CH. XXVIII.] PROTEIN CLEAVAGE PRODUCTS 423 



Two amino-acids are linked together as shown in the following 

 formula : 



HNH . R . CO i OH . H ' NH . R . COOH 



What happens is that the hydroxyl (OH) of the carboxyl (COOH) 

 group of one acid unites with one atom of the hydrogen of the next 

 amino (HNH) group, and water is thus formed, as shown within the 

 dotted lines: this is eliminated and the rest of the chain closes up. 

 In this way we get a dipeptide. The names glycyl, alanyl, leucyl, 

 etc., are given by Fischer to the NH 2 . E . CO groups which replace 

 the hydrogen of the next NH 2 group. Thus glycyl-glycine, glycyl- 

 leucine, leucyl-alanine, alanyl-leucine, and numerous other combina- 

 tions are obtained. If the same operation is repeated we obtain 

 tripeptides (leucyl-glycyl-alanine, alanyl -leucyl-tyrosine, etc.); then 

 come the tetrapeptides, and so on. In the end, by coupling the 

 chains sufficiently often and in appropriate order, Fischer has already 

 obtained substances which give some of the reactions of peptone. 



Hausmann's Method. This is a short and trustworthy procedure, by which 

 an approximate knowledge of the nitrogen distribution in the protein molecule 

 is ascertained. 



It is^ shortly as follows : The whole nitrogen of the protein is estimated by 

 Kjeldahl's method. A weighed amount is then hydrolysed by means of hydro- 

 chloric acid, and then the cleavage products are separated into three classes and 

 the nitrogen estimated in each, as 



1. Ammonia nitrogen. This comprises the nitrogen of the protein molecule 

 which is easily split off as ammonia, and is determined by distilling off the 

 ammonia with magnesia. 



2. Diamino-N. The fluid, free from ammonia, is precipitated by phospho- 

 tungstic acid, and the nitrogen present in the precipitate determined. This 

 represents the nitrogen of the diamino-acids (lysine, arginine, etc.). 



3. Mono-amino-N is then estimated in the residual fluid. 



The method has proved useful for the differentiation of proteins, and interest- 

 ing deductions as to their food value has been drawn from its results. 



Solubilities. The proteins are insoluble in alcohol and ether. 

 Some are soluble in water,* others insoluble. Many of the latter are 

 soluble in weak saline solutions. Some are insoluble, others soluble 

 in concentrated saline solutions. 



All proteins are soluble with the aid of heat in concentrated 

 mineral acids and alkalis. Such treatment, however, decomposes as 

 well as dissolves the protein. Proteins are also soluble in gastric and 

 pancreatic juices ; but here, again, they undergo a change, as we have 

 already seen. 



Heat Coagulation. Most native proteins, such as white of egg, 



* The proteins are not truly soluble in water.; they are in a state of colloidal 

 solution* a condition intermediate between true solution and suspension. Many of 

 their properties are due to this fact. 



