PEOTOPLASM 9 



It is these acids with the benzene ring which give the xantho- 

 proteic test an orange colour when ammonia is added to a 

 protein heated with nitric acid. 



In most proteins the di-amino acids are less abundant than 

 the mon-amino, and in some they are absent. But in a simple 

 form of protein, discovered by Kossel, linked with nucleic acid 

 in the heads of spermatozoa and called by him Protamine, these 

 diamino acids constitute about 80 per cent, of the molecule. 



While these amino acids are normally converted to urea 

 before being excreted, in certain states of the metabolism they 

 appear in the urine. 



The amides are always present in small amount, linked 

 together as in biuret H 2 N CO NH CO NH 2 , and it is these 

 which give the biuret test the pink or violet when sodic 

 hydrate is added to the protein with which a trace of cupric 

 sulphate has been mixed. 



The pyrrhol derivatives are ring formations of four carbons 

 linked by an amidogen, 



H C C H 



II II 

 H C C H 



\/ 

 N 



H 



They are seen in indol and its derivatives, where a phyrrhol 

 ring is attached to the benzene ring, thus 



\ 



N 



They are apt to split off from the protein still linked to amino- 

 acetic acid (see p. 381). 



The presence of indol in the protein molecule explains the 

 purple reaction when a protein is treated with glyoxylic acid 

 and sulphuric acid Adamkiewicz's reaction. 



