Difference structure-factor normalization for heavy-atom or anomalous-scattering substructure determinations

Procedures are described for normalizing structure-factor difference magnitudes, |Δ|F||_SIR = ||F_Der| − |F_Nat|| ≤ |F_Heavy| or |Δ|F||_SAS = ||F_+h| − |F_−h|| ≤ 2|F′′|, to prepare data for probabilistic direct methods phasing to determine heavy-atom or anomalous-scattering substructures in SIR (single-derivative isomorphous replacement) or SAS (single-wavelength anomalous scattering) cases. Applications of the procedures in several recent determinations of multi-selenium substructures in selenomethionyl proteins via SnB direct-methods phasing are briefly summarized.