42 PROTEINS. 



glacial acetic acid, and adding strong sulphuric acid coloured 

 rings are formed at the junction of the two liquids. This reaction 

 is due to the presence of glyoxylic acid or formaldehyde in the 

 acetic acid, which compounds give a blue or bluish-violet colour 

 with tryptophane. Hopkins and Cole, to make the test more 

 certain, add glyoxylic acid, while, as Rosenheim has shown, 

 formaldehyde is equally serviceable. 



5. Lieberman's Reaction. Proteins which have been ex- 

 tracted with ether give a blue or bluish-violet colour on boiling 

 with strong hydrochloric acid. This reaction is really the same 

 as the above, and it is due to the presence of glyoxylic acid or 

 other aldehydic compounds in the ether. 



6. Ehrlich's Diazo Reaction. On adding a diazonium salt 

 to a soluble protein, and making alkaline, a red colour is pro- 

 duced, if histidine or tyrosine be present in the molecule. A 

 diazotised solution of sulphanilic acid is convenient. Other 

 radicles give a yellow colour. 



7. Richmond and Miller's Diazo Reaction. On diazo- 

 tising a solution of a protein, and adding an alkaline solution of 

 /?-naphthol, a colour (usually yellow) is produced, and gas is 

 given off in the cold. This test proves the presence of aromatic 

 as well as other ammo -groups. 



8. The Halogen Reaction. Chlorine and bromine give 

 insoluble compounds with all soluble proteins. Iodine gives a 

 brown coloration. 



9. Aldehyde Reaction. On adding a solution of formalde- 

 hyde to a solution of a protein neutral to phenol-phthalein it 

 becomes acid. This is characteristic of a-amino -acids, the basic 

 amino -group being converted into a very feebly basic methylene- 

 imino-group. 



10. Ehrlich's Aromatic Aldehyde Reaction. Certain 

 aromatic aldehydes when added to proteins in acid solution 

 give w r ell-marked coloured condensation products, p-dimethyl- 

 amino -benzaldehyde and vanillin (p - hydroxy - m - methoxy - 

 benzaldehyde) give a red colour (the latter, however, tinged 

 with blue) and p - nitro - benzaldehyde a green colour. This 

 reaction appears to be characteristic of the tryptophane 

 radicle. 



11. On boiling most proteins with an alkali, a portion of the 

 sulphur is transformed into sulphide, which may conveniently be 

 demonstrated by the black colour given on adding a solution of 

 a lead salt. 



The presence of protein may be considered as proved if re- 

 actions 1, 8, and 9 are given; many, though not necessarily 

 all, of the other reactions will also be obtained should proteins 

 be present. 



