New techniques for applying anomalous-scattering and isomorphous-replacement data incorporated in ANOMIR - a general...

A computer package ANOMIR is described which can derive phases from anomalous scattering and/or isomorphous-replacement data in any combination. For anomalous scattering it incorporates five methods of applying one-wavelength data and three methods for multiple-wavelength data including SPIN, reported here for the first time. In addition there are three procedures for multiple-wavelength data - the first modifying data for different wavelengths to make them mutually consistent, the second estimating the contributions of the anomalous scatterers alone and the third which finds anomalous differences. For single isomorphous replacement or one-wavelength anomalous scattering the phase ambiguity can be resolved by the direct method [Fan, Han, Qian & Yao (1984). Acta Cryst. A40, 489- 495] but for multiple isomorphous replacement the main method is an adaptation of the probability-curve method [Blow & Crick (1959). Acta Cryst. 12, 794-802]. A new statistical method is described for estimating the standard error in measuring magnitudes which is independent of having subsets of centric reflections. A method is described whereby the weights associated with phase estimates are used to generate probability curves, through which it is possible to combine estimates from different methods and to produce a 'best phase' and figure-of-merit for every reflection. ANOMIR procedures are also available for handling combinations of one-wavelength anomalous scattering with single- or multiple-isomorphous replacement. A final process, which is always beneficial, is a single parallel application of the tangent formula. The ANOMIR package has been designed for easy use and is controlled throughout by KEYWORDS. Results for several structures are given and compared with those found from the MLPHARE program in the CCP4 package.