Anomalous Scattering in the Determination of the Protein Phase Angles and the Absolute Configuration

Abstract

Anomalous scattering is not a new subject. It was already introduced in Chapter 7. There, you learned that anomalous scattering by an atom is due to the fact that its electrons cannot be regarded as completely free electrons. This effect depends on the wavelength, but it is in general stronger for the heavier atoms than for the light atoms in the upper rows of the periodic system. If heavy atoms are present in a protein structure, the consequence of their anomalous scattering is that the intensities of a reflection h k l and its Bijvoet mate h̄ k̄ l̄ are no longer equal. In Chapter 7 this effect was used in combination with the isomorphous replacement differences in the search for the heavy atom positions and in the refinement of these positions. In this chapter it will be shown how anomalous scattering information can help to determine the phase angle of the protein reflections and the absolute configuration of the protein structure. Moreover, it will be discussed how anomalous scattering is exploited for protein phase angle determination by the multiple wavelength anomalous dispersion (MAD) method.