Diagram of a quasi-racemic macromolecular crystal and a Harker diagram for phasing. (a) A cartoon of a crystal containing both enantiomeric hands of a protein molecule, with anomalous scattering atoms (red) incorporated into only one hand. The dotted region is intended to convey the possibility that conformational differences might exist between the two molecules, which will, in addition to the differences contributed by anomalous scattering atoms, break the center of symmetry in the crystal. The arrangement shown is in space group P1 (pseudo-P). (b) A phase diagram illustrating information from single-wavelength anomalous dispersion (SAD) data. Protein structure-factor phase circles for the plus (blue) and minus (green) reflections of a Friedel pair are offset by vectors related to the anomalous scattering contributions (f′′) that they contain. Black dashed lines indicate the two possible ambiguous phase choices based on SAD data, assuming perfect data. The black dashed–dotted line indicates the SAD `best' phase obtained from a statistical weighting of the possible phases, which is used in a typical SAD analysis. The red vectors indicate choices for the protein phase that take into account the expectation that the protein structure factor for a quasi-racemic crystal should be approximately centric (0° or 180°). The two vectors shown describe distinct strategic choices discussed in the text. The phase diagram shown is simplified by omitting the dispersive (f′) contribution to the total structure factor, which amounts to considering this contribution to be part of the protein structure factor FP.