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

The direct-method program MULTAN88 has been applied to solve a known protein, pseudoazurin, space group P6(5), unit-cell parameters a = b = 50.0 (1), c = 98.5 (3) A, with 917 protein atoms, a Cu atom and 93 solvent water molecules in the asymmetric unit (>6,000 non-H atoms in the unit cell) and data at 1.55 A resolution. One of several trials with sets of initially random phases yielded phase estimates for 1,000 reflections with a mean phase error of 68.3 degrees that was recognized as the best available solution by the figure of merit being used. Phase extension to 2,000 largest Es showed a distorted tetrahedral geometry around the Cu site. Density modification was applied to improve the phases and the quality of the maps, starting with phases calculated from the atomic positions indicated by the first map.