418 



THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



acetic acid (CH. ( ) 2 CH . CH. 2 (CH . NHo)COOH. It crystallises in spheroidal clumps 

 of crystals, as shown on the left-hand side of fig. 318. 



FIG. 313. Crystals of leucine and tyrosine. x 216. 



All the five amino-acids mentioned (glycine, alanine, serine, valine, 

 and leucine) are found among the final products of most proteins. 



A second group of amino-acids is obtained from fatty acids, 

 which contain two carboxyl (COOH) groups in their molecules. 

 The most important of the amino-derivatives obtained from these 

 dicarboxylic acids are : 



Amino-succinamic acid (asparagine), 

 Amino-succinic acid (aspartic acid), 

 Amino-pyrotartaric acid (glutamic acid). 



The third group of amino-acids is a very important one ; these 

 are termed the aromatic amino-acids ; that is, amino-acids united to 

 the benzene ring, and of these we will mention three, namely, phenyl- 

 alanine, tyrosine, and a nearly related substance called tryptophane. 



Phenyl-alanine is alanine or amino-propionic acid in which an 

 atom of hydrogen is replaced by phenyl (C 6 H 5 ). 



Propionic acid has the formula C 2 H 5 . COOH. 



Alanine (amino-propionic acid) is C 2 H 4 NH 2 . COOH. 



Phenyl-alanine is C 2 H 3 . C 6 H 5 . NH 2 . COOH. 



The formula of phenyl-alanine may also be written another way. 

 The graphic formula of benzene (C 6 H 6 ) is : 



H 



H C 



H C 



C H 

 C H 



