Variable wavelength anomalous dispersion methods and applications

Variable wavelength anomalous dispersion methods and applications 

Anomalous dispersion effects have been used for a considerable time, to some extent, in crystal structure analyses. In combination with a tunable synchrotron X-ray source there is considerable scope for a dramatically increased use and benefit of these small but significant effects. 

The scope for utilising anomalous dispersion is considerable in conjunction with the synchrotron. SR has a high intensity over the X-ray wavelength range of interest and can access the absorption edges of (a) commonly occurring metals in metallo-proteins, (b) metals which are used in heavy atom derivatives, (c) selenium in specially grown selenoproteins (see section 9.8) and (d) bromine in brominated nucleotides. 

Another application of anomalous dispersion and SR is the case of a protein containing a cofactor of two metal atoms of similar atomic number. In such a case the native electron density map would not clearly indicate which metal atom is which. However, the anomalous dispersion / values could be made markedly different by suitable choice 

Unlike the normal atomic scattering factor, f0, the values of/ and/ do not decrease with scattering angle because the radius of the inner absorbing electron shell is much less than the wavelength of the radiation used (table 9.1). This actually aids the signal to noise of the anomalous effects at high resolution (equation (2.22)). 

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Variable wavelength anomalous dispersion methods and applications 339... 

Variable wavelength anomalous dispersion methods and applications Combining equations (9.6) and (9.8)... 

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