On Integrating the Techniques of Direct Methods with Anomalous Dispersion. IV. A Simplified Perturbation Treatment for SAS Phasing

Abstract

Results from probabilistic theory for the singlewavelength anomalous-scattering (SAS) Friedel pair, two-phase structure invariants, ~n = ~°H + 9-s, are used to show that the SAS three-phase structure invariants, ~Prm = 9n + 9K + 9-n-~, tend to positive values that are easily estimated. Appropriate averages of the estimates provide SAS perturbation corrections in the form of positive origin shifts for the probability distribution of apHK values and for the tangent formula. The theoretical probabilistic results are verified by empirical statistical analyses of model-calculated phases and experimentally measured structure-factor magnitudes for a small-molecule and a protein crystal structure.

Now we’re cooking: new successes for shake-and-bake

“…The main limitations of shake-and-bake in its current form are the requirement for atomic-resolution data and the large amounts of computer time consumed by the SnB algorithm. Hauptman and coworkers are addressing the first limitation by developing procedures that use less than atomic-resolution data and/or by including anomalous-scattering data [26,27]. Anomalous-scattering data has been successfully used in many crystallographic phasing procedures: Woolfson and coworkers have successfully applied direct methods to single wavelength anomalous-scattering measurements [28,29].…”