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Application of the sparse density principle: a statistical evaluation of the phase ambiguity in the single-wavelength anomalous scattering method (SAS)

The method of single-wavelength anomalous scattering (SAS) allows the determination of structure-factor phases apart from a two-fold phase ambiguity. A statistical procedure is described, based on the joint probability distribution of three structure-factors, in which a priori the influence of partial structural information is introduced (Giacovazzo, C., Acta Crystallogr. A39 (1983) 685-692), which permits to calculate the relative probabilities of the two SAS solutions for each phase. The relation of this procedure with the sparse density principle (i.e. the assumption that ρ(r)ρ(-r) ≃ 0) is indicated. A possible application is the use of extra structural information, e. g. information on atomic positions in a protein backbone, in the course of a protein crystal structure determination