CHAPTER XXlt 



THE PRODUCTION OF OPTICALLY ACTIVE ORGANIC COMPOUNDS 

 BY FERMENTATION. 



137. Isomers of Lactic Acid. 



THE details given in the preceding chapter with regard to the physiological 

 activity of the lactic ferments require supplementing in one important particular. 

 Mention has been made of lactic acid, and always without qualification or 

 reference to the fact that there are several isomeric lactic acids. This question 

 can now be considered from a higher standpoint, and the occasion utilised for 

 dealing with the production of optically active substances in general, through the 

 activity of micro-organisms. 



Stereo-chemistry teaches that all optically active organic substances contain 

 in the molecule at least one asymmetric carbon atom, i.e. one whose four atom- 

 fixing powers or affinities are connected with four different elements, or groups of 

 atoms (radicals). On the other hand, experience shows that there are; compounds 

 (such as racemic acid and mesotartaric acid) which contain one or more asym- 

 metric carbon atoms, but are nevertheless optically inactive. The structural 

 formula of mesotartaric acid is COOH CHOH CHOH COOH. This same 

 formula, however, is also adopted for two other acids, named from their optical 

 properties respectively dextro-tartaric acid and levo-tartaric acid. To explain this 

 fact it is necessary to refer to the hypothesis promulgated in 1874 by VAN 'T 

 HOFF (L), that the four equivalents of a carbon atom are arranged at the angles 

 of a tetrahedron, in the centre of which the carbon atom itself is situated, whereas 

 the atoms or radicals combined therewith occupy the angles. For example, let 

 Cabcd be a compound containing one asymmetric carbon atom C, with which are 

 combined the four (different) atoms or radicals a, b, c, d. By employing the 

 tetrahedron scheme, the arrangement of these latter can be effected in two 

 different ways : 



These two formulae are not identical, but stand in the mutual relation of an 

 image and its reflection. In order to get from b over c to d, it is necessary 

 (looking from a) in the one instance to move in the same direction as the hands 

 of a clock ; but in the other case the movement is reversed. 



The compound Cabcd is thus obtained in two modifications which have the 



i74 



