Multiple wavelength Anomalous Diffraction.



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Multiple wavelength Anomalous Diffraction.

 

The method of Multiple wavelength Anomalous Diffraction (MAD) is most applicable to problems where there are no available separate native protein diffraction data, e.g. for metallo-proteins where a heavy atom is already bound in the native structure, or to cases where derivative crystals are non-isomorphous and are therefore unsuitable for phasing via isomorphous replacement.

Changes in the heavy atom structure factor are induced by tuning the X-ray energy (wavelength) to different points in the absorption spectrum of the heavy atom, thus changing the values of the anomalous scattering factors and which in turn effect the magnitudes and . As with the MIR and SIRAS phasing methods a minimum of three different measurements of the overall structure factor are necessary where the values of and are different for each. In MAD this may be achieved by the measurement of diffraction data at at least two X-ray energies providing the minimum requirement of a Bijvoet pair measurement at one of the energies and a single Bijvoet mate at the other. Given that we are now dealing only with `derivative' structure factors we shall alter our notation and denote the Bijvoet pair structure factors for the protein plus heavy atom structure measured at the th X-ray energy by and .

For a structure containing protein atoms and heavy atoms all of the same type we can write

 

The only X-ray energy dependent terms in Eq. gif are and . Fig. gif illustrates how changes in the anomalous scattering factors of the heavy atoms can in theory lead to a solution to the phase problem.

  
Figure: Harker construction for the MAD case. Only structure factors for one X-ray energy are labelled but phase circles have been drawn for four observations indicated by the vectors , , and . The observed structure factor amplitudes arise from the measurement of Bijvoet pairs at each of two X-ray energies. The correct solution is given by the point . The changes in the anomalous scattering factors have the effect of offsetting the centres of the phase circles. The vector OC would represent the phase of the native structure without the heavy atoms present.

The magnitudes of the and components in Fig. gif are greatly exaggerated with respect to and in reality have magnitudes which are only a few percent of . Hendrickson et al. [54] have shown that estimated anomalous diffraction ratios (the ratio between the rms anomalous difference and the rms structure factor magnitude) may be calculated for a particular type and number of anomalous scatterer present in a protein containing non-hydrogen atoms. The Bijvoet difference ratio is given by

 

and the dispersive ratio by

 

where an estimate of may be obtained using

 

There are two main approaches to phasing via multiple wavelength methods. The first is an extension of the MIR approach discussed earlier and the second is an algebraic approach which has been developed more recently by Karle [64] and Hendrickson [48] [54].





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Gwyndaf Evans
Fri Oct 7 15:42:16 MET 1994