PEPTIDES AMONG PROTEIN HYDROLYSIS 143 



they may be soluble, in which case they are precipitated on boiling off 

 the ammonia. 



Under conditions involving dehydration, as for example heating, or 

 treatment of the esters with alcoholic ammonia, the dipeptides are 

 converted into anhydrides which are ring-compounds, designated 

 Dikelopiperazines, and having the general formula: 



/CH.R.CCK 



NH</ NNH 



NDO.R.CH/ 



Under similar conditions the polypeptides are modified in an 

 analogous manner, yielding anhydrides with a ring-structure. 



When hydrolyzed, the polypeptides break down into their constit- 

 uent amino-acids, the imino-groups in the polypeptide molecule being 

 converted, by taking up the elements of water, into amino-groups, the 

 reaction being: 



-COHN- + H 2 O = -COOH -f H 2 N- 



A very large number of the polypeptides are hydrolyzed by the pro- 

 tein-splitting enzymes, Pepsin, Trypsin, etc., and, in some cases, at all 

 events, it is certain that the hydrolysis takes place in stages, as it does 

 with the proteins and peptones. We will revert in more detail to this 

 question in a later chapter (Chapter X) . . 



THE OCCURRENCE OF PEPTIDES AMONG THE PRODUCTS 

 OF PROTEIN HYDROLYSIS. 



The fact that hydrolyzing agents, when they act upon proteins, 

 lead to the production of the same substances, amino-acids, that the 

 polypeptides yield when hydrolyzed, and the fact that the synthetic 

 polypeptides resemble the native and derived proteins so closely in 

 their physical and chemical characteristics, afford a very strong basis 

 for the supposition that the proteins are, in fact, chains of ammo-acid 

 radicals, linked together as they are in the polypeptides by the mutual 

 neutralization of NH 2 - and COOH-groups. Confirmation of this 

 view is afforded by the fact that di,- tri- and tetrapeptides have been 

 frequently found and identified among the products of the incomplete 

 hydrolysis of proteins. Thus glycyl-d-alanine, glycyl-1-tyrosine, and 

 a tetrapeptide built up by the union of two molecules of glycine, one 

 of d-alanine and one of 1-tyrosine have been isolated from among the 

 cleavage-products of Fibroin, glycyl-1-leucine, 1-leucyl-d-alanine, glycyl- 

 d-valine and a dipeptide yielding d-alanine and* 1-proline on hydrolysis, 

 have been obtained from Elastin, glycyl-1-proline has been obtained from 

 Gelatin, and a lysyl-glycyl peptide from Egg-albumin. From Gliadin 

 a dipeptide has been split off which yields phenylalanine and proline 

 when completely hydrolyzed. 



