228 OPTICALLY ACTIVE FERMENTATION PRODUCTS. 



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 same structural formula, and behave similarly from 

 a chemical point of view, but differ in a physical sense (crystalline 

 habit, solubility, behaviour towards polarised light). Such com- 

 pounds, whose different behaviour can only be explained by the 

 assumption of a difference of grouping in the molecule, are desig- 

 nated stereo-isomers. Of the compound Cabcd there may exist 

 one modification capable of deviating polarised light towards the 

 right hand, and a second producing a left-handed rotation; and 

 one part by weight of the dextro-rotatory modification gives rise 

 to just as great a deviation towards the right hand as one 

 part by weight of the levo-modification does towards the left 

 hand. 



If now a compound containing such an asymmetric carbon 

 atom be artificially prepared from substances devoid of asymmetric 

 atoms, the probability is that just as many levo-rotatory as dextro- 

 rotatory molcules will be produced ; and assuming that each mole- 

 cule of the one kind coalesces with a molecule of the other class to 

 form a double molecule, then an optically inactive compound will 

 result. There may therefore be presumably obtained from one 

 combination, Cabcd, containing a single asymmetric carbon atom, 

 three different modifications, two of which (one dextro-, the other 

 levo-rotatory) are optically active and the third inactive. An 

 example of such a compound is afforded by ethylidene lactic acid 



CH 3 H CH 3 



V I 



C = CH.OH 



/\ I 



HO COOH COOH 



of which one optically inactive form is known, and in chemical 

 text-books is generally named fermentation lactic acid. The 

 dextro-rotatory modification has also long been known to chemists 

 under the name of sarco-lactic or para-lactic acid, whilst the third 

 modification, levo-lactic acid, was only discovered and produced 

 a few years ago by fermentation physiologists. This will be again 

 referred to later on. 



