Ab initio structure determination of a small protein, rubredoxin, by direct methods

The direct-methods program SAYTAN has been applied successfully to a known protein, rubredoxin, which contains 52 amino-acid residues including an FeS₄ unit, a sulfate ion and 102 solvent water molecules. Starting with initially random phases, useful sets can be obtained from multiple trials and selected by figures of merit at different resolutions. Phase extension followed by weighted Fourier recycling reveals a recognizable structure of rubredoxin. The model is refined against 1 Å resolution data to an R factor of 14.5% using the program SHELXL93.