Combining direct methods with isomorphous replacement or anomalous scattering data. VII. Ab initio phasing of one-wavelength anomalous scattering data from a small protein

The previously proposed method [Fan, Han & Qian (1984). Acta Cryst. A40, 495-498; Fan & Gu (1985). Acta Cryst. A41, 280-284] has been improved and tested for ab initio phasing of the one-wavelength anomalous scattering data at 2 Å, resolution from a known small protein, avian pancreatic polypeptide. Positions of the anomalous scatterers were found by the direct-method program SAPI and used to normalize the anomalous differences before calculating the phase doublets. 2108 independent reflections and about 790 000 Σ₂ relationships were involved in the phase derivation. The 'best phases' and the figures of merit obtained from the direct method were used to calculate a Fourier map, which revealed the essential feature of the structure.