32 THE GENERAL CHARACTERS OF THE PROTEINS 



(a) The Molisch-Udransky Reaction. Concentrated sulphuric 

 acid is added to a solution of protein containing a few drops 

 of an alcoholic solution of a-naphthol. A violet colour is 

 produced which turns yellow on addition of alcohol, ether 

 or sodium hydroxide. If thymol be employed instead of 

 a-naphthol a carmine-red colour is produced. 



(b) BiaFs Modification of the Orcin Reaction. A small quantity of 



dried protein is added to 5 c.c. of fuming hydrochloric acid, and 

 the mixture is then warmed. When the protein is nearly 

 all dissolved a little solid orcin is added, and then a drop of 

 ferric chloride solution. After warming for some time a 

 green coloration is produced, which is soluble in amyl alcohol. 



VII. Sulphur Reaction. On warming a protein solution with 

 sodium hydroxide in the presence of a lead salt (lead acetate) a 

 black coloration is produced owing to the presence of sulphur in 

 the protein molecule. 



As already mentioned, the above tests are not common to all 

 the proteins, and they serve, therefore, for qualitatively distinguishing 

 between them in certain cases. Thus, for example, hydroferrocyanic 

 acid gives only a faint precipitate with gelatin ; with the proteoses 

 it gives a precipitate which disappears on boiling but reappears on 

 cooling the solution ; with peptones it gives no precipitate. 



Nitric acid also gives a precipitate with the proteoses, which 

 dissolves on boiling and reappears on cooling ; the peptones are not 

 precipitated. 



The alkaloidal reagents precipitate the majority of the proteins 

 in acid solution only ; the strongly basic protamines, however, can 

 be precipitated in alkaline solution. The peptones are not pre- 

 cipitated by picric or trichloracetic acids, or by potassio-mercuric 

 iodide ; they are precipitated, however, by tannic, phosphomolybdic 

 and phosphotungstic acids. The colour reactions, as already men- 

 tioned, are due to certain specific groups, which are not common 

 to all proteins. All give the biuret reaction, the peptones giving a 

 characteristic pink coloration. 



The Millon reaction, which is due to the presence of tyrosine, is 

 given only very faintly by gelatin ; the reaction in this case may be due 

 to an impurity, but according to Morner the reaction occurs normally 

 if too much reagent be not present. The reaction is not given by 

 reticulin, nor by the protamines, with the exception of cyclopterine. 



The Adamkiewicz (Hopkins-Cole) reaction varies also in in- 

 tensity with the different proteins. It is not given at all by gelatin, 

 which does not yield tryptophane as a product of hydrolysis. 



An interesting example of the application of the colour reactions 

 is afforded by Pick, who found considerable differences in the colour 

 reactions of the various fractions of Witte's peptone, obtained by the 

 method which has been already discussed (p. 14). 



SECTION XIII. THE CHEMICAL COMPOSITION OF PROTEINS. 

 THE NITROGEN CONTENT AND DISTRIBUTION. 



Proteins sometimes occur in nature combined with other organic 

 complexes, which have been designated by Hoppe-Seyler as "pros- 

 thetic" groups, from which, by gentle chemical treatment (e.g., by 



