Amino acids conglomerate racemates

The geology of the Scripps cliff has been discussed previously by Carter (44) and by Karrow and Emerson (45). The cliff is composed of Pleistocene alluvial and colluvial sediments overlying the Ardath Shale and Scripps Formation which are Eocene in age. There is a marine fossili-ferous terrace deposit at the contact between the Pleistocene sediments and the Eocene conglomerate. Shells obtained from this terrace, 40-50 m north of the area where the cliff collapsed, have been analyzed by amino acid racemization and corresponded in age to a period of high sea level during the last interglacial, i.e., 120,000 years B.P. (46,47). This... 

A racemate can be resolved with ease if it happens that the enantiomers form separate crystals - a so-called conglomerate. It becomes possible to separate, physically, the enantiomorphic crystals - a process sometimes referred to as triage. This is what Pasteur famously achieved in 1848 with a sample of ammonium sodium tartrate [6]. Such good fortune is quite rare and is in any case not a synthetic method in the strictest sense (nor is it practical on a large scale). Other physical processes (alone, without any attendant synthetic process) such as evaporation or sublimation can be used to increase the enantiomeric excess of organic compounds, including amino acids [7]. 

This fixed composition results from the phase rule and is also called a eutectic point. The enantiomeric composition in solution can be higher or lower than initially employed, depending on the eutectic point. In the solid phase it will be the other way roimd, as no new enantiomers are made or consumed. For amino acids in water, the whole range from a low to high eutectic point is foimd threonine is a conglomerate, which is always racemic at the eutectic point, whereas serine exhibits a eutectic point of >99% ee, being basically... 

In contrast to the models discussed above for conglomerates, this time it is the solution phase not the solid that can become enantiomerically eruiched, a possibility that had already been pointed out by Morowitz in 1969 (36). From the values shown in Figure 3, it seems at first that only a few selected amino acids allow for high ee s in solution. But further experiments with amino acids revealed ways to influence the eutectic composition. Valine for example has a medium eutectic of 47% ee in water at 25°C. Addition of fumaric acid, itself achiral, results in a rise of the eutectic to >99% ee (37). As it was shown, fumaric acid forms cocrystals with both racemic and enantiopure valine, strongly decreasing the solubility of racemic valine while not changing that of enantiopure valine much (Figure 4). 

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