Evasion and Correction of Racemization Tendencies

To secure constancy of optical purity, and to evade for as long as possible the ultimately inevitable racemization (which Kuhn tentatively envisages as part, at least, of the process of biological aging), nature appears to employ several selective mechanisms. 

In the first place, in an enzymic catalysis, the ratio of the reaction velocities leading to the two antipodal forms of the new dissymmetric molecule is very high, often of the order of 1000 1 or more. In nonenzymic asymmetric catalysis, on the other hand—for example, in the work of Bredig and Fiske— the ratio is very much less, generally of the order of only 2 1. Correspondingly the equilibrium constant is greater, and the optical stability of the product higher. 

Nevertheless, if the final racemic end point is to be avoided for very long, this initial evasive mechanism requires supplementation by a corrective mechanism. Krebs (63) and Kisch (60) have confirmed that such a corrective mechanism exists whereby the initial preponderance of one configurational series could be maintained by selective destruction of the unwanted series. They found that oxidative deamination of a-amino acids by a deaminase present in animal liver and kidney tissues is markedly optically selective, the Tinnatirral d-series being deaminated more rapidly than the natural i-series. This process will continuously correct and regulate the inevitable tendency for at least some of the unwanted antipode to appear in a true catalytic asymmetric synthesis. 

Another such corrective process has been discussed by Langenbeck and Triem (71). They showed theoretically and confirmed experimentally that, if two optically impure substances, A and B, undei o a process of combination which is interrupted before completion, the four possible dia-stereoisomerides are not found in equivalent amounts. If, to quote an example, [(—)-A] [(-l-)-A] and [(—)-B] [(- -)-B] in the original mixture, then on interruption of the reaction before completion we will have  

In other words, the optical purity of the transformed material is increased, though that of the residual untransformed material is correspondingly decreased. 

---