31 



PROTEINS 



Hausmann,* and has since been modified by Giimbel ; f it de- 

 pends on the fact that di-amino acids, in virtue of their strongly 

 basic character, are precipitated from solution by the addition 

 of phosphotungstic acid, whereas mono-amino acids are not. 



The substance to be examined is first hydrolysed by boiling 

 with concentrated hydrochloric acid for several hours under a 

 reflux condenser. The amount of amide nitrogen and ammonia 

 in the resulting mixture is then determined by distillation with 

 magnesia in vacuo at 40° C. 



2. The di-amino acid nitrogen is next determined by 

 precipitating the residue in the flask with excess I of phospho- 

 tungstic acid and estimating the amount of nitrogen in the 

 precipitate by Kjeldahi's method (see p. 340). 



3. The nitrogen combined as mono-amino acids may be 

 determined directly in the filtrate or by the difference between 

 the total nitrogen and the sum of the nitrogens separately 

 determined by the above methods. 



The fact that proteins on hydrolysis yield such a large 

 number of amino acids, all of which have the amino group 

 attached to the a-carbon atom (i.e., the carbon atom adjacent 

 to the carboxyl), has led to the conclusion that the protein 

 molecule is really composed of a long chain of these acids 

 linked together in some such way as is represented below. 

 — NHCH . CO— NH . CH . CO . NH . CH . CO . NH . CH . COOH 



Lysine residue 



Such a compound would, of course, give the biuret reaction 

 and contain but few free carboxyl groups or amino groups, 

 which is entirely in agreement with the properties of proteins. 

 Acting on this assumption, Fischer has synthesized a number 

 of compounds containing such a structure, with the object of 

 studying their properties and comparing them, if possible, 



* Hausmann : " Zeit. physiol. Chem.," 1899, 27, 95 ; 1900, 29, 136. 

 t Giimbel : " Beitr. chem. Phys. u. Path.," 1904, 5, 297. 

 X In order to ensure complete precipitation of arginine. 



