InternationalCrystallography of biological macromoleculesTables for Crystallography Volume F Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F, ch. 14.1, p. 295
## Section 14.1.6. Anomalous scattering |

All atoms, particularly those used in preparing heavy-atom isomorphs, give rise to anomalous scattering, especially if the energy of the scattered X-rays is close to an absorption edge. The atomic scattering factor of the atom in question can be expressed as

where is the normal scattering factor far from an absorption edge, and Δ*f* ′ and *f* ″ are the correction terms which arise due to dispersion effects. The quantity Δ*f* ′, in phase with , is usually negative, and *f* ″, the imaginary part, is always ahead of the phase of the real part . It may be noted that by using different wavelengths, the term Δ*f* ′ is equivalent to a change in scattering power of the heavy atom and produces intensity differences similar to a normal isomorphous replacement, except that in this case the isomorphism is *exact* (Ramaseshan, 1964). This is the basis of the multiwavelength-anomalous-dispersion (MAD) method (Hendrickson, 1991) discussed in Chapter 14.2
. Here we focus on measurements based on a single wavelength, traditionally referred to as the `anomalous-scattering method'.

The anomalous scattering of a heavy atom is always considerably less than the normal scattering (for Cu *K*α radiation, ranges from about 0.24 to 0.36), but there are several factors which tend to offset this reduction in magnitude (*e.g.* see Blow, 1958; North, 1965).

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