What happens when the signs of anomalous differences or the handedness of substructure are inverted?

Wang, J.; Wlodawer, A.; Dauter, Z.

doi:10.1107/S0907444907025620

Figure 2
An illustration of SAD phasing. All vectors are colored as in Fig. 1

. (a) If only single-wavelength data are available and the anomalous substructure is located, there are two possible solutions for the structure-factor phase. Since it is not known a priori which alternative is correct, the average phase φ^o_SAD is used with the average amplitude (weighted by the figure of merit, which corresponds to the cosine of the half-angle between the two possible total blue vectors). The average (black) vector F_SAD is always parallel to the purple vectors representing the anomalous scattering contribution of the substructure. (b) If the handedness of the substructure is inverted through the center of symmetry, the phase of the substructure is reversed, φ_A^c = −φ_A^o, which results in different alternative solutions of the SAD phases; in effect, the average SAD phase changes, φ^c_SAD = π − φ^o_SAD. (c) If only the signs of the anomalous differences are inverted, but the substructure has the correct handedness, the red/purple vectors are oriented in the same direction as originally, but the amplitudes of F_T⁺ and F_T⁻ (the radii of the blue circles) are exchanged, resulting in the SAD phase φⁱ_SAD = π + φ^o_SAD. (d) If both the handedness of the substructure and the signs of the anomalous differences are wrong, the average SAD phase is negative with respect to the correct value, φ^ic_SAD = −φ^o_SAD. (e) If the substructure is centrosymmetric, its normal structure factor does not have an imaginary component and the red vectors are horizontal. Two alternative SAD solutions lie symmetrically on both sides of the imaginary axis of the Argand diagram and the average vector is vertical with φ^o_SAD = ±π/2. Inversion of the substructure by the center has no effect (since it is already centrosymmetric), but exchange of the signs of anomalous differences changes the average phase to the negative value, φⁱ_SAD = −φ^o_SAD, therefore inverting the handedness of all map features.

BIOLOGICAL
CRYSTALLOGRAPHY

ISSN: 1399-0047

Volume 63| Part 7| June 2007| Pages 751-758

doi:10.1107/S0907444907025620

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