446 



PROTEINS 



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

 strongly basic character, are precipitated from solutions by 

 the addition of phosphotungstic acid, whereas mono-amino 

 acids are not. 



A method for obtaining an insight into the composition of 

 proteins on a comparatively small quantity of material, was 

 devised by van Slyke * and has since been modified from time 

 to time.f 



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 $ of phospho- 

 tungstic acid and estimating the amount of nitrogen in the 

 precipitate by Kjeldahl's method. 



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 ad- 

 jacent 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 



I 



I 



CH2 

 C6H4OH 



Tyrosine 

 residue 



Leucine residue 



Lysine residue 



* Van Slyke : "J. Biol. Chem.," 1911, 10, 15. 



t Plimmer and Lowndes : " Biochem. Journ.," 1927, 21, 247, 



X In order to ensure complete precipitation of arginine. 



