Abstract

Both of the conventional 'double-difference' or 'residual' syntheses used in protein crystallography to detect errors in refined heavy-atom parameters are approximations to the desired vector difference synthesis between the observed and calculated heavy-atom structure factors. By using the measured anomalous differences between Friedel-related pairs of reflexions from the parent-plus-heavy-atom crystal, it is possible to calculate the coefficients for the vector difference synthesis. Tests indicate that this synthesis can provide a better signal-to-noise ratio than the conventional syntheses. In the Appendix it is shown that PH parent-plus-heavy-atom structure amplitude in the absence of anomalous scattering, is properly calculated as the mean of the structure amplitudes of the Friedel-related reflexions, not the root mean square.