LESSON IV 

 THE PROTEINS 



1. Tests for Proteins. The following tests are to be tried with a mixture of 

 one part of white of egg to ten of water. (Egg-white contains a mixture of 

 albumin and globulin.) 



(a) Heat Coagulation. Faintly acidulate with a few drops of 2-per-cent. 

 acetic acid and boil. The protein is rendered insoluble (coagulated protein). 



(b) Precipitation with Nitric Acid. The addition of strong nitric acid to 

 the original solution also produces a white precipitate. 



(c) Xanthoproteic Reaction. On boiling the white precipitate produced 

 by nitric acid it turns yellow ; after cooling add ammonia ; the yellow becomes 

 orange. 



(d) Millon's Test. Millon's reagent (which is a mixture of the nitrates 

 of mercury containing excess of nitric acid ; see p. 5) gives a white precipitate, 

 which turns brick -red on boiling. 



(e) After the addition of a few drops of 20-per-cent. acetic acid, potassium 

 ferrocyanide gives a white precipitate. 



(/) Rose's or Piotroivski' s Test. Add one drop of a 1-per-cent. solution 

 of cupric sulphate to the original solution and then caustic potash, and a 

 violet solution is obtained. 



Kepeat experiment (/) with a solution of commercial peptone, and note 

 that a rose-red solution is obtained. This is called the biuret reaction. 



(g) Rosenheim's Formaldehyde -Reaction. Add to the solution of com- 

 mercial peptone a very dilute solution of formaldehyde (1 : 2,500), and then 

 about one third of the volume of strong sulphuric acid containing (as most 

 commercial specimens of the acid do) a trace of an oxidising agent such as 

 ferric chloride or nitrous acid. A purple ring develops at the surface of contact. 

 This reaction probably plays a part in the original 



(h) Adamkiewicz Reaction, in which glacial acetic acid was used instead 

 of the formaldehyde. Most commercial specimens of glacial acetic acid con- 

 tain hydrogen peroxide as an impurity ; the oxidising action of this on the 

 acetic acid leads to the formation of traces of glyoxylic acid arid formalde- 

 hyde ; the necessary factors for the occurrence of the formaldehyde reaction 

 are thus present. (According to Hopkins glyoxylic acid itself with pure 

 sulphuric acid gives the test with proteins.) 



The same reactions (g and h) are given by the solution of egg-white, but 

 not so markedly. 



2. Action of Neutral Salts. (a) Saturate the solution of egg-white with 

 magnesium sulphate by adding crystals of the salt and grinding it up 

 thoroughly in a mortar. A white precipitate of egg-globulin is produced. 

 Filter. The nitrate contains egg-albumin. The precipitate of the globulin 

 is very small. 



(6) Half saturate the solution of egg-white with ammonium sulphate. 

 This may be done by adding to the solution an equal volume of a saturated 



