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, ribonuclease (RNAP1), containing 808 non-H atoms, including five S atoms, plus 83 ordered solvent water molecules. Phase sets with mean phase errors between 69 and 75 degrees were selected by modified figures of merit for trials with the full data at 1.17 A resolution and also with restricted data at 1.25 and 1.5 A resolution. These figures of merit had previously only been applied to protein structures containing heavy atoms, and this is the first demonstration of their usefulness with no heavy atom present. An initial set of 1091 phases from a 1.17 A 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.