A direct-method ab initio phasing of a protein, pseudoazurin, at 1.55 angstrom resolution

The direct-method program MULTAN88 has been applied to solve a known protei n, pseudoazurin, space group P6(5), unit-cell parameters a = b = 50.0 (1), c = 98.5 (3) Angstrom, with 917 protein atoms, a Cu atom and 93 solvent wat er molecules in the asymmetric unit (>6000 non-H atoms in the unit cell) an d data at 1.55 Angstrom resolution. One of several trials with sets of init ially random phases yielded phase estimates for 1000 reflections with a mea n phase error of 68.3 degrees that was recognized as the best available sol ution by the figure of merit being used. Phase extension to 2000 largest Es showed a distorted tetrahedral geometry around the Cu site. Density modifi cation was applied to improve the phases and the quality of the maps, start ing with phases calculated from the atomic positions indicated by the first map.