78 PHYSIOLOGY 



It will be noticed that in the fatty acids with more than two atoms of carbon 

 the position of the NH 2 group may be varied. Thus, 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 ^-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 , 

 fi, y, S, e - and so on. Practically all the amino-acids which occur as 

 constituents of the protoplasmic molecule belong to the 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 



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 by synthesis are inactive, and special means 

 have to be devised in order to obtain from the artificially formed racemic 

 amino-acid either the d- or Z-amino-acid. 



If more than one hydrogen atom in an organic acid be replaced by NH 2 



