GENERAL CHEMICAL CHARACTERS OF PROTEINS 67 



protein derivatives can be prepared with fixed halogen content, which 

 could serve for the characterisation of individual proteins. The 

 bromination method of Hopkins and Pinkus, and the iodination 

 method of Blum and Vaubel, might, under certain stringent specified 

 conditions, be employed, and in this way a factor obtained for the 

 characterisation analogous somewhat to the " Hiibl number " for 

 fats. The suggestion of Cross, Bevan and Briggs for a "chlor- 

 amine " number is also worthy of further attention. 



SECTION XX. THE ACTION OF NITROUS ACID ON PROTEINS. 



When proteins are treated with nitrous acid a considerable froth- 

 ing and evolution of nitrogen takes place. This action has formed 

 the subject of many investigations. Schiff, on treatment of egg- 

 proteins with nitrous acid, obtained a product which no longer gave 

 the biuret reaction, which was insoluble in water, and to which he 

 gave the name desamido-albumin, on the assumption that nitrous 

 acid had destroyed the amido groups (z>., groups containing the 

 complex CO . NH 2 ). The action of nitrous acid on proteoses and 

 peptones formed the subject of investigations by Paal and Schrotter, 

 who obtained products differing in many properties from the original 

 substances. In more recent times the action of nitrous acid has 

 formed the subject of investigations by Levites, and more especially 

 by Skraup and his pupils. Both these investigators failed to confirm 

 the observation of Schiff, that the product of the reaction yields no 

 biuret reaction. Levites found, moreover, that the products contained 

 as much amide-nitrogen as the original bodies. Skraup and his 

 pupils have prepared these desamido-proteins from several pro- 

 teins (caseinogen, gelatin, serum-globulin), and subjected them to 

 hydrolysis. They found that, with one exception, the hydrolysis pro- 

 ducts did not differ very essentially from the original proteins. They 

 failed, however, to isolate lysine from the desamido-proteins, although 

 this base was obtainable from the proteins themselves. The ele- 

 mentary compositions also did not differ very greatly from those 

 of the proteins ; the ratios of the carbon, hydrogen and nitrogen 

 were approximately the same; in the case of caseinogen thedesamido 

 body contained considerably less phosphorus than the protein from 

 which it was derived. The yield of desamido body varied consider- 

 ably with the different proteins, although approximately the same 

 methods of preparation were employed (mixture of sodium nitrite 

 and protein solution treated with acetic acid) ; from gelatin the 

 yield was about 100 per cent, of that of the protein, from caseinogen 

 70 per cent., and from crystallised egg-albumin about 50 per cent. 

 Obermayer, and more recently Treves and Salomone, have stated 

 that diazo compounds can be obtained from proteins by the action 

 of nitrous acid, and from these dye-stuffs can be prepared. 



The Amino-Index. 



As to the mechanism of the action of nitrous acid little is yet 

 known. E. Fischer and Koelker have shown that nitrous acid, when 

 acting on polypeptides, causes an evolution of nitrogen, the quantity 

 of which bears no very definite atomic ratio to the nitrogen of the 

 polypeptide. It is possible from the colour of the products that 



