International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 14.1, p. 297
Section 14.1.11. Use of anomalous-scattering data in heavy-atom location
aInstitute of Molecular Biology, Howard Hughes Medical Institute and Department of Physics, University of Oregon, Eugene, OR 97403, USA |
A relation exactly analogous to equation (14.1.10.1) can be used to approximate the anomalous heavy-atom scattering amplitude, namely, (see Fig. 14.1.7.1b). As noted above, if all the heavy atoms are the same, . Thus, a Patterson function with coefficients should also show the desired heavy-atom–heavy-atom vector peaks (Blow, 1957; Rossmann, 1961).
For each individual reflection, however, and as is also the case for phase determination, the information that is provided by the isomorphous-replacement difference is exactly complementary to that provided by the anomalous-scattering measurement . By combining both sets of experimental measurements, it is possible to obtain a much better estimate of the heavy-atom scattering, , for every reflection (Kartha & Parthasarathy, 1965a,b; Matthews, 1966a; Singh & Ramaseshan, 1966). One formulation (Matthews, 1966a) can be written as where and w is a weighting factor (from 0 to 1) that is an estimate of the relative reliability of the measurements of compared with .
References
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