International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F, ch. 14.1, p. 295   | 1 | 2 |

Section 14.1.6. Anomalous scattering

B. W. Matthewsa*

aInstitute of Molecular Biology, Howard Hughes Medical Institute and Department of Physics, University of Oregon, Eugene, OR 97403, USA
Correspondence e-mail: brian@uoxray.uoregon.edu

14.1.6. Anomalous scattering

| top | pdf |

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

[{\bf f} = f_{0} + \Delta f' + if'' = f' + if'', \eqno(14.1.6.1)] where [f_{0}] is the normal scattering factor far from an absorption edge, and Δf ′ and f ″ are the correction terms which arise due to dispersion effects. The quantity Δf ′, in phase with [f_{0}], is usually negative, and f ″, the imaginary part, is always [\pi/2] ahead of the phase of the real part [(\;f_{0} + \Delta f')]. It may be noted that by using different wavelengths, the term Δf ′ 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[link]). This is the basis of the multiwavelength-anomalous-dispersion (MAD) method (Hendrickson, 1991[link]) discussed in Chapter 14.2[link] . 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 Kα radiation, [2f''/f'] 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[link]; North, 1965[link]).

References

Blow, D. M. (1958). The structure of haemoglobin. VII. Determination of phase angles in the non-centrosymmetric [100] zone. Proc. R. Soc. London Ser. A, 247, 302–336.Google Scholar
Hendrickson, W. A. (1991). Determination of macromolecular structures from anomalous diffraction of synchrotron radiation. Science, 254, 51–58.Google Scholar
North, A. C. T. (1965). The combination of isomorphous replacement and anomalous scattering data in phase determination of non-centrosymmetric reflexions. Acta Cryst. 18, 212–216.Google Scholar
Ramaseshan, S. (1964). The use of anomalous scattering in crystal structure analysis. In Advanced methods of crystallography, edited by G. N. Ramachandran, pp. 67–95. London: Academic Press.Google Scholar








































to end of page
to top of page