CHAPTER XXII. 



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 sub- 

 stances contain in the molecule at least one asymmetric carbon 

 atom, i.e. one whose four atom-fixing powers or affinities are con- 

 nected 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 asymmetric carbon atoms, but are nevertheless optically inac- 

 tive. 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 (I.), 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 

 , &, c, d. By employing the tetrahedron scheme, the arrangement 

 of these latter can be effected in two different ways : 



a 



