The ab initio crystal structure solution of proteins by direct methods. III. The phase extension process

In two preceding papers [Giacovazzo, Siliqi and Ralph (1994). Acta Cryst. A50, 503-510; Giacovazzo, Siliqi and Spagna (1994). Acta Cryst. A50, 609-621], a direct-phasing process was described which proved to be potentially able to solve ab initio crystal structures of proteins. The method uses the diffraction data of the native and of one isomorphous derivative. The main limitation of the approach was the small number of phased reflections rather than the quality of the assigned phases. In this paper, it is shown that the phasing process can be extended to about 40% of the measured reflections (up to the derivative resolution) without reducing significantly the quality of the new phases. Of the four test proteins examined, in one case it was possible to obtain fully interpretable electron-density maps.