Anisotropy of anomalous scattering.



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Anisotropy of anomalous scattering.

One additional effect which may need to be considered seriously in the future if a very narrow band pass approach is to be used is the Anisotropy of Anomalous Scattering (AAS) from a heavy atom in a protein. AAS occurs as a result of anisotropy of the local environment around the scatterer. The effects of AAS on the structure factor equation have been analysed in two articles by Fanchon and Hendrickson [32] and Kirfel et al [67].

The effect of AAS on the structure of the anomalous scattering factors in the region of absorption edges can be very significant. One example is at the of Pt in crystals of where the ions have square planar coordination. Anomalous scattering factor have been measured [106] with the incident beam polarisation vector parallel and perpendicular to the four-fold axis of the ion. The results indicated that the inflection point of the edge shifts in energy by as much as between the two orientations. The maximum value of at the absorption edge was also seen to change by about . AAS effects have also been observed in the selenobiotinyl complex with core streptavidin [52]. Shifts in the observed position of the Se edge energy of were observed when fluorescence was measured with the X-ray's electric field vector parallel to the crystallographic and axes. The local environment of the selenium atoms in this case is highly anisotropic due to the bonding.

Such marked AAS adds complications to the MAD method. Anomalously scattering heavy atoms with highly anisotropic environments which sit with different orientations within a crystal will tend to contribute to the diffraction pattern with difference anomalous scattering factors at particular energies in the region of their absorption edges. However the observed fluorescence spectrum from such a crystal sample would appear as the average of the two individual spectra due to the differently oriented heavy atom sites. This makes optimised energy selection more difficult.

The measurement of the anomalous scattering factors for a heavy atom bound in a protein are made even more difficult in the case of heavy atom derivatives prepared by soaking. In such cases the level of binding is generally weak and is maintained by an equilibrium between heavy atom in the protein solvent and bound heavy atoms. The presence of heavy atoms in the solvent is inevitable and will therefore contribute to the fluorescence measured from the protein crystal. The local environments of the heavy atoms bound to the protein and those in the solvent will differ in general, meaning that the observed fluorescence will not correspond to that expected from the bound heavy atoms alone. On the other hand, in cases such as in seleno-methionine containing proteins and metallo-proteins, the observed fluorescence should correspond to the bound heavy atoms alone since they are an intrinsic part of the protein and are not present in solvent. Also in the case of Hg binding to cysteinyl S, for example, the covalent binding should be strong enough to allow the Hg containing solvent to be replaced with a Hg free solvent without reducing the number of bound Hg atoms.

Anisotropic effects may be avoided by the use of suitable heavy atom complex ions, i.e. those which are tetrahedrally or octahedrally coordinated to their ligands; one such example is the ion used to prepare the lysozyme derivative used in this work. If isotropic metal ion complexes can be used the problem of ions bound to the protein and ions in the solvent having different fluorescence spectra is minimised. However it should be noted that there is a possibility that the form of the fluorescence spectrum changes on introduction to a protein crystal. Some evidence for this has been observed during this work where the maximum measured value at the white line of the iridium edge in changed from e to e on introduction to the protein environment.



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