84 



PHYSIOLOGY 



The constitution of arginine is analogous to that of creatine, one of the 

 most abundant nitrogenous extractives of muscle, which has the formula 

 HN = C | N(CH 3 )CH 2 COOH 



It is methyl guanidine acetic acid. On boiling creatine with baryta water 

 it takes up a molecule of water and splits in the situation of the dotted line 

 in the formula, giving 



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



This later 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 ornithine. 



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 



|C 6 H 4 | 



CH 2 CH.NH 2 COOH 



It is paraoxyphenyl a-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 FIG. 18. Tyrosine crystals. (PLIMMEK.) 



