CH. II.] 



COLOUR TESTS. 



41 



groups attached to one another, to the same nitrogen atom, or to the same 

 carbon atom. Thus it is given by 



CONK,, CONH a CONH 2 



CONH a NH CH 2 



CONH 2 CONH 2 ' 



Oxamide. Biuret (See Ex. 345). Malonamide. 



The cause of the reaction with proteins is the presence of one or more groupings 

 of the last type, formed by the condensation of the carboxylic group of an 

 amino-acid with the amino group of another amino-acid. The linkage thus 



formed is known as the " pep tide " linkage, 

 polypeptide, glycyl-alanyl-tyrosine 



Thus it would be given by the 



25. The sulphur reaction. Boil a little undiluted egg-white 

 or serum with some 40 per cent, sodium hydroxide for two minutes, 

 and then add a drop or two of lead acetate. The solution turns deep 

 black. 



NOTES. i . This reaction is due to the fact that the sulphur of the protein 

 is liberated as sodium sulphide when boiled with the strong alkali. The 

 sulphide gives a black colour or precipitate of lead sulphide when the solution 

 is subsequently treated with lead acetate. 



2. The reaction does not succeed with caseinogen, peptones, and certain 

 other proteins. 



3. The sulphur in the protein is mainly combined as 



CH 2 .SH 

 CH.NH 2 



COOH 

 Cystein 



or 



CH 2 .S.S.CH 2 



CH.NH 2 CH.NH 2 



I I 



COOH COOH 

 Cystine 



26. Molisch's reaction. Treat 5 cc. of the diluted solution 

 with three or four drops of a I per cent, solution of alpha-naphthol or 

 of thymol in alcohol. Mix, and run about 5 cc. of concentrated 

 sulphuric acid under the fluid. A violet ring is formed at the 

 junction of the two liquids. 



NOTES. i. The reaction is due to the presence of a carbohydrate group 

 (glucosamine) in the protein. This is converted by the acid to furfurol, which 

 condenses with the alpha-naphthol or the thymol to give the purple colour. 

 (See notes to Exs. no and 114.) 



2. A green ring is often seen in addition to the violet ring. This is due 

 to the action of the sulphuric acid on the alpha-naphthol. 



