Bijvoet anomalous scattering, heavy atoms

When anomalous scattering from Bijvoet pairs of a heavy metal atom derivative of a protein occurs, equation (6.5) now needs to be written ... 

But absolute configurations can be obtained from an analysis of small differences in diffraction intensities by a method developed by J.M. Bijvoet. The method makes use of extra phase shifts that occur when the frequency of the X-rays approaches an absorption frequency of atoms in the compound. The phase shifts are called anomalous scattering and result in different intensities in the diffraction patterns of different enantiomers. See Section 2.3.7(b) of the 7th edition of this text for an explanation of the origin of this anomalous phase shift. The incorporation of heavy atoms into the compound makes the observation of the extra phase shift easier to observe, but with very seasitive modern diffractometers this is no longer strictly neces.sary. 

It is well known that the AC of a chiral compound was first determined in 1951 by J. M. Bijvoet and co-workers using the anomalous scattering effect of heavy atoms in X-ray crystallography. The outline of the Bijvoet method is as follows. 

The AC determination by the Bijvoet method is exemplified in the case of cyclophostin (—)-4, an acetylcholinesterase inhibitor isolated from Streptomyces lavendulae, as shown in Figure 55.3 and Table 55.1. Compound 4 has a phosphorous atom, which was useful as a heavy atom to generate enough of an anomalous scattering effect, where the Cu-Ka X-ray was employed. The final R-value for the (3aR,6S) configuration is smaller than that of the mirror image structure, and therefore, the AC of (—)-4 was determined as shown. 

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