Multi-wavelength anomalous dispersion

Multi-wavelength anomalous dispersion (sometimes Multi-wavelength anomalous diffraction; abbreviated MAD) is a technique used in X-ray crystallography that facilitates the determination of the structure of proteins or other biological macromolecules (e.g. DNA) by allowing the solution of the phase problem. This is possible if the structure contains one or more atoms that cause significant anomalous scattering from incoming X-rays at the wavelength used for the diffraction experiment. Atoms in proteins which are suitable for this purpose are sulfur or heavier atoms, for example metal ions in metalloproteins. The most commonly used atom for phase determination via MAD, however, is selenium, since it is usually possible to replace the natural sulfur containing amino acid methionine by selenomethionine. The use of the MAD technique in an experiment utilizing different wavelengths of X-rays generated at a synchrotron relieves the crystallographer from the traditional method of phase determination via Multiple isomorphous replacement (MIR), which involves the preparation of heavy atom derivatives in a trial-and-error approach.

Anomalous Dispersion

 * Multi-wavelength anomalous dispersion (MAD)
 * Single wavelength anomalous dispersion (SAD)

Isomorphous Replacement
Two methods for providing the needed phasing information by introducing heavy atoms into isomorphous crystals:
 * Multiple isomorphous replacement (MIR); and
 * Single isomorphous replacement with anomalous signal (SIRAS)