6. PERP(PHASE EXTENSION AND REFINEMENT PROGRAM)

Several techniques for extending and refining phases for macromolecula r structures have been incorporated into a program package PERP. In ad dition to previously employed techniques such as solvent flattening an d histogram matching, PERP includes a new way of applying the Sayre eq uation [Refaat, Tate & Woolfson (1995). Acta Cryst. D51, 1036-1040], l ow-density elimination [Shiono & Woolfson (1992). Acta Cryst. A48, 451 -456] and two double-histogram methods [Refaat, Tate & Woolfson (1996) . Acta Cryst. D52, 252-256]. PERP is an easy-to-use package controlled by keywords and provided with default parameters that usually give ne ar-optimum results. Examples are given of refinement, and also extensi on and refinement, for six known protein structures with a variety of characteristics. In each case PERP gives a very satisfactory outcome a s measured by improvements in the mean-phase-error and conventional ma p-correlation coefficient. The main conclusion is that the several met hods used in sequence give more effective extension and refinement tha n using any single method alone.