H,N 



HN 



X 



THE PROTEINS 



(urea) and NH(CH 3 )CH 2 COOH (methyl glycine). 



83 



This latter substance is known as sarcosine and is derived from glycine by 

 the replacement of one atom of hydrogen by a methyl group CH 3 . 



Arginine has a similar formula. On the left-hand side of the dotted line 

 the formula would be identical with that of creatine. On the right-hand 

 side the sarcosine group is replaced by a diamino-acid of the fatty series, 

 diamino-valerianic acid or orni thine. 



DIAMINO-TRIOXYDODECOIC acid is, as its name implies, a derivative of 

 a twelve carbon acid. Its constitutional formula has not yet been made out. 



B. AMINO-ACIDS CONTAINING AN AROMATIC NUCLEUS 



The best known of these is TYROSINE, which has the formula 



OH 

 /\ 



CH 2 CH.NH 2 COOH 



It is paraoxyphenyl ct-alanine. It is one of the first of the amino-acids to be 



split off from the protein molecule under the influence of hydrolytic agents. 



Owing to its insolubility it 



rapidly separates out as 



bundles of fine needle-shaped 



crystals at the sides and 



bottom of the vessel. 



When tyrosine is treated 

 with an acid solution of 

 mercuric nitrate containing a 

 little nitrous acid, a precipi- 

 tate is produced, and on 

 boiling, the precipitate and 

 the supernatant fluid assume 

 a deep red colour. This re- 

 action is given by all benzene 

 derivatives in which one atom 

 of hydrogen in the ring is re- 

 placed by one OH group. This 

 is known as Hoffmann's test, 

 but is identical with Millon's reaction, which is given by all proteins con- 

 taining tyrosine in their molecules. 



Closely allied to the foregoing compound is another aromatic amino-acid 

 namely, PHENYL u-ALANINE ; 



FIG. 18. Tyrosine crystals. (PLIMMER.) 



