THE PROTEINS 93 



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 



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

 H 2 N 



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 ornithine. 



DIAMINO-TRIOXYDODECOIC acid is, as its formula 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 -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 solu- 

 tion of mercuric nitrate 

 containing a little nitrous 

 acid, a precipitate is 

 produced, and on boiling, 

 the precipitate and the 

 supernatant fluid assume 

 a deep red colour. This 

 reaction is given by all FIG. 18. Tyrosine crystals, (PUMMER.) 



