Combining direct methods with isomorphous replacement or anomalous scattering data. IV. Test in the SIR case with the replacing atoms in the centrosymmetric arrangement

Abstract

The method described in the preceding papers has been applied to the single isomorphous replacement (SIR) case with the replacing atoms in a centrosymmetric arrangement. Two kinds of phase ambiguities simultaneously occurred in this example. One is inherent in the SIR method and was resolved by calculating the probabilities P+(ΔϕH). The other comes from the special arrangement of the replacing atoms and was treated by a multi-solution procedure with random starting sign sets. A new figure of merit was used to predict the quality of the solutions. The method has been verified using a set of error-free data from the model structures of a protein and its heavy- atom derivative.