Multiple anomalous scattering

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... 

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. 

Reciprocal lattices and therefore, diffraction patterns are generally centrosymmetric regardless of whether the corresponding direct lattices are centrosymmetric or not Thus, pairs of reflections with the opposite signs of indices, hkl) and hkl) -ihe so-called Friedel pairs - usually have equal intensity. Yet, they may be different in the presence of atoms that scatter anomalously (see section 2.11.4) and this phenomenon should be taken into account when multiplicity factors are evaluated comprehensively. Relevant details associated with the effects of anomalous scattering on the multiplicity factor will be considered below in section 2.12.2. 

Earlier, we considered the effect of lattice symmetry on the multiplicity factors in powder diffraction only in terms of the number of completely overlapping equivalent Bragg peaks (section 2.10.3). Because Friedel s law is violated in non-centrosymmetric structures in the presence of anomalously scattering chemical elements, the evaluation of multiplicity factors should also include Eqs. 2.114 and 2.115. Thus, the multiplicity factor,maybe comprehensively defined as the number of equivalent reflections ihkT), which satisfy Eq. 2.114 in addition to having identical lengths of the... 

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. 

As mentioned at the beginning of this section, anomalous scattering based methods (SAD and MAD) have become very important. This is due to two reasons. First, unlike in the isomorphous replacement methods, the methods do not require multiple crystals, one with and one without a heavy atom. All the measurements are made from one crystal, so there is no problem with isomorphicity,- a crystal is by definition isomorphous with itself. As long as the protein has a 3rd transition row or heavier element bound, MAD and SAD experiments can be performed. Second, modern techniques have made the method extremely easy to use in most cases, it is not even necessary to introduce a heavy atom into the protein after crystallization. Selenium has a K-edge at 12.6578 keV, or 0.9795 A, a very good energy for data collection at a synchrotron and cells can be grown on... 

In calculating the value of R for a particular diffraction line, various factors should be kept in mind. The unit cell volume v is calculated from the measured lattice parameters, which are a function of carbon and alloy content. When the martensite doublets are unresolved, the structure factor and multiplicity of the martensite are calculated on the basis of a body-centered cubic cell this procedure, in effect, adds together the integrated intensities of the two lines of the doublet, which is exactly what is done experimentally when the integrated intensity of an unresolved doublet is measured. For greatest accuracy in the calculation of F, the atomic scattering factor f should be corrected for anomalous scattering by an amount A/ (see Sec. 13-4), particularly when Co Ka radiation is used. The value of the temperature factor can be taken from the curve of Fig. 4-20. 

Protein structures are now more often determined by using multiple anomalous dispersion (MAD) techniques. In this method, scattering that does not obey the kinematic theory is used. The kinematic theory deals with single scatter-... 

Single (or multiple) isomorphous replacement with optimised anomalous scattering (Helliwell 1977a Helliwell et al 1984a) can be... 

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. 

The RMC method is more general than this simple algorithm in that any set(s) of data that can be directly related to the structure can be modelled. It can be applied to isotopic substitution in neutron diffraction, or equivalently to anomalous scattering in X-ray diffraction, to EXAFS (multiple edges) and possibly to NMR data. All data sets can be modelled simultaneously simply by adding the respective y1 values. 

The advent of synchrotron radiation as a highly collimated and tunable source of X-rays has opened new prospects for the application of anomalous scattering. Several different data sets can be collected. In multiple-beam scattering methods, synchrotron radiation also offers advantages. 

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