Facing the phase problem

Hendrickson, W.A.

doi:10.1107/S2052252523006449

Figure 2
Harker phasing diagrams illustrating the phase ambiguity inherent in SIR phasing (a) and SAD phasing (c), and its resolution by two derivatives in MIR (b) or by SIRAS (d). The `native' amplitude circles (heavy lines) each have radius |F_P|. The single isomorphous pair from SIR derivative 1 yields options at Q or R, whereas derivative 2 gives alternatives Q and R′. The anomalous scattering from derivative 1 gives alternatives Q and Q′. Neglecting experimental error, in principle either MIR or SIRAS resolves the phase ambiguity. Adapted from Figs. 4 and 6 of Hendrickson (1981

) with permission from Macmillan.