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Experience with phase extension and ab initio phase determination in macromolecular crystallography using maximum-entropy methods.

Three procedures, or 'tools', have been developed and tested for applying maximum-entropy methods to phase extension and to ab initio phase determination. The phase expander tool has been used in connection with the solution of two previously unknown macromolecular structures. An efficient algorithm for the determination of an electron-density distribution that is everywhere positive and that agrees with observed structure amplitudes (tools II and III) has been used to determine the phases of X-ray diffraction data from recombinant bovine chymosin, a protein with 323 amino-acid residues in the molecular chain, the structure of which was recently determined using replacement methods. By use of the same maximum-entropy methods, the structure amplitudes from the unknown structure of bovine heart creatine kinase, a protein with 381 amino-acid residues, have been phased ab initio to 2.7 A resolution. The phases of the centric reflections have also been confirmed by a satisfactory solution of the Patterson map of a mercury derivative. The current status of the structure interpretation is presented. This technique has also been applied to a test case where 48 centric reflections from bovine prothrombin fragment 1 data were phased ab initio and subsequently used in the determination of Patterson solutions for a heavy-atom derivative data set.