On the application of phase relationships to complex structures. XXXVI: some experiments with a small protein without heavy atoms

The direct-methods program MULTAN88 has been applied to a known protein, ri bonuclease (RNAP1), containing 808 non-H atoms, including five S atoms, plu s 83 ordered solvent water molecules. Phase sets with mean phase errors bet ween 69 and 75 degrees were selected by modified figures of merit for trial s with the full data at 1.17 Angstrom resolution and also with restricted d ata at 1.25 and 1.5 Angstrom resolution. These figures of merit had previou sly only been applied to protein structures containing heavy atoms, and thi s is the first demonstration of their usefulness with no heavy atom present . An initial set of 1091 phases from a 1.17 Angstrom trial was developed by an objective procedure to give the full structure with a residual of 0.21, which agrees well with the published structure.