Ab initio phase determination for spherical viruses: parameter determination for spherical-shell models

The structure determination of canine parvovirus depended on the extension of phases calculated initially from a spherical-shell model [Tsao, Chapman, Wu, Agbandje, Keller & Rossmann (1992). Acta Cryst. B48, 75-88]. Such ab initio phasing holds the promise of obviating initial experimental phasing by isomorphous or molecular replacement, thereby expediting the structure determinations of spherical virus capsids. In this paper, it is shown how parameters such as radii, DNA density and particle positions may be determined and refined from diffraction data with sufficient precision to start phase extension from 20 Å resolution for a virus of approximately 122 Å radius.