Bijvoet difference method

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. 

If two isomorphous derivatives in which the replaceable atoms have considerably different diffracting powers are available, an improved form of this method can be used. Strychnine sulphate pentahydrate and the corresponding selenate form isomorphous monoclinic crystals of space group C2 Bokhoven, Schoone, and Bijvoet (1951) solved the centrosymmetric b projection by the straightforward method first used... 

In 1951, Johannes Martin Bijvoet used such differences in intensity, resulting from anomalous scattering by an atom in a noncentrosymmetric crystal, to determine the chirality (absolute configuration) of the tartrate ion. Details of this method, which has been used extensively for finding the absolute configurations of natural products and for determining macromolecular structures, are given in Chapter 14. 

The experimental method used to determine the chirality or absolute structure of a molecule or crystal structure involves the use of the anomalous dispersion of X-rays by one or more atoms in the structure. We will now describe this effect and how Bijvoet used it to determine the absolute configuration of (-l-)-tartaric acid from the differences in the intensities of the hkl and iM Bragg reflections. 

If the protein has anomalous scatters in its molecule, the difference in intensity between the Bijvoet pairs, Fm/( + ) and Fa /( —), can be used for the phase angle determination. In the MAD method the wavelength... 

---