The application of one-wavelength anomalous scattering. II. An analytical approach for phase determination

An analytical method has been developed by which phase estimates may be uniquely determined from one-wavelength anomalous-scattering data; the method as described can be applied to structures containing one type of anomalous scatterer. The method has been tested on two structures. The first is an Hg derivative of a small protein, avian pancreatic polypeptide (App), crystallizing in space group C2 with one molecule of 36 amino-acid residues in the asymmetric unit. The second is a Pt derivative of ribonuclease Sa (RNA), crystallizing in space group P2₁2₁2₁ with two molecules of 96 amino-acid residues in the asymmetric unit. The phases for App give an electron density map which can easily be interpreted in terms of a model. For RNA the map is less clear but has strong similarities with the true map and could probably be interpreted. If anomalous scatterers are centrosymmetrically arranged then the analysis shows that there is a simple alternative process for calculating a map, called a B_p map, which directly shows a structural image.