THE PROTEINS 



C. ALKALOIDAL REACTIONS 



Proteins, like the polypeptides and the ammo-acids of which they a 

 composed, may function either as weak acids or as weak bases acconl, 

 they are treated with bases or acid radicals respectively. I n the prea 

 strong acids therefore, proteins act like organic bases, and are thrown down 

 m an insoluble form by the various alkaloidal precipitants With ml u, 

 proteins, such as the protamines, where there is a preponderance of basic 

 groups, it is not necessary to add mineral acid in order to ensure the precipita 

 tion. The following are the principal alkaloidal precipitants which mav be 

 employed : 



(a) Phosphotungstic acid. 

 (6) Phosphomolybdic acid. 



(c) Tannic acid. 



(d) Potassium mercuric iodide. 



(e) Acetic acid and potassium ferrocyanide. 



(/) Trichloracetic acid. (In order to precipitate all the coagulable 

 proteins from a solution it is treated with an equal volume of 10 

 per cent, trichloracetic acid, well shaken and filtered.) 



(g) Metaphosphoric acid. 



(Ti) Salicyl-sulphonic acid. 



These two latter are generally employed in a 5 per cent, solution. 



(i) Picric acid. 



A mixture of picric and citric acids is largely employed, under the 

 name of Esbach's reagent, as a precipitant for coagulable proteins 

 in the urine. 



D. TESTS DEPENDING ON THE COLLOIDAL CHARACTER 

 OF THE PROTEIN 



(1) HEAT COAGULATION. On boiling proteins in a very slightly acid 

 solution some are coagulated and form an insoluble white precipitate. 

 This test is applicable to albumins, globulins, and under certain conditions 

 to the derived albumins. In order that the separation of protein in this way 

 may be complete it is necessary to provide for the presence of neutral salts 

 and also for the maintenance of a slight acidity. The best method of carry- 

 ing out this test, therefore, is to boil the protein in slightly alkaline or neutral 

 solution after the addition of 2-5 per cent, of sodium chloride or sodium 

 sulphate. While the solution is in active ebullition 1 per cent, acetic acid is 

 added drop by drop until the reaction is just acid to litmus. By this means 

 a nearly perfect separation of all the coagulable proteins may be effected. 



(2) HELLER'S TEST. On pouring a solution of protein carefully down 

 the side of a test-tube containing strong nitric acid so as to form a lay.-r 

 on the top, a white layer of coagulated protein is produced at the junction 

 of the two fluids. A similar coagulative effect is produced by other strong 

 mineral acids. 



