APPLICATION OF THE SPARSE DENSITY PRINCIPLE - A STATISTICAL EVALUATION OF THE PHASE AMBIGUITY IN THE SINGLE-WAVELENGTH ANOMALOUS SCATTERINGMETHOD (SAS)

The method of single-wavelength anomalous scattering (SAS) allows the determination of structure-factor phases apart from a two-fold phase a mbiguity. A statistical procedure is described, based on the joint pro bability distribution of three structure-factors, in which a priori th e influence of partial structural information is introduced (Giacovazz o, C., Acta Crystallogr. A39 (1983) 685-692), which permits to calcula te 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 rho(r)rho(-r) almost-equal-to 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.