An approach to solving the phase problem in protein structure determination by comparing structure factors collected at different wavelengths, including the absorption edge of a heavy-atom scatterer. Also known as multiple-wavelength anomalous diffraction or multiwavelength anomalous dispersion.
The 'normal' atomic scattering factor \(f^0\) describes the strength of X-rays scattered from the electrons in an atom assuming that they are free oscillators. Because the scattering electrons are in fact bound in atomic orbitals, they act instead as a set of damped oscillators with resonant frequencies matched to the absorption frequencies of the electron shells. The total atomic scattering factor f is then a complex number, and is represented by the sum of the
\[f = f^0 + f' + if''.\]
A consequence of the
The technique, often using tunable synchrotron radiation, is particularly well suited to proteins where methionine residues can be readily replaced by selenomethionine derivatives; selenium has a sufficiently strong anomalous scattering effect that it allows phasing of a macromolecule.
This technique was introduced by W. Hendrickson (Hendrickson, W. A., 1991, Determination of macromolecular structures from anomalous diffraction of synchrotron radiation.
MAD and MIR. J. L. Smith, W. A. Hendrickson, T. C. Terwilliger and J. Berendzen. International Tables for Crystallography (2006). Vol. F, ch. 14.2, pp. 299-309