Direct methods enantiomorph

The a vaiues are a measure of the electron-density variation in the protein and solvent regions, and the ratio of these numbers is a measure of the contrast between the two regions. Since anomalous dispersion data were used to phase the maps, the map for the correct hand will show greater contrast. In this case, the original direct-methods sites give rise to greater contrast thereby indicating that these sites do correspond to the correct enantiomorph. 

For instance, three-beam diffraction (one transmitted and two diffracted beams) is used for determination of triplet phases (see Direct Methods) and provides also a means of resolving the enantiomorphism problem without the need for anomalous scattering. 

For many years it was not known which of a pair of enantiomorphs rotated the plane of polarized light to the right, and which to the left. The problem was solved in 1951 by a crystal-structure determination of glyceraldehyde, using X-ray methods. It was found in this way that the enantiomorph of glyceraldehyde that rotates the plane of polarized light in a positive, or clockwise, direction has the following structure ... 

The first method is the classical technique of Pasteur, which entails the mechanical separation of enantiomorphic crystals formed simultaneously while the mother liquor remains racemic. Enantiomer separation by this particular method is extremely time-consuming and is really impossible unless the crystals form with well-defined hemihedral faces. Nevertheless, it is often the method of choice to obtain the seed crystals required for other direct crystallization procedures. When a particular system has been shown to be a conglomerate, and the crys-... 

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