28 OPTICAL ACTIVITY OF BIOL. MATERIAL 



Substances. As has been said, the primary organic sub- 

 stances are obligatorily asymmetric and the secondary 

 substances are optionally asymmetric. To this character- 

 istic property one should add another which might be called 

 the "replaceability" or "non-replaceability" of a given 

 optic isomer by its antipode. Substances possessing ob- 

 ligatory asymmetry are found in nature in the form of one 

 only of the two optical isomers, whilst the secondary sub- 

 stances are found as well in the dextrorotatory as in the 

 laevorotatory form, often as inactive racemates. We shall 

 describe this property as exclusiveness or non-exclusive- 

 ness of the asymmetry-sign. 



Exclusiveness of the asymmetry-sign in primary sub- 

 stances is a well established fact. In amino-acids, no 

 exception has ever been recorded. Only dextrorotatory 

 alanine, laevorotatory leucine, dextrorotatory valine, 

 laevorotatory histidine, laevorotatory aspartic acid, etc., 

 have been isolated from animal or plant tissues. All 

 apparent exceptions to this rule could be traced to some 

 experimental error as shown by Pringsheim (1910). 



The same holds true for the carbohydrates which pos- 

 sess obligatory asymmetry. Only dextrorotatory glucose, 

 laevorotatory fructose, etc., can be found in living material. 



The isomer which is present in the biological material 

 is often called "natural," whilst its antipode which is pre- 

 pared synthetically is considered unnatural, but it is evi- 

 dent that the term "natural" as a synonym of "biological" 

 is somewhat improper. 



6. N on-Exdusiveness of the Asymmetry-Sign in Sec- 

 ondary Substances. Turning now to the substances in 

 which optical purity is not obligatory, we find that one 

 optical isomer is found in one species of plants and its 

 antipode in another. 



Let us consider first the optionally asymmetric carbohy- 

 drates. Arabinose, which is found in organic nature in the 

 racemic state, can also be present in the form of the rela- 

 tively pure dextrorotatory and of the relatively pure lae- 

 vorotatory isomers. The left form is the most widely 



