ORGANIC CHEMISTRY 21 



compound. By oxidising the end carbon atom to a primary 

 alcohol serine is formed and by replacing the hydroxyl 

 group (OH) of the primary alcohol by a sulphhydrate (SH) 

 group cysteine is formed. Two molecules of cysteine united 

 through their sulphur atoms form cystine. 



The union of alanine with aromatic and heterocyclic 

 compounds gives rise to important amino acids. Phenyl 

 alanine is formed by the union of benzene and alanine. On 

 oxidising benzene phenol or carbolic acid is formed ; this 

 united with alanine forms tyrosine. By uniting alanine with 

 indole tryptophane results. 



PROTEINS 



The proteins are formed by chains of these amino acids. 

 The amino acids are united to each other by the NH 2 and 

 COOH groups with the loss of one molecule of water. 



RCOOH + NH 2 R = RCO-NHR + H 2 O. 



By repeated linkages of this sort long chains have been 

 produced which behave like some of the simpler proteins, so 

 there is small doubt that the more complicated proteins will 

 be synthesised in the same way. 



NH 2 CH 2 COOH + NH 2 CH 2 COOH = 



Glycine. Glycine. 



NH 2 CH 2 CO-NHCH 2 COOH+H 2 



Glycyl-glycine. 



The colour tests for proteins depend firstly on the presence 

 of CONH groups which occur whenever two amino acids 

 unite with each other and secondly on the chemical nature 

 of the amino acids contained in the molecule. 



Biuret Reaction. To a solution of protein add a trace of 

 copper sulphate, then make the solution alkaline with sodium 

 or potassium hydroxide. A purple or pink colour is produced, 

 depending on the nature of the protein. This test is due to 

 the presence of a number of CONH groups united to each 

 other in various ways.* Owing to the pink colour given by 

 biuret with copper sulphate and alkali this test is called 

 the biuret test. 



Xanthoproteic reaction. On heating a protein solution with 

 a few drops of concentrated nitric acid a yellow colour is 

 produced. This test is due to the nitric acid forming nitro- 

 compounds with the benzene groups of the aromatic amino 

 acids.* The addition of alkali causes the colour to become 



* H. Schiff, Ber., 1896, vol. 29, p. 298. 



