10 THE AMINO ACIDS 



and peptones are characterized chiefly by their greater 

 solubility and by the fact that, unlike most other pro- 

 teins, they are diffusible through suitable membranes. 

 They represent stages in gastric, pancreatic and bac- 

 terial digestions of protein and the peptones are 

 regarded as products of greater cleavage than the 

 proteoses. There are several proteoses, as protopro- 

 teose, heteroproteose and deuteroproteose and prob- 

 ably there may be several types of peptones. The 

 proteoses are distinguished from the peptones prin- 

 cipally in being precipitated from solutions by satura- 

 tion with ammonium or zinc sulphate. 



The peptides are "definitely characterized combina- 

 tions of two or more amino acids, the carboxyl 

 (COOH) group of one being united with the amino 

 (NH 2 ) group of the other with the elimination of a 

 molecule of water." For example, if two molecules 

 of glycocoll (glycine) amino-acetic acid are con- 

 densed, a peptide, glycyl-glycine, will result. Thus 



NHH NH a 



CH,.CO OH 



NH 



H 



CH a .CO.NH 



CH a .COOH 



CH 9 . COOH glycyl-glycine . 



The peptides are designated di-tri-tetra-peptides, etc., 

 according to the number of amino acids in combina- 

 tion. The name polypeptides is also applied to these 



