484 PROTOPLASM 



A protein is thus positive in tin acid medium and negative in an 

 alkaline one. 



The foregoing story can be retold to advantage by using a 

 simple protein derivative. The simplest amino acid is glycine; 

 in the presence of an acid (hydrochloric), it forms glycine hydro- 

 chloride, as follows: 



H H 



^C— C— NH2 + HCl = ^C— C— NH3-C1 = CI- + 

 OH Ij OH Ij 



(H3NCH2COOH)+ 



In the presence of a base (sodium hydroxide), glycine forms 

 sodium glycinate and water: 



H H 



^C— C— NH2 + NaOH = ^C— C— NH2 + H2O = 



OH Jj ONa ^ 



Na+ + (H2NCH2COO)- + H2O 



As the acid tendencies of proteins are contribvited by the 

 carboxyl radical (COOH) and the basic ones by the amino radical 

 (NH2), the individual amino acids themselves should exhibit 

 amphoteric properties within the protein molecule, in that they 

 join, one to another, by linking the amino group of one amino 

 chain with the carboxyl group of another; thus, the diamino 

 molecule of glycyl-glycine is HsN-CHs-CO— NH-CH2-C00H, 

 and the triamino molecule of digylcyl-glycine is H2N-CH2-C0 — ■ 

 NHCH2CO— NHCHs-COOH. Both are formed by linkage 

 of the carboxyl radical of one amino acid with the amino radical 

 of another amino acid of the same kind, a molecule of water 

 being split off. In this way, an amphoteric molecule may show 

 a tendency toward internal salt formation; that is to say, the 

 acid and the basic radicals may mutually satisfy each other. 

 The open chain would then be converted into a ring, and chemi- 

 cally active glycine 



H 



^C— C— NH2 

 OH Ij 



