# Multiwavelength anomalous diffraction (MAD)

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**.

### Discussion

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.

### History

This technique was introduced by W. Hendrickson (Hendrickson, W. A., 1991, *Determination of macromolecular structures from anomalous diffraction of synchrotron radiation*. * Science*,

**254**, 51–58.)

### See also

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