54 ORIGIN AND MAINTEN. OF OPT. ACTIVITY 



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LEVEL OF PROTOPLASMIC STABILITY 



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LEVEL OF THERMODYNAMIC STABILITY 



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Fig. 4. Diagram representing the processes involved in the origin and 

 maintenance of optical purity in living matter. The arrows A represent the 

 Langenbeck and Triem mechanism; the upward arrows B, the Krebs mecha- 

 nism; the downward arrows C, the Kuhn mechanism; the horizontal arrow 

 3 illustrates Curie 's view according to which asymmetry is simply transmitted 

 from one asymmetric molecule to another. 



selection. So the racemic state is stable in inorganic 

 nature and unstable in living matter. 



For living systems to pass from the level of thermo- 

 dynamic to that of protoplasmic stability and to stay at 

 the latter level requires a series of mechanisms of which 

 two have been described : the Langenbeck and Triem 

 mechanism by which the optical purity is increased in a 

 series of interrupted reactions (upward arrows in plain 

 lines, A^ in the diagram) and the Krebs mechanism by 

 which the unnatural optical isomers are removed (upward 

 arrows in dotted lines, B). 



Since the racemic state represents a state of thermo- 

 dynamic equilibrium, the initial asymmetry will have a 

 tendency to disappear (Kuhn's mechanism, downward 

 arrows, C, in the diagram). 



The eifect of circularly polarized light in inducing some 

 asymmetry will probably not be sufficient to maintain the 

 high degree of optical purity exhibited by protoplasm. 



It is possible also that the Langenbeck and Triem 

 mechanism, which probably is not efficient enough to main- 

 tain the almost absolute purity of protoplasm as we know 



