Ab initio structure determination using dispersive differences from multiple-wavelength synchrotron-radiation powder diffraction data

The purpose of this paper and a test case study is to assess a method of ab initio structure solution from powder diffraction data using f' difference techniques. A theoretical foundation for the approach used is first provided. Then, with a test case ( nickel sulfate hexahydrate), it is shown that both the position of the anomalous scatterer (Ni) can be determined and the structure can be developed in full. Specifically, synchrotron-radiation data were collected at two wavelengths close to the K edge for Ni and three wavelengths remote from the Ni absorption edge, at 1.3, 1.8 and 2.16 angstrom. These five wavelengths then allowed various combinations to be tried to establish which wavelength pairs gave the optimum signal in the Patterson maps using dispersive amplitude differences. The initial phases derived from the metal-atom position then allowed the structure to be fully developed by difference Fourier cycling. The relevance of these developments to structure-solution possibilities for proteins via powder dispersive difference data is then outlined.