Properties of an electron-density map derived from a limited number of experimentally determined triplet phases

In a previous communication [Weckert et al. (1999). Acta Cryst. D55, 1320-1328], the feasibility of the measurement of a large set of triplet phases by three-beam interference was demonstrated. This paper reports the methodology for the calculation of an electron-density map from this limited amount of experimental phase information and the map's properties with respect to model building and refinement. The tetragonal form of hen egg-white lysozyme (HEWL) was chosen as a test structure for the development of this method. The quality of the electron-density map obtained from all measured triplet phases allows a straightforward and nearly complete interpretation. The starting model was refined to a final R value of 17.4%. In a second step, the minimum number of phased reflections needed for the interpretation of an electron-density map was investigated, applying criteria based on |F| and resolution.