PROTEIDS AND THEIR CLASSIFICATION. 877 



copper sulphate are added carefully so as to avoid an excess. A 

 purple color is obtained. Some proteids (peptones) give a red 

 purple, others a blue purple. If only a blue color, without any 

 mixture of red, is obtained, no proteid is present. At present 

 this reaction gives the best single test for proteid. It obtains 



its name from the fact that it is given by biuret HN <CC)NH 2 ' & 

 compound that may be formed by heating urea. Two molecules 

 of urea give off a molecule of ammonia and form biuret. 



2. The Millon reaction. The proteid solution is boiled with Millon'a 



reagent. The solution or the precipitate, if one is formed, takes 

 on a reddish color, which varies in intensity with different proteids. 

 Millon's reagent consists of a solution of mercuric nitrate in nitric 

 acid containing some mercurous nitrate. This reaction is supposed 

 to be given by the tyrosin grouping in the proteid molecule, and 

 fails, therefore, with those proteids in which tyrosin is not 

 present. 



3. The xanthoproteic reaction. Nitric acid is added to strong acid 



reaction and the solution is then boiled. After cooling ammonia 

 is added. The ammonia causes the development of a deep-yellow 

 color if proteid is present. This reaction is supposed to be due 

 to the presence in the molecule of the groupings belonging to the 

 aromatic series. 



4. Adamkiewicz's reaction. A mixture is made of one volume of con- 



centrated sulphuric and two volumes of glacial acetic acid; if the 

 proteid solution is added to this mixture and warmed a reddish- 

 violet color is obtained. According to Hopkins and Cole, the re- 

 action depends upon the presence of glyoxylic acid in the acetic 

 acid. This reaction seems to be due to the tryptophan grouping 

 in the proteid molecule. 



5. Liebermann's reaction. Dry proteid purified with alcohol and ether 



gives a blue color upon boiling with strong hydrochloric acid. 



6. The lead sulphid reaction. The proteid solution is boiled with a 



solution of a lead salt made strongly alkaline with soda or potash. 

 A black precipitate or black or brown coloration results according 

 to the amount of proteid. The color is due to the splitting off of 

 sulphur and formation of lead sulphid. It is given, therefore, by 

 the sulphur-containing groups in the proteid molecule. 



7. The Molisch reaction. A few drops of an alcoholic solution of a- 



naphthol are added to the proteid solution and then strong sul- 

 phuric acid. A violet color is obtained. This reaction is given 

 by the carbohydrate grouping in the proteid molecule. The strong 

 acid f orms f urf urol from this group, which then reacts with the naph- 

 thpl. The reaction is not given by those proteids that do not con- 

 tain a carbohydrate group. 



Classification of the Proteids. No classification of the proteids has 

 been proposed which is entirely satisfactory. Eventually a classification 

 will be obtained based upon the chemical structure of the various proteids, 

 but our knowledge at present is much too incomplete for this purpose. We 

 must be content with a less satisfactory system based upon empirical reac- 

 tions which have gradually been recognized in the course of physiological 

 investigations. 



I. Simple proteids 



Albumins. 



Globulins. 



Albuminates or derived albumins. 



Nucleo-albumins (phosphoproteids) , 



Proteoses and peptones. 



Histons. 



Protamins. 



Coagulated proteids. 



