THE PROTEINS 33 



C 6 H 9 N 3 2 is probably also a diamino-acid, but its exact constitution 

 has not yet been made out with certainty. 



These substances we have hitherto described as acids, but they 

 may also play the part of bases, the introduction of a second amino- 

 group into the fatty-acid molecules conferring upon them basic pro- 

 perties. The three substances 



Lysine (C 6 H 14 N 2 O 2 ) 

 Arginine (C 6 H 14 N 4 2 ) 

 Histidine (C 6 H 9 N 3 2 ) 



are in fact often called the hexone bases because each of them contains 

 six atoms of carbon, as the above empirical formulae show. 



But there is still an important group of amino-acids to be con- 

 sidered, and these are termed the aromatic amino-acids : that is, amino- 

 acids united to the benzene ring; and of these we shall mention 

 three : namely, phenyl-alanine, tyrosine, and a nearly related sub- 

 stance 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 H4.NH 2 .COOH. 

 Phenyl-alanine is C 2 H 3 .C 6 H 5 .NH2.COOH. 



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

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



H 



A 



^ \ 



C C H 



H 



T 



H 



If the H placed lowermost in the above formula is replaced by 

 CH 2 .CH.NH 2 .COOH we obtain the formula of phenyl-alanine : 



CH 2 .CHNH 2 COOH 



the remainder of the benzene ring which is unaltered being repre- 

 sented, as usual, by a simple hexagon. 



D 



