86 PHYSIOLOGY 



instead of alanine we may have another amino-propionic acid, 

 namely : 



CH 2 NH 2 



CH 2 



COOH 



This acid would be spoken of as /3-amino-propionic acid, alanine 

 being a-amino-propionic acid. This nomenclature is always used to 

 distinguish the position of the NH 2 group, so that we may have mono- 

 amino- acids a , ft, y-> S, e and so on. Practically all the amino- 

 acids which occur as constituents of the protoplasmic molecule belong 

 to the a group. 



On inspection of the formula of glycine it is evident that only one 

 isomer of this body is possible. In alanine, however, the carbon atom 

 to which NH 2 is attached is asymmetric, since its four combining 

 affinities are each attached to different groups. Thus : 



C 

 H C NH 2 



C 



In this case, therefore, there is a possibility of stereoisomerism, and 

 alanine must have an influence on polarised light. If the compound 



CH 3 

 HCNH 2 



COOH 



is dextro-rotatory, then its stereoisomer 



CH 3 



I 

 H 2 NCH 



COOH 



will be laevo-rotatory, and it will be possible to obtain a racemic 

 modification without any influence on polarised light by mixing 

 equal molecules of these two isomeric forms. All the amino- acids 

 derived from proteins are optically active, whereas those obtained 



