CONSEQUENCES OF THE POLYPEPTID STRUCTURE 29 



have to be opened up. The internal-salt formation may go a 

 step further, with the formation of anhydrides such as 



NH 



such anhydride formation being frequently observed in the poly- 

 peptids. Then two molecules of an amino-acid, and therefore, 

 of a poly-amino-acid such as protein, may unite with one another 

 in either of two ways, thus : 



/NH 2 HOOC. /NH.OC.R.NH 2 



R' + ^R = R' + H 2 



X COOH R 2 ^ / N COOH 



or 



y NH 2 HOOC V /NH.OC, 



R' + ^R = R' X R + 2H 2 



X COOH HJX' X CO.HN X 



the product being, in the first instance, a poly-amino-acid of a 

 higher order and greater molecular weight, and in the second 

 an " internal salt" or anhydride.* Another type of anhydride 

 which may be formed is that of a diketopiperazin, of the general 



formula: 



/ CH(R).C(X 

 NH/ X NH 



O.CH(R). 



and these anhydrides may exist in two isomeric forms, the keto 

 and enol forms, the former being represented by the above for- 

 mula, and the latter by the general formula : 



The enol form carrying with it the possibility (in the presence 

 of water) of the transient formation of salts with bases before the 

 ring breaks up.f 



* Such as leucyl-glycin anhydride. 



t Cf. R. H. Aders Plimmer (53), Part 2, p. 36. 



