152 THE CARBOHYDRATES. [cH. V. 



/-adrenaline : ^-asparagine has a sweet taste, whilst /-aspara- 

 gine is insipid : /-nicotine is far more poisonous than 

 ^-nicotine. Further, if d/-phenyl-aminoacetic acid be 

 administered to an animal, only the /- variety is found in 

 the urine, the body having the power to destroy the 

 d-acid. Many other similar instances have been described. 



Proteins, like casein, can be racemized by treatment 

 with dilute alkalies. Such proteins are not attacked by 

 the pro teoly tic enzymes, and when administered to dogs 

 can be recovered quantitatively from the faeces. The 

 relationship between configuration and enzyme action 

 is discussed on p. 1 84. 



In a few cases substances having two, four or six 

 asymmetric carbon atoms are optically inactive, and can- 

 not be resolved into two components. The optical 

 inactivity is due to internal compensation, the molecule 

 possessing a plane of symmetry. The simplest example 

 of this is that of mesotartaric acid. We can represent the 

 formulae of the tartaric acids in one plane as follows : 



COOH COOH COOH 



I ! I 



HO C H H C OH H C OH 



HO C H H C OH 



I I I 



COOH COOH COOH 



A. B. C. 



d-Tartaric Acid. /-Tartaric Acid. Mesotartaric Acid. 



The dotted line in the structural formula of Mesotartaric 

 Acid indicates the plane of symmetry. A can be regarded 

 as the mirror image of B. Admixture of these in equal 

 parts will be inactive through external compensation. If 

 the upper carbon atom of C be regarded as dextro-rotatory, 

 then the lower one can be regarded as its mirror image 

 and will therefore be laevorotatory. The whole molecule 

 will therefore be optically inactive, and the compound is 

 incapable of being resolved into two constituents. 



