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Figure 2
The conventional Harker construction for SAD phasing. The total structure factors for F+ and F* (the complex conjugate of F) are sums of complex numbers (which can be represented as vectors) with common components. In the Harker construction we represent F+ as the vector sum of the imaginary contribution from the anomalous scatterers (FH+′′), the real contribution from the anomalous scatterers (FH + FH′) and the unknown contribution from the rest of the protein (FP, represented with two possibilities in solid and dashed arrows). Since the amplitude of the total structure factor, |Fo+|, is known, the FP vector must end up on the blue circle, which is centred on the tail of the FH+′′ vector and has a radius of |Fo+|. Similarly, F* is represented as a vector sum, starting with its imaginary contribution from the anomalous scatterers (FH′′) and then sharing the remaining real scattering components. The red circle, which is centred on the tail of the FH′′ vector and with a radius of |Fo+|, crosses the blue circle at the two possible values for FP; the shorter of the two possible vectors is more probable. If the structure factor will be used for a map containing the anomalous scatterers, the origin of the Harker construction is taken at the base of the vector for the real contribution from the anomalous scatterers, indicated by a cross.

Journal logoBIOLOGICAL
CRYSTALLOGRAPHY
ISSN: 1399-0047
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