Multiple-wavelength anomalous scatterers

Currently, use of selenium methionine, recombinant protein crystals, or crystals of wild type protein into which have been introduced heavy atom anomalous scatterers, has made single and multiple wavelength anomalous scattering the method of choice for phase determination. In addition, the ways it can be used, and the increasing opportunities for its application are dramatically expanding its popularity. 

The more automatic solution of crystal structures using variable or multiple wavelength anomalous scattering measurements is an important development especially when conventional, isomorphous, heavy atom derivatives cannot be made. The phasing power of the anomalous scattering component of specific atoms can be fully optimised with SR. 

A teclmique that employs principles similar to those of isomorphous replacement is multiple-wavelength anomalous diffraction (MAD) [27]. The expression for the atomic scattering factor in equation (B1.8.2h) is strictly accurate only if the x-ray wavelength is well away from any characteristic absorption edge of the element, in which case the atomic scattering factor is real and Filiki) = Fthkl V- Since the diffracted... 

The multiple-wavelength anomalous diffraction method (MAD) relies upon sufficiently strong anomalously scattering atoms in the protein structure itself In this method, diffraction data must be collected at a number of different wavelengths, usually requiring data collection with... 

Alternative methods of solving the phase problem are also used now. When a transition metal such as Fe, Co, or Ni is present in the protein, anomolous scattering of X-rays at several wavelengths (from synchrotron radiation) can be used to obtain phases. Many protein structures have been obtained using this multiple wavelength anomalous diffraction (MAD phasing) method.404 407 408 Selenocysteine is often incorporated into a protein that may be produced in... 

SOLVE/RESOLVE is a program system that permits automation of all the steps between processed data and interpretation of phased maps. These include scaling of data measured at multiple wavelengths, location of anomalous scatterers. 

The anomalous components of the total scattering are wavelength dependent and the use of radiation close to an absorption edge may increase or optimise the contribution due to the anomalously scattering atoms. Ramaseshan (1962) pointed out that data collected at multiple wavelengths optimising the anomalous dispersion effects would improve the quality of phase determination. 

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