THE PKOTEINS 93 



(6) REACTIONS INDICATING THE PRESENCE OF CARBOHYDRATES. 



Molisch's test is applied as follows. A few drops of alcoholic solution of a-naphthol 

 and then strong sulphuric acid are added to a protein solution. A violet colour is 

 produced, which on addition of alcohol, ether, or potash turns yellow. The reaction 

 is determined by the presence, either as an impurity or a constituent part of the mole- 

 cule, of a carbohydrate radical which, under the influence of strong sulphuric acid, is 

 converted into furfurol. The furfurol gives the colour reaction with the a-naphthol. 

 Another test for the carbohydrate radical is the orcin reaction. A small quantity 

 of the dried albumin is added to 5 c.c. of fuming hydrochloric acid, and the mixture is 

 then warmed. When the albumin is nearly all dissolved, a little solid orcin is added 

 on the point of a knife, and then a drop of ferric chloride solution. After warming 

 this mixture for some minutes, a green colour is produced which is soluble in amyl 

 alcohol and gives a definite absorption spectrum. 



B. METALLIC SALTS 



The following metallic salts form double insoluble compounds with proteins, and 

 therefore cause a double precipitation when added to solutions of these bodies : ferric 

 chloride, copper sulphate, mercuric chloride, lead acetate, zinc acetate. 



C. ALKALOIDAL REACTIONS 



Proteins, like the polypeptides and the amino-acids of which they are composed, 

 may function either as weak acids or as weak bases, according as they are treated with 

 bases or acid radicals respectively. In the presence of _ strong acids therefore, 

 proteins act like organic bases, and are thrown down in an insoluble form by the various 

 alkaloidal precipitants. With certain 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 precipitation. The following are the principal alkaloidal precipitants 

 which may be employed : 



(a) Phosphotungstic acid. 



(b) 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. 



(h) 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 carrying 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. 



