PROTEINS. 75 



The reaction cannot be obtained in solutions containing free 

 alkali. It is best to use such an amount of bromine as will produce 

 a permanent yellow color in the cold. The use of a less amount of 

 bromine than this produces a weak coloration whereas an excess of 

 bromine prevents the reaction. The test is not very delicate, but 

 a characteristic reaction may always be obtained in 1 : 1000 solu- 

 tions. The only histidine derivative which yields a similar colora- 

 tion is imidazolethylamine, and the reaction in this case is rather 

 weak as compared with the color obtained with histidine or histi- 

 line salts. 



Valine, Cgll^NG^. The amino-valerianic acid obtained from 

 proteins is a-amino-isovalerianic acid, and as such bears the follow- 

 ing formula : 



CH 3 NH 2 



H-C C-COOH. 



OH, H 



-3 



It closely resembles leucine in many of its properties, but is more 

 soluble in water. It is a difficult matter to identify valine in the 

 presence of leucine and isoleucine inasmuch as these amino acids 

 crystallize together in such a way that the combination persists even 

 after repeated recrystallizations. Valine is dextro-rotatory. 



Arginine, C 6 H 14 N 4 O 2 . Arginine is guanidine-a-amino-valeri- 

 anic acid and possesses the following structural formula: 



H H H NH 2 



NH-C-C-C-C-COOH. 



_d i U A 



NHo 



It has been obtained from every protein so far subjected to decom- 

 position. The arginine obtained from proteins is dextro-rotatory, 

 and has pronounced basic properties, reacts strongly alkaline to 

 litmus, and forms stable carbonates. Because of these facts, some 

 investigators consider arginine to be the nucleus of the protein 

 molecule. It is obtained in widely different amounts from different 

 proteins, over 85 per cent of certain protamines having been obtained 

 in the form of this amino acid. It is claimed that in the ordinary 



