Progress on the direct-methods solution of macromolecular structures using single-wavelength anomalous-dispersion (SAS) data

In the past few years, a number of strategies have been outlined to resolve the SAS phase ambiguity given that unique estimates (h, k) of the triple invariants are available. A new least-squares method is described that can in principle resolve the phase ambiguity to determine macromolecular phases provided that (h, k) estimates are unbiased. Limi­tations of the method in practical applications are discussed. An example is given where the correct solution can be identified by use of the SAS tangent formula in the instance that traditional SAS phasing methods have lead to an incorrect heavy-atom sub­structure.